research paper on biofuel from algae

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Biofuel production from microalgae: challenges and chances

• Published: 02 May 2022
• Volume 22 , pages 1089–1126, ( 2023 )

Cite this article

• Anh Tuan Hoang 1 ,
• Ranjna Sirohi 2 , 3 ,
• Ashok Pandey 2 , 4 , 5 ,
• Sandro Nižetić 6 ,
• Su Shiung Lam 7 ,
• Wei-Hsin Chen   ORCID: orcid.org/0000-0001-5009-3960 8 , 9 , 10 ,
• Rafael Luque 11 , 12 ,
• Sabu Thomas 13 ,
• Müslüm Arıcı 14 &
• Van Viet Pham 15  

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The inherent capability and increased efficiency of microalgae to convert sunlight into solar chemical energy are further enhanced by the higher amount of oils stored in microalgae compared to other land-based plant species. Therefore, the widespread interest in producing biofuels from microalgae has gained considerable interest among leading energy experts and researchers due to the burgeoning global issues stemming from the depletion of fossil fuel reserves, future energy security, increasing greenhouse gas emissions, and the competition for limited resources between food crops and conventional biomass feedstock. This paper aims to present the recent advances in biofuel production from microalgae and the potential benefits of microalgae in the energy and environmental sectors, as well as sustainable development. Besides, bottlenecks and challenges mainly relating to techniques of cultivation and harvesting, as well as downstream processes are completely presented. Promising solutions and novel trends for realizing strategies of producing biofuels from microalgae on an industrial and commercial scale are also discussed in detail. Alternatively, the role of microalgae in the circular economy is thoroughly analyzed, indicating that the potential of scaling up current microalgae-based production could benefit from the waste-to-energy strategy with microalgae as a key intermediate. In the future, further research into combining different microalgae biomass pretreatment techniques, separating the microalgae feedstock from the cultured media, developing new species, and optimizing the biofuel production process should be carried out to reduce the prices of microalgae biofuels.

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Hoang, A.T., Sirohi, R., Pandey, A. et al. Biofuel production from microalgae: challenges and chances. Phytochem Rev 22 , 1089–1126 (2023). https://doi.org/10.1007/s11101-022-09819-y

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Latest development in microalgae-biofuel production with nano-additives

• Nazia Hossain   ORCID: orcid.org/0000-0001-7925-0894 1 ,
• T. M. I. Mahlia 2 &
• R. Saidur 3 , 4  

Biotechnology for Biofuels volume  12 , Article number:  125 ( 2019 ) Cite this article

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Microalgae have been experimented as a potential feedstock for biofuel generation in current era owing to its’ rich energy content, inflated growth rate, inexpensive culture approaches, the notable capacity of CO 2 fixation, and O 2 addition to the environment. Currently, research is ongoing towards the advancement of microalgal-biofuel technologies. The nano-additive application has been appeared as a prominent innovation to meet this phenomenon.

The main objective of this study was to delineate the synergistic impact of microalgal biofuel integrated with nano-additive applications. Numerous nano-additives such as nano-fibres, nano-particles, nano-tubes, nano-sheets, nano-droplets, and other nano-structures’ applications have been reviewed in this study to facilitate microalgae growth to biofuel utilization. The present paper was intended to comprehensively review the nano-particles preparing techniques for microalgae cultivation and harvesting, biofuel extraction, and application of microalgae-biofuel nano-particles blends. Prospects of solid nano-additives and nano-fluid applications in the future on microalgae production, microalgae biomass conversion to biofuels as well as enhancement of biofuel combustion for revolutionary advancement in biofuel technology have been demonstrated elaborately by this review. This study also highlighted the potential biofuels from microalgae, numerous technologies, and conversion processes. Along with that, the study recounted suitability of potential microalgae candidates with an integrated design generating value-added co-products besides biofuel production.

Conclusions

Nano-additive applications at different stages from microalgae culture to end-product utilization presented strong possibility in mercantile approach as well as positive impact on the environment along with valuable co-products generation into the near future.

Biofuel has caught substantial attention worldwide nowadays as an alternative fuel due to its capability to adapt with gasoline for a maximum 85% blend without any engine modification. Subsequently, the suitability of various candidates for biofuel is being continuously quested by the researchers and environmentalists [ 1 , 2 , 3 , 4 , 5 ]. In this recent era, one of the most sophisticated technologies, nano-technology integration with bioenergy application by the nano-energy sector has brought a revolutionary impact on biofuel conversion processes and enhancement of engine performances. Nano-technology is defined as designing a device or material in nano-scale (10 −9 m). To accelerate the biofuel yield and improve the efficiency of biofuel utilization in petrol and diesel, nano-technology has been initiated via nano-additives such as nano-magnets, nano-crystals, nano-fibres, nano-droplets, and others [ 6 , 7 , 8 ]. Figure  1 presents the perspectives of nano-additives on microalgae cultivation to microalgal-biofuel implementation.

Nano-additive applications for the enhancement of microalgae cultivation to biofuel implementation

On this eve of the quest for suitable biomass for biofuel, the concept of microalgae cultivation appeared to the spotlight for biofuel manufacturing due to several positive perspectives such as (i) they do not clash with human or animal food chains, (ii) very rich with carbohydrate, protein, and oil content, (iii) can grow in aqueous media such as wastewater, freshwater, saline water, and assimilate nutrients from brackish water, salt water, or highly polluted water, (iv) demand low water, (v) sustain capability to grow whole year naturally with sunlight presence, (vi) can be cultivated in the waste dump area, sea, ponds, rivers, industrial, and municipal waste drainage, wet bare lands especially in cold regions, (vii) develop sustainable O 2 generation system, and (viii) diminish CO 2 by up taking it for photosynthesis respiration [ 9 , 10 , 11 , 12 , 13 , 14 ]. In addition, microalgae contain very short harvesting life cycle and yield nascent biomass that drives higher productivity of the desired biofuel. Interestingly, microalgae carry a prodigious amount of carbohydrates, protein and lipid, the sole components of biofuel conversion [ 13 , 15 , 16 , 17 ]. Nano-technology applications have been implemented to biofuel industries, since the existing controversial approaches of traditional microalgae culture-biofuel production contain a number of limitations such as inconsistent industrial-scale microalgae production, high microalgae production and harvesting cost, energy consumption for biofuel production from microalgae, and the increase of greenhouse gas intensity in environmental [ 18 ]. Nano-technology applications can be entailed in different stages from microalgae cultivation to microalgae-biofuel application in fuel engines due to durability, recyclability, adsorption efficiency, catalytic performance, stability, crystallinity, economical advantage, high storage capacity, excellent biofuel yield, and environment-friendly characteristics. According to the previous studies, nano-technology application enhanced microalgae cultivation, the maximum yield of numerous microalgae biofuels as well as microalgae-biofuel implications in petrol and diesel engines. Various nano-materials, e.g., nano-fibres, nano-particles, nano-tubes, nano-sheets, and other nano-structures, have been investigated as effective nano-catalysts in direct and indirect approaches in biofuel (e.g., bioethanol, biodiesel, biomethane, and others) yield enhancement [ 19 , 20 , 21 , 22 ]. For instance, magnetic nano-particles were used as a carrier for enzyme immobilization for bioethanol and biodiesel generation effectively. Owing to high coercivity and powerful paramagnetic characteristics, magnetic nano-particles were also preferred for methanogenesis to produce biomethane [ 21 ].

To authors’ best knowledge, no review study has been performed on numerous biofuel productions from microalgae integrated with the nano-additive application so far. The closest review with this study was conducted on the bioenergy production from lignocellulosic biomass (agricultural residues), industrial waste (sludge) as well as algae (microalgae and macroalgae) with the nano-scale optimization which has merely emphasized on the mechanism of nano-particles, biomass characteristics, and nano-particle application on biomass growth [ 23 ]. Compared with that, the current review contextualized the numerous biofuel productions from pure microalgae and optimization with nano-additive application on biomass growth to end-product application. Therefore, the major objectives of this review work are (i) to determine the array of the techniques and methods associated with nano-particles incorporation with microalgae culture as well as microalgal biofuel, (ii) to demonstrate divergent nano-additive applications on microalgae cultivation, biomass conversion to biofuels, and biofuel combustion, (iii) to identify the potential sources of microalgae, especially the carbohydrate, protein, and lipid-enriched microalgae types for biofuel production and determine the possible microalgae biofuels, biomass conversion technologies, and processes to biofuels, and (iv) to assess the future prospects of the process development planning along with integrated design of some other value-added products besides biofuel.

General perspective of microalgae

Microalgae are referred as photosynthetically driven single or multi-cellular living being, the habitat of moist environment either on the solid mud or float on various water types, e.g., fresh water, marine water, wastewater with the presence of sunlight, or artificial light. The scientific consensus is that through photosynthesis respiration, they convert CO 2 to O 2 and generate large amounts of cellular energy content embedded with sugar, protein, and lipid [ 24 , 25 , 26 , 27 ].

Nowadays, industrialization and urbanization threaten the existing ecosystem severely by dumping heavy metal waste containing phosphorus, nitrogen, sulfur and others as well as exhaling high amount CO 2 to the free air. Another knocking threat to the energy sector is rapid depletion of fossil fuel worldwide due to excessive energy uses [ 28 , 29 , 30 ]. With this circumstance, microalgae cultivation in the wastewater, unused fresh, and saline water, drainage is considered as suitable scientific solution for green energy due to some favorable aspects such as multi-functionality, genuine conversion competency biologically and flexibility with growth system, wastewater accumulation, CO 2 sequestration, and large amount of carbohydrate–lipid–protein content. To note, carbohydrate–lipid–protein are the main components to generate divergent biofuels (e.g., bio-oil, biodiesel, biobutanol, and others) and biogas (e.g., bio-hydrogen) [ 31 , 32 ]. The cellular components of microalgae are composed of huge fraction of lipid, protein, and carbohydrates resulting in the driving factors of biofuel production. Table  1 presents some well-known potential microalgae candidates for biofuels. These species were extensively researched in the laboratory and large-scale applications so far. Type and description of these species have been tabulated to present a detailed view of selected species, suitable growth conditions (such as water type and region for cultivation), availability, and cellular specifications. Table  2 represents prime microalgae cellular component composition of several well-known microalgae species for biofuel production [ 24 , 33 , 34 ].

Biofuels from microalgae

Numerous biofuels, e.g., bioethanol, biodiesel, bio-oil, biomethane, bio-hydrogen, and others, have been extracted from microalgae [ 46 , 47 ]. Nano-particles’ incorporation with microalgae cultivation (e.g., cell suspension, cell separation, and cell harvesting), biofuel conversion technologies, and biofuel application have amplified the overall yield in every stage [ 22 ]. According to the previous studies, a very small amount of colloidal hydrous iron(III) oxide particles boosted almost 100% microalgae cell suspension; magnetic particles incorporated with aluminum sulfate were very effective for cell separation from the mixed culture of Anabaena and Aphanizomenon microalgae species; silver nano-particles application on Chlamydomonas reinhardtii and Cyanothece 51142 microalgae harvesting increased 30% higher biomass productivity; and calcium-oxide nano-particles escalated the large-scale biodiesel conversion yield up to 91% via catalytic transesterification [ 18 , 22 , 48 ]. This study summarized the overall microalgae cultivation integrated with nano-particles until biofuel production in Fig.  2 . Different biofuels from microalgae and conversion processes are diagrammed in Table  3 .

Process flow diagram of carbon capture, water treatment and biofuels production from microalgae incorporated with nano-particles [ 18 , 25 , 49 , 50 , 51 , 52 , 53 , 54 ]

Preparing techniques of nano-additives for microalgae biofuel

Magnetic nano-particle (NP) powder has been enumerated to the microalgae cell suspension in the photobioreactor cultivation process to flocculate cells for uniform distribution of nutrients and light all over the reactor. Immunomagnetic detection and modification of microalgae cell by NPs are another well-practiced method for cell suspension enhancement. Nano-liquid has been injected to the cell culture for microalgae harvesting and bio-separation through this technique. Silver nano-materials have also been implemented on the photobioreactor surface coating for higher light accessibility [ 22 ]. Along with microalgae culture and harvesting, sphere nano-particles have been enacted during hydrolysis, lipid extraction, transesterification, and biofuel purification from microalgae via irradiation and ultra-sonication methods and much higher biofuel yield have been obtained. Another established method of nano-materials application includes enzymatic nano-catalyst, lipase carrier. The reactant diffusion rate enhancement by the NPs to the active side of lipase has been determined by Eq.  1 [ 55 ]:

where R df  = diffusion rate to the active sides of lipase and D  = diffusion path diameter of reactant to the access of lipase active side.

NPs for biofuel doping can be formulated by either physical or chemical methods. For instance, plasma-arcing, sol–gel method has been presented as chemical method and ball mill process (agitation rate: 450 rpm) was presented as a physical method for NPs’ preparation in the previous studies [ 56 , 57 , 58 ]. Subsequently, NPs were doped with microalgal biofuel (e.g., microalgae oil, biodiesel, bioethanol, and others) with different doses (e.g., 25 ppm, 50 ppm, 100 ppm, and others) via ultra-sonication processing by the presence of magnetic stirrer and implemented on compression ignition (CI), direct ignition (DI) engines without any engine modification. NPs are dispersed in a base fuel and smoothen potential agglomerate into nano-scale due to its’ larger surface area and surface energy [ 58 , 59 , 60 , 61 ]. The ultra-sonication method was conducted with various parameters such as frequency (e.g., 20 kHz, 40 kHz, and 45 kHz), power (e.g., 120 W and 220 W), and time (30 min and 60 min) [ 60 , 62 , 63 ]. Cationic surfactants, e.g., tetra methyl ammonium hydroxide, cetyl trimethyl ammonium bromide, have been incorporated on the nano-particle surface for a negative-charge envelope formation to resist NPs’ sedimentation [ 56 , 64 ]. After biofuel-NPs’ doping, the NP-blended biofuel was preserved under the static condition to stabilize for fuel purpose [ 59 ]. Several potential NPs–microalgae-biofuel blends are tabulated in Table  4 . The morphology and crystalline phases after NP-doping were analysed through a scanning electron microscope and X-ray diffraction, respectively [ 61 ]. Botryococcus braunii oil was doped with almost 50 nm sized titanium dioxide (TiO 2 ) and silicon dioxide (SiO 2 ) incorporated with biodiesel (B20) of different doses for enhanced fuel efficiency in CI engine [ 60 ]. Caulerpa racemosa green algae biodiesel (B20) was doped with 50 nm sized zirconium dioxide (ZrO 2 ) by the different doses for CI engine [ 59 ].

Future applications of nano-additives for microalgae-biofuel

Nano-additive application on microalgae-biofuel enhancement has been categorized into several stages from raw material production to end-product implications. The stages are:

nano-additives for microalgae cultivation;

nano-additives for microalgae biomass conversion to biofuels;

nano-additives for microalgae-biofuel applications.

Nano-additives for microalgae cultivation

Improvement of the microalgae biomass productivity with the minimum area requirement is considered as the main purpose of nano-additive application in the microalgae culture. Nano-technology is being applied for enzyme immobilization, since nano-structures broaden the immobilization surface area causing high loading power of enzymes and stability of immobilized enzymes. Enzyme immobilization can be performed in different approaches such as electrospun nanofibers, covalently attached enzymes into nano-fibres, and enzyme aggregate coatings on nanofibers. The enzyme immobilization was investigated on various carbon nano-particles, e.g., graphene oxide (GO), multi-walled carbon nano-tubes (MWNTs), oxidized-MWNTs (O-MWNTs), and fullerene (C60). Among these nano-structures, O-MWNTs yielded the highest, and C60 yielded the lowest [ 21 , 22 ]. Nano-particles’ (NPs’) application was implemented on several microalgae species harvesting and yielded outstanding in each phase of the application. Application of nano-particles on microalgae harvesting claimed 20–30% microalgae production cost in large-scale application [ 22 ]. Table  5 presents the harvesting efficiency of several microalgae species cultivated with various nano-particles. Nano-particles also boosted the light conversion efficiency in photobioreactor (PBR) during the microalgae culture period. It is also worth mentioning that PBR is run by artificial light sources consuming additional energy and cost. However, during biomass growth, light sources do not reach in culture broth inadequately due to self-shading and biofilm formation on the PBR surface. To achieve desired illumination properties and photo-conversion efficiency in the PBR, various light-emitting diodes (LEDs) equipped with nano-materials fabrication are being implemented recently. Gallium aluminum arsenide (GAA)-fabricated LEDs have been experimented on laboratory scale algae culture so far. It was evident that the application of optical fibres in algal culture saves much energy, additional light cost, and increase efficiency [ 18 ]. Another latest development of nano-particle, integration of metallic nano-particles (MNPs) with localized surface plasmon resonance (LSPRs) amplifies the light scattering at certain wavelength [ 65 ]. An experimental study revealed that silver nano-particles’ (Ag-NPs’) suspension in plasmonic mini-PBRs backscatter blue light strongly. The blue light increased the photosynthetic efficiency significantly for green microalgae, Chlamydomonas reinhardtii, and blue–green microalgae, Cyanothece 51142, and 30% higher microalgae biomass have been obtained [ 48 ]. Nano-particles addition in microalgae cultivation can also improve the yield of the CO 2 absorption from the atmosphere and CO 2 sequestration that can boost the biomass growth. For instance, nano-bubbles in microalgae culture remained stable for a longer time. Nano-bubbles also floated algae biomass into the culture, ensured high mass transfer efficiency, and improved biomass density by sufficient accumulation of CO 2 , O 2 stripping, and minor buoyancy. Moreover, nano-bubbles suspended the biomass around airlift-loop bioreactor (ALB) and required less energy than micro-bubbles. Uniform nonporous membrane of ALB was also capable of producing 100 nm sized bubbles for this purpose [ 18 , 66 , 67 ]. The previous studies also delineated that nano-additives played a significant role in flocculation and separation process before biofuel production besides microalgae harvesting [ 22 ].

Nano-additives for microalgae biomass conversion to biofuels

Among microalgae biofuel, biodiesel has been appeared as the most popular and commercial biofuel in the mobile fuel market. For the case of biodiesel production, applications of acidic and basic nano-catalyst spheres can substitute the chemical compounds such as sodium methoxide by reacting with the free fatty acids and oils. Additional advantages of these nano-catalysts are recyclability and positive economical impact. Moreover, reactions can take place with low temperature and pressure as well as this approach reduces the contaminant release to the environment borne by sodium methoxide [ 6 ]. An industrial biodiesel study demonstrated that commercial CaO-NPs presented 91% biodiesel conversion efficiency during scaled-up catalytic transesterification [ 18 ]. Experimental study of microalgae cultivation with spherical nano-particles composed with sand (silica) and calcium compounds revealed that microalgae cellular growth increased drastically without harming harvesting as well as biofuel production from vegetable oil. The best way to address this issue was described as one of the major driving factors for commercial biofuel, biofuel production cost dropped effectively [ 6 , 7 , 8 , 68 ]. The experimental study mentioned that mesoporous silica nano-catalyst, Ti-loaded SBA-15 presented ten times higher free fatty acids (FFA) and water tolerance level than any other catalysts for biodiesel production from vegetable oil as well as this nano-catalyst performed three times better than other effective nano-catalysts titanium silicalite-1 (TS-1) and titanium dioxide silicate (TiO 2 -S) [ 69 ].

Moreover, Ti-loaded Santa Barbara Amorphous-15 (SBA-15) nano-catalyst application reduced the chemical (alkaline catalyst, NaOH) cost of transesterification process for biodiesel production by recycling the nano-catalyst as well as this process is more environment-friendly [ 6 , 69 ]. On the other hand, sulfate incorporated Ti-SBA-15 also performed as biocatalyst to convert vegetable oil to 100% esterified bio-lubricant. In consequences, this nano-particle can be expected to produce bio-lubricant from bio-oil of microalgae [ 70 ]. Other study showed that Niobia (N 2 O 5 ) incorporated with SBA-15 application on biodiesel production from biomass through esterification presented a significant scenario for microalgae-biodiesel yield [ 71 ]. Another study delineated that the enzyme extracted from Pseudomonas cepacia conjuncts with the nano-particles such as polyacrylonitrile (PAN) nanofibre, Fe 3 O 4, and nanoporous gold; silica nano-particles with lipase enzyme from Rhizopus miehei ; ferric silica and magnetic nano-particles with lipase from Burkholderia sp., polyacrylonitrile nano-fibre bound with lipase from Thermomyces lanuginosa has performed very effectively to produce biodiesel from various bio-oil by the transesterification process [ 7 ]. Furthermore, nano-magnetic biocatalyst of KF/CaO–Fe 3 O 4 , Li(lithium)-doped CaO, Fe 2 O 3 –CaO, sulfate (SO 4 − ) incorporated Zi (zirconium), sodium titanate and carbon-based nano-tubes and nano-particles reached up to 95% or above biodiesel yield from diverse types of biomass and biodegradable waste [ 7 , 72 ]. Besides enhancement of biodiesel-yield efficiency, a type of NP, zeolite (an alumina silicate mineral), has been used as commercial absorbent during the transesterification process. Zeolites absorbed the undesirable moisture content (4–6%) and produced pure glycerine as co-product besides biodiesel. Mesoporous nano-particles also presented a vital capability for continuous microalgal-biofuel generation from biomass without cell lysis. Zeolites also removed lipids from the microalgae cell membrane [ 18 , 73 ]. Table 6 presented the applications of nano-additives for biodiesel-yield enhancements during microalgae to biofuel conversion, suitable conversion processes, and efficiencies.

Nano-particles were efficiently capable to perform as immobilizing beds for valuable enzymes due to their large surface area-to-volume ratio. This capability of NPs broke down the long chains of complex sugar of microalgae, converted it to simple sugar, and consequently turned into bioethanol via the fermentation process. Due to the large surface area, the interaction between the surface of the nano-particles and fuel surrounded by them achieved adequate stability to overcome density variations. Nano-particles prepared by carbon nano-tubes doped with iron-oxide nano-particles presented excellent biocatalytic efficiency in a bioreactor, recyclable option enzyme applications, less capital cost as well as better enzyme loading for this purpose [ 6 , 7 , 74 ]. A catalytic study mentioned that mesoporous niobium oxide (N 2 O 5 ) application on complex sugar (sucrose) possessed both Lewis acid (LA) and Bronsted acid (BA) sites to convert fructose to 5-hydroxymethylfurfural (HMF) with the highest yield so far. The synergistic catalytic effect from a large amount of both LA and BA acid site quantities and surface areas played a positive impact on the reaction rate with a few times faster conversions [ 75 ]. Functionalised multiwall carbon-nano-tube (MWCNT) immobilization presented more than 55% initial activity of microalgal hydrolysis for Candida Antarctica. Nano-catalysts such as cobalt–molybdenum fabricated with Si/Al have been experimented on Botryococcus braunii and presented stable hydrocarbons. Another nano-catalyst, mobil composition of matter No. 41 (MCM-41), mesoporous material effectively reduced oxygenated fractions of bio-oil through catalytic pyrolysis [ 18 ].

Nano-catalysts can synthesize biomethane produced from microalgae from wastewater into pure hydrogen and carbon content. In a further step, this methane can produce biogas through anaerobic digestion. Biogas could be used as raw material of bio-fuelled electricity generation further. The elemental carbon can also be utilized as pure nano-graphite for the applications on batteries, aerospace, automobiles, and others [ 6 ]. The latest development conducted by quantum sphere on marine microalgae species evinced biogasification from wet microalgae biomass by metal nano-catalysts [ 18 ]. Besides that, nano-particles such as TiO 2 , CeO 2 were manifested to improve 10–11% of the biogas yield from wastewater treatment. Therefore, these nano-particles can be projected further for the biomethane production from microalgae grown in wastewater [ 7 ]. Apart from that, nano-substances with SiO 2 , nano-particles of platinum (Pt), nickel (Ni), cobalt (Co), and iron (Fe) can increase methane production from biomass up to 70%. Nano-fly ash and nano-bottom ash were proved to increase biomethane yield up to 3.5 times more. Nano-metal oxides, e.g., MgO, CaO, and SrO, nano-materials such as silica, single-walled nano-tubes of carbon-based materials, nano-clay, and nano-zero valence metal applications in biodiesel, bio-hydrogen, and biomethane production from microalgae and other biomass presented outstanding yield. These nano-particles can be projected for large-scale microalgal-biofuel production in the future to obtain revolutionary yield [ 7 , 8 ]. In addition, nano-hybrid catalysts are being commercialized as emulsion stabilizers in industrial applications. For instance, quaternary ammonium salts have been documented as an emulsifying surfactant for separation, extraction, isolation, and purification of biofuels. Carbon nano-tubes with silica fusion, SiO 2 –MgO nanohybrids have been performed as stabilizers on bio-oil in water emulsion due to its inherent hydrophobicity and resulted in full conversion in different emulsion phases [ 18 ].

Nano-additives for microalgae-biofuel applications

Solid nano-particles, nano-fluids, or nano-droplets with biofuel and fossil fuel were proved to improve the fuel lubricity, cetane number, burning rate, chemical reaction, catalytic performance, fire/flash point, heat and mass transfer efficiency and water co-solvency as well as decrease delay period [ 76 , 77 ]. That resulted in more complete and cleaner combustion of microalgae biofuel mixed with fossil fuel in compression ignition (CI), spark ignition (SI), and direct ignition (DI) engines. In line with that, nano-technology applications showed the capability of amplifying microalgal-biofuel combustion efficiency and reduced soot, NO x , smoke, HC, CO 2, and CO emission to the environment up to 72% [ 6 , 76 , 78 ]. Application of solid nano-additives such as alumina (Al 2 O 3 ), CERIA, carbon nano-tubes (CNT), Co 3 O 4 , ZrO 2 , La 2 O 3 , CeO 2 , SiO 2 , Ni 2 O, TiO 2 , ZnO, Fe 2 O 3 , CuO, Ce x Zr (1– x ) O 2 , and amide-doped MWCNTs-CeO 2 boosted the engine power, torque, and brake thermal performance of biodiesel (extracted from microalgae and other biomass) in CI and DI engines up to 11% [ 59 , 76 , 79 , 80 ]. The experimental study of nano-particles on DI engines demonstrated that nano-particles blended with biodiesel as well as diesel–biodiesel mixture performed outstanding. The effectiveness was higher compared to usual catalysts [ 61 , 81 ]. Another study presented that nano-particles of CeO 2 incorporated with an emulsion of biofuel with sol–gel combustion technology performed excellent mono-cylinder 4 stroke direct CI and DI engines without any hardware modification. Nano-particles addition with biofuels escalated the fuel calorific value, fastened evaporation rate, improved brake-specific fuel consumptions and thermal efficiency, reduced greenhouse gases (GHGs) such as CO, NO x , and smoke, and unburned HCs. Chemical reactions between CeO 2 and GHGs gases are presented in Rc. 2 , Rc. 3 , and Rc. 4 [ 82 ]:

In contrast, liquid nano-additive, nano-Al-droplet application (nano-suspension) on biofuel mixture has been manifested more efficient than even micro-suspension. Liquid nano-additives also presented outstanding performance by achieving better suspension than n -decane-based fuels. Nano-Al suspension with ethanol was strong enough for a longer period than other particles, because ethanol tended to form a gel around the nano-particles due to higher viscosity [ 74 ]. Nano-droplets coated a monolayer on the mechanical parts of the engine touched with liquid fuel and improved fuel efficiency [ 18 ]. In addition, NPs such as nano-Al, Al 2 O 3 , CuO, MgO, MnO, and ZnO incorporated with water–diesel–biodiesel (E10) emulsion and bioethanol performed remarkably. Among these NPs, Al 2 O 3 performed the best because of mandate disabling, consistent torque boosting, higher heat of combustion, super-high DTG max value, tiniest size of water droplets, the minimum value of brake-specific fuel consumption, and lowest values of Soot, NO x , CO, and HC [ 19 , 83 ].

Challenges and future prospects

Although nano-additive applications played significant role in microalgae cultivation, harvesting, conversion to biofuel and biofuel applications to enhance the efficiency, yet some challenges remained before the implementation of nano-additives for the mercantile approach. Most of the nano-additives from experimental research were not well-characterized in terms of particle size, shape, and size distribution as well as clustering [ 84 ]. Before large-scale application, well characterization of nano-particles and nano-fluids must be studied comprehensively. Appropriate nano-additive selection, preparation methods, and time for the selected application should be emphasized for optimum productivity. The effect of nano-catalyst implementation for microalgae-biofuel combustion quality, engine performance, and gas emission should be well studied and well-understood before implementation. In line with that, the availability of appropriate nano-additives with large amount might be a challenge for mass application though for laboratory scale, nano-additives are adequately available. Another constraint is cost-effectiveness of nano-catalysts for an industrial application which may hinder the commercial perspective, since many nano-catalysts are quite expensive.

Along with the potential microalgae determination and biofuel generation, integration of a plant design of value-added co-products will be the predominant advantage of the overall project with the economical aspect. This review encouraged biofuel research and development (R&D) sector worldwide to convert their unused, abandoned and wastewater sources, wet, and barren lands into microalgae farm as an eminent source of biofuel production. However, it should be highlighted that based on the existing research experiments, microalgae fuel production still stands at initial stage due to downward economic profile worldwide. Nano-additive applications on microalgal biofuel are yet confined into laboratory and pilot scale which can be counted as a significant limitation. Hence, it is strongly recommended to figure out large-scale process development with nano-additive applications for enhancement of microalgal growth, biofuel transformation processes, and fuel utilization in CI and DI engines. Nano-additive applications at different stages from microalgae culture to end-product utilization have a strong possibility to gain economical feasibility. Therefore, the detailed techno-economic analysis must be commanded to determine whether NP applications on microalgae biofuel are economically favorable or not, since economical issue is one of the most effective factors behind large-scale plant setup. Besides, these applications also have positive impact on the environment with value-added co-product generation into near further. Since the nano-additive utilization manifested itself environment-friendly, still a comprehensive life cycle assessment should be conducted to present the environmental positivities transparently. Besides all these factors, public safety, impact on flora and fauna, and the possibility of bio-hazards are also needed to be analysed extensively before commercialization.

Microalgae utilization for biofuel production is undoubtedly desirable all over the world. Though this approach is energy-efficient and environment-friendly, experts are still looking for an innovation that can boost the microalgae-biofuel yield from primary stage to end product as well as shift the whole process towards a cost-effective fuel solution. Hence, this review was emphasized on the synergistic effect of nano-additive-enhanced microalgal biofuel for mercantile approach and fuel-yield extension. Application of various forms of nano-additives in different phases on microalgae growth to biofuel demonstrated an excellent outcome that may project revolutionary improvement of commercial microalgae biofuel. However, the sustainability analysis of stepwise production rounds for microalgae biofuel still presented a bare need of further research and innovative concepts. These concepts may determine the most appropriate nano-additive for the desired type of biofuel in the context of economical aspect. Since nano-additive application on microalgae is quite new research concept, policy making and implementation of nano-additives will remain as the most vital issues for commercial output especially in developing countries. Therefore, managerial insights are needed to be emphasized further on proper policy, socio-economic impact, advantages and limitations for the overall system to attract the government and non-government fuel industries.

Enhancement of microalgae cultivation and harvesting by nano-bubbles and nano-particles application.

Identification of suitable microalgae species, possible biofuels from microalgae, latest conversion technologies, processes, and required equipments.

Excellent microalgal-biofuel yield by nano-droplet and nano-additives.

Complete and cleaner combustion in fuel engines by nano-emulsion and nano-stabilizers.

Availability of data and materials

Not applicable.

Abbreviations

acetone–butanol–ethanol

silver nano-particles

silver oxide

airlift-loop bioreactor

aluminum oxide/alumina

aluminum droplet

bronsted acid

calcium carbonate

cerium oxide

calcium oxide

calcium oxide nano-particles blends

compression ignition

carbon-dioxide

carbon mono oxide

cobalt oxide

carbon nano-tubes

copper oxide

cerium–zirconium oxide

chitosan/magnetic nano-particles

direct ignition

deep eutectic solvent

maximum derivative thermogravimetry

free fatty acids

ferric oxide

ferric chloride

gallium aluminum arsenide

greenhouse gases

graphene oxide

hydro-carbon

5-hydroxymethyl furfural

lanthanum oxide

light-emitting diodes

localized surface plasmon resonance

multiwall carbon nano-tubes

mobil composition of matter No 41

magnesium oxide

manganese oxide

metal nano-particles

niobia/niobium oxide

nickel oxide

nitrogen oxide

oxidized MWNTs

polyacrylonitrile

photobioreactor

poly dimethylammonium chloride

research and development

rhodium oxide

santa barbara amorphous-15

supercritical water gasification (SCWG)

surface-functionalized iron-oxide nano-particles

spark ignition

silicon dioxide

simultaneous saccharification and co-fermentation

separate hydrolysis and co-fermentation

separate hydrolysis and fermentation

simultaneous saccharification and fermentation

strontium oxide

titanium dioxide

titanium silicalite-1

titanium dioxide silicate

alumino silicate mineral

zirconium dioxide

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Hossain, N., Mahlia, T.M.I. & Saidur, R. Latest development in microalgae-biofuel production with nano-additives. Biotechnol Biofuels 12 , 125 (2019). https://doi.org/10.1186/s13068-019-1465-0

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Concerns about the rapid depletion of fossil fuels, energy security, climate change due to global warming, environmental pollution, and faster increase of fossil fuel prices have drawn attention to researchers, the scientific community, and government policymakers to develop alternative energy sources for reducing dependence on fossil fuel. In recent years, microalgae culture has received significant attention due to its potential application for bioenergy production, wastewater treatment, industrial CO 2 removal, and production of biochemical compounds that can be used for human and animal health and other benefits. However, large-scale microalgae production and their processing for producing various products and by-products could have environmental impacts beyond energy consumption in the microalgal production process. This article has reviewed the environmental effects of microalgae-based biofuel production on water resources and quality, eutrophication, biodiversity, waterborne toxicant, algal toxicity, wastewater remediation or treatment, waste generation, and greenhouse gas land-use changes, and genetically engineered microalgae.

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REVIEW article

Scope of algae as third generation biofuels.

• Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Kapurthala, Punjab, India

An initiative has been taken to develop different solid, liquid, and gaseous biofuels as the alternative energy resources. The current research and technology based on the third generation biofuels derived from algal biomass have been considered as the best alternative bioresource that avoids the disadvantages of first and second generation biofuels. Algal biomass has been investigated for the implementation of economic conversion processes producing different biofuels such as biodiesel, bioethanol, biogas, biohydrogen, and other valuable co-products. In the present review, the recent findings and advance developments in algal biomass for improved biofuel production have been explored. This review discusses about the importance of the algal cell contents, various strategies for product formation through various conversion technologies, and its future scope as an energy security.

Introduction

The requirement of energy for the mankind is increasing day by day. The major source of energy is based on fossil fuels only. Thus, the scarcity of fossil fuels, rising price of petroleum based fuels, energy protection, and increased global warming resulted in focusing on renewable energy sources such as solar, wind, hydro, tidal, and biomass worldwide ( Goldemberg and Guardabassi, 2009 ; Dragone et al., 2010 ; Rajkumar et al., 2014 ).

Different biomass from various sources like agricultural, forestry, and aquatic have been taken into consideration as the feedstocks for the production of several biofuels such as biodiesel ( Boyce et al., 2008 ; Yanqun et al., 2008 ), bioethanol ( Behera et al., 2014 ), biohydrogen ( Marques et al., 2011 ), bio-oil ( Shuping et al., 2010 ), and biogas ( Hughes et al., 2012 ; Singh et al., 2014 ). However, the environmental impact raised from burning of fuels has a great impact on carbon cycle (carbon balance), which is related to the combustion of fossil fuels. Besides, exhaustion of different existing biomass without appropriate compensation resulted in huge biomass scarcity, emerging environmental problems such as deforestation and loss of biodiversity ( Goldemberg, 2007 ; Li et al., 2008 ; Saqib et al., 2013 ).

Recently, researchers and entrepreneurs have focused their interest, especially on the algal biomass as the alternative feedstock for the production of biofuels. Moreover, algal biomass has no competition with agricultural food and feed production ( Demirbas, 2007 ). The photosynthetic microorganisms like microalgae require mainly light, carbon dioxide, and some nutrients (nitrogen, phosphorus, and potassium) for its growth, and to produce large amount of lipids and carbohydrates, which can be further processed into different biofuels and other valuable co-products ( Brennan and Owende, 2010 ; Nigam and Singh, 2011 ). Interestingly, the low content of hemicelluloses and about zero content of lignin in algal biomass results in an increased hydrolysis and/or fermentation efficiency ( Saqib et al., 2013 ). Other than biofuels, algae have applications in human nutrition, animal feed, pollution control, biofertilizer, and waste water treatment ( Thomas, 2002 ; Tamer et al., 2006 ; Crutzen et al., 2007 ; Hsueh et al., 2007 ; Choi et al., 2012 ). Therefore, the aim of the current review is to explore the scope of algae for the production of different biofuels and evaluation of its potential as an alternative feedstock.

Algae: Source of Biofuels

Generally, algae are a diverse group of prokaryotic and eukaryotic organisms ranging from unicellular genera such as Chlorella and diatoms to multicellular forms such as the giant kelp, a large brown alga that may grow up to 50 m in length ( Li et al., 2008 ). Algae can either be autotrophic or heterotrophic. The autotrophic algae require only inorganic compounds such as CO 2 , salts, and a light energy source for their growth, while the heterotrophs are non-photosynthetic, which require an external source of organic compounds as well as nutrients as energy sources ( Brennan and Owende, 2010 ). Microalgae are very small in sizes usually measured in micrometers, which normally grow in water bodies or ponds. Microalgae contain more lipids than macroalgae and have the faster growth in nature ( Lee et al., 2014a ). There are about more than 50,000 microalgal species out of which only about 30,000 species have been taken for the research study ( Surendhiran and Vijay, 2012 ; Richmond and Qiang, 2013 ; Rajkumar et al., 2014 ). The short harvesting cycle of algae is the key advantage for its importance, which is better than other conventional crops having harvesting cycle of once or twice in a year ( Chisti, 2007 ; Schenk et al., 2008 ). Therefore, the main focus has been carried out on algal biomass for its application in biofuel area.

There are several advantages of algal biomass for biofuels production: (a) ability to grow throughout the year, therefore, algal oil productivity is higher in comparison to the conventional oil seed crops; (b) higher tolerance to high carbon dioxide content; (c) the consumption rate of water is very less in algae cultivation; (d) no requirement of herbicides or pesticides in algal cultivation; (e) the growth potential of algal species is very high in comparison to others; (f) different sources of wastewater containing nutrients like nitrogen and phosphorus can be utilized for algal cultivation apart from providing any additional nutrient; and (g) the ability to grow under harsh conditions like saline, brackish water, coastal seawater, which does not affect any conventional agriculture ( Spolaore et al., 2006 ; Dismukes et al., 2008 ; Dragone et al., 2010 ). However, there are several disadvantages of algal biomass as feedstock such as the higher cultivation cost as compared to conventional crops. Similarly, harvesting of algae require high energy input, which is approximately about 20–30% of the total cost of production. Several techniques such as centrifugation, flocculation, floatation, sedimentation, and filtration are usually used for harvesting and concentrating the algal biomass ( Demirbas, 2010 ; Ho et al., 2011 ).

The algae can be converted into various types of renewable biofuels including bioethanol, biodiesel, biogas, photobiologically produced biohydrogen, and further processing for bio-oil and syngas production through liquefaction and gasification, respectively ( Kraan, 2013 ). The conversion technologies for utilizing algal biomass to energy sources can be categorized into three different ways, i.e., biochemical, chemical, and thermochemical conversion and make an algal biorefinery, which has been depicted in Figure 1 . The biofuel products derived from algal biomass using these conversion routes have been explored in detail in the subsequent sections.

Figure 1. Algal biomass conversion process for biofuel production .

Biodiesel Production

Biodiesel is a mixture of monoalkyl esters of long chain fatty acids [fatty acid methyl esters (FAME)], which can be obtained from different renewable lipid feedstocks and biomass. It can be directly used in different diesel engines ( Clark and Deswarte, 2008 ; Demirbas, 2009 ). Studies to explore the microalgae as feedstock for the production of liquid fuels had been started for the mid-1980s. In order to solve the energy crisis, the extraction of lipids from diatoms was attempted by some German scientists during the period of World War-II ( Cohen et al., 1995 ). The higher oil yield in algal biomass as compared to oil seed crops makes the possibility to convert into the biodiesel economically using different technologies. A comparative study between algal biomass and terrestrial plants for the production of biodiesel has been depicted in Table 1 . The oil productivity (mass of oil produced per unit volume of the microalgal broth per day) depends on the algal growth rate and the biomass content of the species. The species of microalgae such as Kirchneriella lunaris , Ankistrodesmus fusiformis , Chlamydocapsa bacillus , and Ankistrodesmus falcatus with high levels of polyunsaturated FAME are generally preferred for the production of biodiesel ( Nascimento et al., 2013 ). They commonly multiply their biomass with doubling time of 24 h during exponential growth. Oil content of microalgae is generally found to be very high, which exceed up to 80% by weight of its dry biomass. About 5,000–15,000 gal of biodiesel can be produced from algal biomass per acre per year, which reflects its potentiality ( Spolaore et al., 2006 ; Chisti, 2007 ).

Table 1 . Comparative study between algal biomass and terrestrial plants for biodiesel production .

However, there are some standards such as International Biodiesel Standard for Vehicles (EN14214) and American Society for Testing and Materials (ASTM), which are required to comply with the algal based biodiesel on the physical and chemical properties for its acceptance as substitute to fossil fuels ( Brennan and Owende, 2010 ). The higher degree of polyunsaturated fatty acids of algal oils as compared to vegetable oils make susceptible for oxidation in the storage and further limits its utilization ( Chisti, 2007 ). Some researchers have reported the different advantages of the algal biomass for the biodiesel production due to its high biomass growth and oil productivity in comparison to best oil crops ( Chisti, 2007 ; Hossain et al., 2008 ; Hu et al., 2008 ; Rosenberg et al., 2008 ; Schenk et al., 2008 ; Rodolfi et al., 2009 ; Mutanda et al., 2011 ).

Algal biodiesel production involves biomass harvesting, drying, oil extraction, and further transesterification of oil, which have been described as below.

Harvesting and Drying of Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. Harvesting of algal biomass and further drying is important prior to mechanical and solvent extraction for the recovery of oil. Macroalgae can be harvested using nets, which require less energy while microalgae can be harvested by some conventional processes, which include filtration ( Rossignol et al., 1999 ) flocculation ( Liu et al., 2013 ; Prochazkova et al., 2013 ), centrifugation ( Heasman et al., 2008 ), foam fractionation ( Csordas and Wang, 2004 ), sedimentation, froth floatation, and ultrasonic separation ( Bosma et al., 2003 ). Selection of harvesting method depends on the type of algal species.

Drying is an important method to extend shelf-life of algal biomass before storage, which avoids post-harvest spoilage ( Munir et al., 2013 ). Most of the efficient drying methods like spray-drying, drum-drying, freeze drying or lyophilization, and sun-drying have been applied on microalgal biomass ( Leach et al., 1998 ; Richmond, 2004 ; Williams and Laurens, 2010 ). Sun-drying is not considered as a very effective method due to presence of high water content in the biomass ( Mata et al., 2010 ). However, Prakash et al. (2007) used simple solar drying device and succeed in drying Spirulina and Scenedesmus having 90% of moisture content. Widjaja et al. (2009) showed the effectiveness of drying temperature during lipid extraction of algal biomass, which affects both concentration of triglycerides and lipid yield. Further, all these processes possess safety and health issues ( Singh and Gu, 2010 ).

Extraction of Oil from Algal Biomass

Unicellular microalgae produce a cell wall containing lipids and fatty acids, which differ them from higher animals and plants. In the literature, there are different methods of oil extraction from algae, such as mechanical and solvent extraction ( Li et al., 2014 ). However, the extraction of lipids from microalgae is costly and energy intensive process.

Mechanical oil extraction

The oil from nuts and seeds is extracted mechanically using presses or expellers, which can also be used for microalgae. The algal biomass should be dried prior to this process. The cells are just broken down with a press to leach out the oil. About 75% of oil can be recovered through this method and no special skill is required ( Munir et al., 2013 ). Topare et al. (2011) extracted oil through screw expeller by mechanical pressing (by piston) and osmotic shock method and recovered about 75% of oil from the algae. However, more extraction time is required as compared to other methods, which make the process unfavorable and less effective ( Popoola and Yangomodou, 2006 ).

Solvent based oil extraction

Oil extraction using solvent usually recovers almost all the oil leaving only 0.5–0.7% residual oil in the biomass. Therefore, the solvent extraction method has been found to be suitable method rather than the mechanical extraction of oil and fats ( Topare et al., 2011 ). Solvent extraction is another method of lipid extraction from microalgae, which involves two stage solvent extraction systems. The amount of lipid extracted from microalgal biomass and further yield of highest biodiesel depends mainly on the solvent used. Several organic solvents such as chloroform, hexane, cyclo-hexane, acetone, and benzene are used either solely or in mixed form ( Afify et al., 2010 ). The solvent reacts on algal cells releasing oil, which is recovered from the aqueous medium. This occurs due to the nature of higher solubility of oil in organic solvents rather than water. Further, the oil can be separated from the solvent extract. The solvent can be recycled for next extraction. Out of different organic solvents, hexane is found to be most effective due to its low toxicity and cost ( Rajvanshi and Sharma, 2012 ; Ryckebosch et al., 2012 ).

In case of using mixed solvents for oil extraction, a known quantity of the solvent mixture is used, for example, chloroform/methanol in the ratio 2:1 (v/v) for 20 min using a shaker and followed by the addition of mixture, i.e., chloroform/water in the ratio of 1:1 (v/v) for 10 min ( Shalaby, 2011 ). Similarly, Pratoomyot et al. (2005) extracted oil from different algal species using the solvent system chloroform/methanol in the ratio of 2:1 (v/v) and found different fatty acid content. Ryckebosch et al. (2012) optimized an analytical procedure and found chloroform/methanol in the ratio 1:1 as the best solvent mixture for the extraction of total lipids. Similarly, Lee et al. (1998) extracted lipid from the green alga Botryococcus braunii using different solvent system and obtained the maximum lipid content with chloroform/methanol in the ratio of 2:1. Hossain et al., 2008 used hexane/ether in the ratio 1:1 (v/v) for oil extraction and allowed to settle for 24 h. Using a two-step process, Fajardo et al. (2007) reported about 80% of lipid recovery using ethanol and hexane in the two steps for the extraction and purification of lipids. Therefore, a selection of a most suitable solvent system is required for the maximum extraction of oil for an economically viable process.

Lee et al. (2009) compared the performance of various disruption methods, including autoclaving, bead-beating, microwaves, sonication, and using 10% NaCl solution in the extraction of Botryococcus sp., Chlorella vulgaris , and Scenedesmus sp, using a mixture of chloroform and methanol (1:1).

Transesterification

This is a process to convert algal oil to biodiesel, which involves multiple steps of reactions between triglycerides or fatty acids and alcohol. Different alcohols such as ethanol, butanol, methanol, propanol, and amyl alcohol can be used for this reaction. However, ethanol and methanol are used frequently for the commercial development due to its low cost and its physical and chemical advantages ( Bisen et al., 2010 ; Surendhiran and Vijay, 2012 ). The reaction can be performed in the presence of an inorganic catalyst (acids and alkalies) or lipase enzyme. In this method, about 3 mol of alcohol are required for each mole of triglyceride to produce 3 mol of methyl esters (biodiesel) and 1 mol of glycerol (by-product) ( Meher et al., 2006 ; Chisti, 2007 ; Sharma and Singh, 2009 ; Surendhiran and Vijay, 2012 ; Stergiou et al., 2013 ) (Figure 2 ). Glycerol is denser than biodiesel and can be periodically or continuously removed from the reactor in order to drive the equilibrium reaction. The presence of methanol, the co-solvent that keeps glycerol and soap suspended in the oil, is known to cause engine failure ( Munir et al., 2013 ). Thus, the biodiesel is recovered by repeated washing with water to remove glycerol and methanol ( Chisti, 2007 ).

Figure 2. Transesterification of oil to biodiesel . R 1–3 are hydrocarbon groups.

The reaction rate is very slow by using the acid catalysts for the conversion of triglycerides to methyl esters, whereas the alkali-catalyzed transesterification reaction has been reported to be 4000 times faster than the acid-catalyzed reaction ( Mazubert et al., 2013 ). Sodium and potassium hydroxides are the two commercial alkali catalysts used at a concentration of about 1% of oil. However, sodium methoxide has become the better catalyst rather than sodium hydroxide ( Singh et al., 2006 ).

Kim et al. (2014) used Scenedesmus sp. for the biodiesel production through acid and alkali transesterification process. They reported 55.07 ± 2.18%, based on lipid by wt of biodiesel conversion using NaOH as an alkaline catalyst than using H 2 SO 4 as 48.41 ± 0.21% of biodiesel production. In comparison to acid and alkalies, lipases as biocatalyst have different advantages as the catalysts due to its versatility, substrate selectivity, regioselectivity, enantioselectivity, and high catalytic activity at ambient temperature and pressure ( Knezevic et al., 2004 ). It is not possible by some lipases to hydrolyze ester bonds at secondary positions, while some other group of enzymes hydrolyzes both primary and secondary esters. Another group of lipases exhibits fatty acids selectivity, and allow to cleave ester bonds at particular type of fatty acids. Luo et al. (2006) cloned the lipase gene lipB68 and expressed in Escherichia coli BL21 and further used it as a catalyst for biodiesel production. LipB68 could catalyze the transesterification reaction and produce biodiesel with a yield of 92% after 12 h, at a temperature of 20°C. The activity of the lipase enzyme with such a low temperature could provide substantial savings in energy consumption. However, it is rarely used due to its high cost ( Sharma et al., 2001 ).

Extractive transesterification

It involves several steps to produce biodiesel such as drying, cell disruption, oils extraction, transesterification, and biodiesel refining ( Hidalgo et al., 2013 ). The main problems are related with the high water content of the biomass (over 80%), which overall increases the cost of whole process.

In situ transesterification

This method skips the oil extraction step. The alcohol acts as an extraction solvent and an esterification reagent as well, which enhances the porosity of the cell membrane. Yields found are higher than via the conventional route, and waste is also reduced. Industrial biodiesel production involves release of extraction solvent, which contributes to the production of atmospheric smog and to global warming. Thus, simplification of the esterification processes can reduce the disadvantages of this attractive bio-based fuel. The single-step methods can be attractive solutions to reduce chemical and energy consumption in the overall biodiesel production process ( Patil et al., 2012 ). A comparison of direct and extractive transesterification is given in Table 2 .

Table 2 . Comparison of extractive transesterification and in situ methods ( Haas and Wagner, 2011 ) .

Bioethanol Production

Several researchers have been reported bioethanol production from certain species of algae, which produce high levels of carbohydrates as reserve polymers. Owing to the presence of low lignin and hemicelluloses content in algae in comparison to lignocellulosic biomass, the algal biomass have been considered more suitable for the bioethanol production ( Chen et al., 2013 ). Recently, attempts have been made (for the bioethanol production) through the fermentation process using algae as the feedstocks to make it as an alternative to conventional crops such as corn and soyabean ( Singh et al., 2011 ; Nguyen and Vu, 2012 ; Chaudhary et al., 2014 ). A comparative study of algal biomass and terrestrial plants for the production of bioethanol has been given in Table 3 . There are different micro and macroalgae such as Chlorococcum sp., Prymnesium parvum , Gelidium amansii , Gracilaria sp., Laminaria sp., Sargassum sp., and Spirogyra sp., which have been used for the bioethanol production ( Eshaq et al., 2011 ; Rajkumar et al., 2014 ). These algae usually require light, nutrients, and carbon dioxide, to produce high levels of polysaccharides such as starch and cellulose. These polysaccharides can be extracted to fermentable sugars through hydrolysis and further fermentation to bioethanol and separated through distillation as shown in Figure 3 .

Table 3 . Comparative study between algal biomass and terrestrial plants for bioethanol production .

Figure 3. Process for bioethanol production from microalgae .

Pre-Treatment and Saccharification

It has been reported that, the cell wall of some species of green algae like Spirogyra and Chlorococcum contain high level of polysaccharides. Microalgae such as C. vulgaris contains about 37% of starch on dry weight basis, which is the best source for bioethanol with 65% conversion efficiency ( Eshaq et al., 2010 ; Lam and Lee, 2012 ). Such polysaccharide based biomass requires additional processing like pre-treatment and saccharification before fermentation ( Harun et al., 2010 ). Saccharification and fermentation can also be carried out simultaneously using an amylase enzyme producing strain for the production of ethanol in a single step. Bioethanol from microalgae can be produced through the process, which is similar to the first generation technologies involving corn based feedstocks. However, there is limited literature available on the fermentation process of microalgae biomass for the production of bioethanol ( Schenk et al., 2008 ; John et al., 2011 ).

The pre-treatment is an important process, which facilitates accessibility of biomass to enzymes to release the monosaccharides. Acid pre-treatment is widely used for the conversion of polymers present in the cell wall to simple forms. The energy consumption in the pre-treatment is very low and also it is an efficient process ( Harun and Danquah, 2011a , b ). Yazdani et al. (2011) found 7% (w/w) H 2 SO 4 as the promising concentration for the pre-treatment of the brown macroalgae Nizimuddinia zanardini to obtain high yield of sugars without formation of any inhibitors. Candra and Sarinah (2011) studied the bioethanol production using red seaweed Eucheuma cottonii through acid hydrolysis. In this study, 5% H 2 SO 4 concentration was used for 2 h at 100°C, which yielded 15.8 g/L of sugars. However, there are other alternatives to chemical hydrolysis such as enzymatic digestion and gamma radiation to make it more sustainable ( Chen et al., 2012 ; Yoon et al., 2012 ; Schneider et al., 2013 ).

Similar to starch, there are certain polymers such as alginate, mannitol, and fucoidan present in the cell wall of various algae, which requires additional processing like pre-treatment and saccharification before fermentation. Another form of storage carbohydrate found in various brown seaweeds and microalgae is laminarin, which can be hydrolyzed by β-1,3-glucanases or laminarinases ( Kumagai and Ojima, 2010 ). Laminarinases can be categorized into two groups such as exo- and endo-glucanases based on the mode of hydrolysis, which usually produces glucose and smaller oligosaccharides as the end product. Both the enzymes are necessary for the complete digestion of laminarin polymer ( Lee et al., 2014b ).

Markou et al. (2013) saccharified the biomass of Spirulina ( Arthrospira platensis ), fermented the hydrolyzate and obtained the maximum ethanol yield of 16.32 and 16.27% (g ethanol /g biomass ) produced after pre-treatment with 0.5 N HNO 3 and H 2 SO 4 , respectively. Yanagisawa et al. (2011) investigated the content of polysaccharide materials present in three types of seaweeds such as sea lettuce ( Ulva pertusa ), chigaiso ( Alaria crassifolia ), and agar weed ( Gelidium elegans ). These seaweeds contain no lignin, which is a positive signal for the hydrolysis of polysaccharides without any pre-treatment. Singh and Trivedi (2013) used Spirogyra biomass for the production of bioethanol using Saccharomyces cerevisiae and Zymomonas mobilis . In a method, they followed acid pre-treatment of algal biomass and further saccharified using α-amylase producing Aspergillus niger . In another method, they directly saccharified the biomass without any pre-treatment. The direct saccharification process resulted in 2% (w/w) more alcohol in comparison to pretreated and saccharified algal biomass. This study revealed that the pre-treatment with different chemicals are not required in case of Spyrogyra , which reflects its economic importance for the production of ethanol. Also, cellulase enzyme has been used for the saccharification of algal biomass containing cellulose. However, this enzyme system is more expensive than amylases and glucoamylases, and doses required for effective cellulose saccharification are usually very high. Trivedi et al. (2013) applied different cellulases on green alga Ulva for saccharification and found highest conversion efficiency of biomass into reducing sugars by using cellulase 22119 rather than viscozyme L, cellulase 22086 and 22128. In this experiment, they found a maximum yield of sugar 206.82 ± 14.96 mg/g with 2% (v/v) enzyme loading for 36 h at a temperature of 45°C.

Fermentation

There are different groups of microorganisms like yeast, bacteria, and fungi, which can be used for the fermentation of the pretreated and saccharified algal biomass under anaerobic process for the production of bioethanol ( Nguyen and Vu, 2012 ). Nowadays, S. cerevisiae and Z. mobilis have been considered as the bioethanol fermenting microorganisms. However, fermentation of mannitol, a polymer present in certain algae is not possible in anaerobic condition using these well known microorganisms and requires supply of oxygen during fermentation, which is possible only by Zymobacter palmae ( Horn et al., 2000 ).

Certain marine red algae contain agar, a polymer of galactose and galactopyranose, which can be used for the production of bioethanol ( Yoon et al., 2010 ). The biomass of red algae can be depolymerized into different monomeric sugars like glucose and galactose. In addition to mannitol and glucose, brown seaweeds contain about 14% of extra carbohydrates in the form of alginate ( Wargacki et al., 2012 ). Horn et al. (2000) reported the presence of alginate, laminaran, mannitol, fucoidan, and cellulose in some brown seaweeds, which are good source of sugars. They fermented brown seaweed extract having mannitol using bacteria Z. palmae and obtained an ethanol yield of about 0.38 g ethanol/g mannitol.

In the literature, there are many advantages supporting microalgae as the promising substrate for bioethanol production. Hon-Nami (2006) used Chlamydomonas perigranulata algal culture and obtained different by-products such as ethanol and butanediol. Similarly, Yanagisawa et al. (2011) obtained glucose and galactose through the saccharification of agar weed (red seaweed) containing glucan and galactan and obtained 5.5% of ethanol concentration through fermentation using S. cerevisiae IAM 4178. Harun et al. (2010) obtained 60% more ethanol in case of lipid extracted microalgal biomass rather than intact algal biomass of Chlorococcum sp. This shows the importance of algal biomass for the production of both biodiesel and bioethanol.

Biogas Production

Recently, biogas production from algae through anaerobic digestion has received a remarkable attention due to the presence of high polysaccharides (agar, alginate, carrageenan, laminaran, and mannitol) with zero lignin and low cellulose content. Mostly, seaweeds are considered as the excellent feedstock for the production of biogas. Several workers have demonstrated the fermentation of various species of algae like Scenedesmus , Spirulina , Euglena , and Ulva for biogas production ( Samson and Leduy, 1986 ; Yen and Brune, 2007 ; Ras et al., 2011 ; Zhong et al., 2012 ; Saqib et al., 2013 ). The production of biogas using algal biomass in comparison to some terrestrial plants is shown in Table 4 .

Table 4 . Comparative study between algal biomass and terrestrial plants for biogas production .

Biogas is produced through the anaerobic transformation of organic matter present in the biodegradable feedstock such as marine algae, which releases certain gases like methane, carbon dioxide, and traces of hydrogen sulfide. The anaerobic conversion process involves basically four main steps. In the first step, the insoluble organic material and higher molecular mass compounds such as lipids, carbohydrates, and proteins are hydrolyzed into soluble organic material with the help of enzyme released by some obligate anaerobes such as Clostridia and Streptococci . The second step is called as acidogenesis, which releases volatile fatty acids (VFAs) and alcohols through the conversion of soluble organics with the involvement of enzymes secreted by the acidogenic bacteria. Further, these VFAs and alcohols are converted into acetic acid and hydrogen using acetogenic bacteria through the process of acetogenesis, which finally metabolize to methane and carbon dioxide by the methanogens ( Cantrell et al., 2008 ; Vergara-Fernandez et al., 2008 ; Brennan and Owende, 2010 ; Romagnoli et al., 2011 ).

Sangeetha et al. (2011) reported the anaerobic digestion of green alga Chaetomorpha litorea with generation of 80.5 L of biogas/kg of dry biomass under 299 psi pressure. Vergara-Fernandez et al. (2008) evaluated digestion of the marine algae Macrocystis pyrifera and Durvillaea antarctica marine algae in a two-phase anaerobic digestion system and reported similar biogas productions of 180.4 (±1.5) mL/g dry algae/day with a methane concentration around 65%. However, in case of algae blend, same methane content was observed with low biogas yield. Mussgnug et al. (2010) reported biogas production from some selected green algal species like Chlamydomonas reinhardtii and Scenedesmus obliquus and obtained 587 and 287 mL biogas/g of volatile solids, respectively. Further, there are few studies, which have been conducted with microalgae showing the effect of different pre-treatment like thermal, ultrasound, and microwave for the high production of biogas ( Gonzalez-Fernandez et al., 2012a , b ; Passos et al., 2013 ).

However, there are different factors, which limit the biogas production such as requirement of larger land area, infrastructure, and heat for the digesters ( Collet et al., 2011 ; Jones and Mayfield, 2012 ). The proteins present in algal cells increases the ammonium production resulting in low carbon to nitrogen ratio, which affects biogas production through the inhibition of growth of anaerobic microorganisms. Also, anaerobic microorganisms are inhibited by the sodium ions. Therefore, it is recommended to use the salt tolerating microorganisms for the anaerobic digestion of algal biomass ( Yen and Brune, 2007 ; Brennan and Owende, 2010 ; Jones and Mayfield, 2012 ).

Biohydrogen Production

Recently, algal biohydrogen production has been considered to be a common commodity to be used as the gaseous fuels or electricity generation. Biohydrogen can be produced through different processes like biophotolysis and photo fermentation ( Shaishav et al., 2013 ). Biohydrogen production using algal biomass is comparative to that of terrestrial plants (Table 5 ). Park et al. (2011) found Gelidium amansii (red alga) as the potential source of biomass for the production of biohydrogen through anaerobic fermentation. Nevertheless, they found 53.5 mL of H 2 from 1 g of dry algae with a hydrogen production rate of 0.518 L H 2 /g VSS/day. The authors found an inhibitor, namely, 5-hydroxymethylfurfural (HMF) produced through the acid hydrolysis of G. amansii that decreases about 50% of hydrogen production due to the inhibition. Thus, optimization of the pre-treatment method is an important step to maximize biohydrogen production, which will be useful for the future direction ( Park et al., 2011 ; Shi et al., 2011 ). Saleem et al. (2012) reduced the lag time for hydrogen production using microalgae Chlamydomonas reinhardtii by the use of optical fiber as an internal light source. In this study, the maximum rate of hydrogen production in the presence of exogenic glucose and optical fiber was reported to be 6 mL/L culture/h, which is higher than other reported values.

Table 5 . Comparative study between algal biomass and terrestrial plants for biohydrogen production .

Some of microalgae like blue green algae have glycogen instead of starch in their cells. This is an exception, which involves oxidation of ferrodoxin by the hydrogenase enzyme activity for the production of hydrogen in anaerobic condition. However, another function of this enzyme is to be involved in the detachment of electrons. Therefore, different researchers have focused for the identification of these enzyme activities having interactions with ferrodoxin and the other metabolic functions for microalgal photobiohydrogen production. They are also involved with the change of these interactions genetically to enhance the biohydrogen production ( Gavrilescu and Chisti, 2005 ; Hankamer et al., 2007 ; Wecker et al., 2011 ; Yacoby et al., 2011 ; Rajkumar et al., 2014 ).

Bio-Oil and Syngas Production

Bio-oil is formed in the liquid phase from algal biomass in anaerobic condition at high temperature. The composition of bio-oil varies according to different feedstocks and processing conditions, which is called as pyrolysis ( Iliopoulou et al., 2007 ; Yanqun et al., 2008 ). There are several parameters such as water, ash content, biomass composition, pyrolysis temperature, and vapor residence time, which affect the bio-oil productivity ( Fahmi et al., 2008 ). However, due to the presence of water, oxygen content, unsaturated and phenolic moieties, crude bio-oil cannot be used as fuel. Therefore, certain treatments are required to improve its quality ( Bae et al., 2011 ). Bio-oils can be processed for power generation with the help of external combustion through steam and organic rankine cycles, and stirling engines. However, power can also be generated through internal combustion using diesel and gas-turbine engines ( Chiaramonti et al., 2007 ). In literature, there are limited studies on algae pyrolysis compared to lignocellulosic biomass. Although, high yields of bio-oil occur through fluidized-bed fast pyrolysis processes, there are several other pyrolysis modes, which have been introduced to overcome their inherent disadvantages of a high level of carrier gas flow and excessive energy inputs ( Oyedun et al., 2012 ). Demirbas (2006) investigated suitability of the microalgal biomass for bio-oil production and found the superior quality than the wood. Porphy and Farid (2012) produced bio-oil from pyrolysis of algae ( Nannochloropsis sp.) at 300°C after lipid extraction, which composed of 50 wt% acetone, 30 wt% methyl ethyl ketone, and 19 wt% aromatics such as pyrazine and pyrrole. Similarly, Choi et al. (2014) carried out pyrolysis study on a species of brown algae Saccharina japonica at a temperature of 450°C and obtained about 47% of bio-oil yield.

Gasification is usually performed at high temperatures (800–1000°C), which converts biomass into the combustible gas mixture through partial oxidation process, called syngas or producer gas. Syngas is a mixture of different gases like CO, CO 2 , CH 4 , H 2 , and N 2 , which can also be produced through normal gasification of woody biomass. In this process, biomass reacts with oxygen and water (steam) to generate syngas. It is a low calorific gas, which can be utilized in the gas turbines or used directly as fuel. Different variety of biomass feedstocks can be utilized for the production of energy through the gasification process, which is an added advantage ( Carvalho et al., 2006 ; Prins et al., 2006 ; Lv et al., 2007 ).

Conclusion and Future Perspectives

Recently, it is a challenge for finding different alternative resources, which can replace fossil fuels. Due to presence of several advantages in algal biofuels like low land requirement for biomass production and high oil content with high productivity, it has been considered as the best resource, which can replace the liquid petroleum fuel. However, one of its bottlenecks is the low biomass production, which is a barrier for industrial production. Also, another disadvantage includes harvesting of biomass, which possesses high energy inputs. For an economic process development in comparison to others, a cost-effective and energy efficient harvesting methods are required with low energy input. Producing low-cost microalgal biofuels requires better biomass harvesting methods, high biomass production with high oil productivity through genetic modification, which will be the future of algal biology. Therefore, use of the standard algal harvesting technique, biorefinery concept, advances in photobioreactor design and other downstream technologies will further reduce the cost of algal biofuel production, which will be a competitive resource in the near future.

Conflict of Interest Statement

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Acknowledgments

The authors are thankful to Prof. Y. K. Yadav, Director, NIRE, Kapurthala for his consistent support to write this review paper. The authors greatly acknowledge the Ministry of New and Renewable Energy, New Delhi, Govt. of India, for providing funds to carry out research work.

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Keywords: algae, microalgae, biofuels, bioethanol, biogas, biodiesel, biohydrogen

Citation: Behera S, Singh R, Arora R, Sharma NK, Shukla M and Kumar S (2015) Scope of algae as third generation biofuels. Front. Bioeng. Biotechnol. 2 :90. doi: 10.3389/fbioe.2014.00090

Received: 31 July 2014; Accepted: 29 December 2014; Published online: 11 February 2015.

Reviewed by:

Copyright: © 2015 Behera, Singh, Arora, Sharma, Shukla and Kumar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY) . The use, distribution or reproduction in other forums is permitted, provided the original author(s) or licensor are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.

*Correspondence: Sachin Kumar, Biochemical Conversion Division, Sardar Swaran Singh National Institute of Renewable Energy, Jalandhar-Kapurthala Road, Wadala Kalan, Kapurthala 144601, Punjab, India e-mail: sachin.biotech@gmail.com

Disclaimer: All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article or claim that may be made by its manufacturer is not guaranteed or endorsed by the publisher.

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Algae: Biomass to Biofuel

Affiliations.

• 1 Sustainable Materials and Catalysis Research Laboratory (SMCRL), Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, India.
• 2 Sustainable Materials and Catalysis Research Laboratory (SMCRL), Department of Chemistry, Indian Institute of Technology Jodhpur, Jodhpur, India. [email protected].
• PMID: 34009581
• DOI: 10.1007/978-1-0716-1323-8_3

Worldwide demand for ethanol alternative fuel has been emerging day by day owing to the rapid population growth and industrialization. Culturing microalgae as an alternative feedstock is anticipated to be a potentially significant approach for sustainable bioethanol biofuel production. Microalgae are abundant in nature, which grow at faster rates with a capability of storing high lipid and starch/cellulose contents inside their cells. This process offers several environmental advantages, including the effective utilization of land, good CO2 sequestration without entering into "food against fuel" dispute. This chapter focuses on the methods and processes used for the production of bioethanol biofuels from algae. Thus, it also covers significant achievements in the research and developments on algae bioethanol production, mainly including pretreatment, hydrolysis, and fermentation of algae biomass. The processes of producing biodiesel, biogas, and hydrogen have also been discussed.

Keywords: Bioethanol; Biofuel; Biogas; Biohydrogen; Biomass; Microalgae.

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Sustainable production of biofuels from the algae-derived biomass

Tehreem mahmood.

1 Department of Biotechnology, Quaid-i-Azam University, Islamabad, 45320 Pakistan

Nazim Hussain

2 Center for Applied Molecular Biology (CAMB), University of the Punjab, Lahore, Pakistan

Areej Shahbaz

Sikandar i. mulla.

3 Department of Biochemistry, School of Allied Health Sciences, REVA University, Bangalore, 560064 India

Hafiz M.N. Iqbal

4 Tecnologico de Monterrey, School of Engineering and Sciences, 64849 Monterrey, Mexico

Muhammad Bilal

5 Faculty of Chemical Technology, Institute of Chemical Technology and Engineering, Poznan University of Technology, Berdychowo 4, 60695 Poznan, Poland

Associated Data

Not applicable.

The worldwide fossil fuel reserves are rapidly and continually being depleted as a result of the rapid increase in global population and rising energy sector needs. Fossil fuels should not be used carelessly since they produce greenhouse gases, air pollution, and global warming, which leads to ecological imbalance and health risks. The study aims to discuss the alternative renewable energy source that is necessary to meet the needs of the global energy industry in the future. Both microalgae and macroalgae have great potential for several industrial applications. Algae-based biofuels can surmount the inadequacies presented by conventional fuels, thereby reducing the ‘food versus fuel’ debate. Cultivation of algae can be performed in all three systems; closed, open, and hybrid frameworks from which algal biomass is harvested, treated and converted into the desired biofuels. Among these, closed photobioreactors are considered the most efficient system for the cultivation of algae. Different types of closed systems can be employed for the cultivation of algae such as stirred tank photobioreactor, flat panel photobioreactor, vertical column photobioreactor, bubble column photobioreactor, and horizontal tubular photobioreactor. The type of cultivation system along with various factors, such as light, temperature, nutrients, carbon dioxide, and pH affect the yield of algal biomass and hence the biofuel production. Algae-based biofuels present numerous benefits in terms of economic growth. Developing a biofuel industry based on algal cultivation can provide us with a lot of socio-economic advantages contributing to a publicly maintainable result. This article outlines the third-generation biofuels, how they are cultivated in different systems, different influencing factors, and the technologies for the conversion of biomass. The benefits provided by these new generation biofuels are also discussed. The development of algae-based biofuel would not only change environmental pollution control but also benefit producers' economic and social advancement.

Graphical abstract

Introduction

Commonly it is assumed that algae are photosynthetic autotrophs that mostly live in water, evolve oxygen, and are either made up of single cells or live in colonies or filamentous forms [ 1 ]. Algae are comprised of a large number of photosynthetic living beings that mostly inhabit aquatic surroundings. According to the size and morphological characteristics, algal species are usually classified into macroalgae and microalgae. Macroalgae which are also called seaweeds are made up of a large number of cells and can be seen with a naked eye. As compared to macroalgae, microalgal species can only be visualized with the help of a microscope and are highly important in the field of micro nanomedicine [ 2 ]. Based on existing pigments, brown algae, blue–green algae, and red algae are the three classes of macroalgae [ 3 ]. Although blue–green algae and bacteria share some common structural characteristics, blue–green algae were placed in the algal class because of the presence of chlorophyll and correlated complexes [ 4 ]. One more class of algae comprises the red algae. Species that belong to this class of Rhodophyta are eukaryotes that contain chloroplasts and phycobilins [ 5 ].

Brown algae named brown seaweeds are typically large macroalgae and have the comparatively immense ability to convert photons as a result of which biomass can be synthesized much more quickly. Brown algae are given more attention for the development of maintainable biofuels because their efficiency is considerably higher in contrast to that of cyanobacteria or red algae [ 6 ]. Microalgae have also arisen as a probable feedstock for the production of biofuels because a large number of microalgal strains have the ability of lipid accumulation, with a higher growth rate of biomass and greater photosynthetic production as compared to their counterparts that exist on land [ 7 ].

Difficulty to sustain and persistent debilitating of non-sustainable petroleum derivatives gave rise to the significance of inexhaustible fuel sources [ 8 ] a worldwide temperature alteration further amounts to the difficulties previously confronted [ 9 ]. These days, to move in the promising direction developed and underdeveloped countries are thinking about environmentally friendly power sources [ 10 ]. Biofuel is referred to as any fuel that is obtained from biomass that is either a plant, algae, or animal manure [ 11 ]. Biofuels are accepted to be the most natural amicable energy source. Biomass got from trees, agro backwoods buildups, marine or land plants, grasses, and harvests is the adaptable and significant sustainable feedstock for the development of biofuels [ 12 ]. The utilization of biomass as fuel is one of a handful of genuine systems to decrease the effects that greenhouse gases are causing. Contrasted with petroleum products, biomass ignition fundamentally diminishes CO 2 and CO 2 outflows and essentially lessens the debris obtained after burning [ 13 ]. As the conventional fuel resources are being depleted at a high rate, there is more focus towards the employment of alternative sources.

More than 50 years ago, the concept of employing algae as a source of food, feed, and energy was first proposed. During the energy crisis of the 1970s, when programs were started to manufacture gaseous fuels (hydrogen and methane), the production of methane gas from algae received a significant boost [ 14 ]. Our knowledge of cultivating algae for fuel has greatly benefited from the researchers' work on open pond algae growth [ 15 ]. The effects of various nutrient and CO 2 concentrations were documented, the engineering difficulties of mass-producing algae were addressed, and a strong basis for algae-fuel research was established through the isolation and testing of thousands of distinct species. But in 1995, US Department of Energy’s (DOE) Office decided to end the initiative due to budgetary restrictions and low oil prices. Everything has altered in recent years. Concerns about "peak oil", the rising effects of atmospheric CO 2 , the United States' increasing reliance on fuel imports, and the associated hazards to energy security have all contributed to a resurgence in interest in biofuels in general and algae-based biofuels in particular [ 16 ]. Advantages in biotechnology have opened up new possibilities that were not possible during the years of former research, such as the ability to genetically modify algae to produce more oils and convert solar energy more effectively. The United States has seen the majority of activity in algae research and commercial production. Algae biofuels are currently being explored globally in both established and developing countries in Europe, Asia, and other regions. The US-based Algal Biomass Organization serves as the industry's chief spokesperson and a resource for data on the businesses pioneering the technology [ 17 ]. This article outlines the third-generation biofuels, how they are cultivated in different systems, different influencing factors, and the technologies for the conversion of biomass. The benefits provided by these new generation biofuels are also discussed. Given these factors, it is essential to remove the current bottlenecks to use microalgae for commercial purposes.

Biofuel generations

The only renewable energy sources that can directly replace fossil fuels for current and future energy shortages are biofuel and biomass. These sources are environmentally favorable and renewable [ 18 ]. Generally, there are three generations of biofuels categorized based on their sources. Biofuels which are obtained directly from the food source are termed as first generation biofuels such as those that have been manufactured from the biomass comprising sugar, starch, and vegetable fats and oils [ 19 ]. The biofuels which fall under the category of the second generation are those that are produced from the plant biomass, which is mostly comprised of lignocellulosic materials, as this builds up most of the economical and ample nonfood compounds accessible from plants. But, in the present situation it is not economical to produce these fuels as there is a large number of mechanical obstacles that need to be avoided before their perspectives can be considered [ 20 ]. Second-generation biofuels, for instance, ethanol and methanol created from woody biomass, are more energy productive and more adaptable concerning their feedstock. The likelihood to utilize cellulosic and heterogeneous biomass recommends lower costs [ 21 ]. In any case, the ecological effect raised from biofuel combustion extraordinarily affects the carbon cycle (carbon balance), which is connected with the ignition of petroleum derivatives. Furthermore, the weariness of various existing biomass without suitable compensation brought about colossal biomass shortage, arising ecological issues like deforestation and biodiversity loss [ 22 – 24 ]. In an inquiry for feasible and practical options in contrast to non-renewable energy sources, past investigations have detailed the predominant abilities of green algae inferred biomass for the development of a better form: the third-age biofuels [ 25 ].

Algal biofuels

Algae-derived biofuels are progressed sustainable fuels obtained from algal feedstock utilizing different conversion systems. This is because of the oil-rich arrangement of this feedstock that can be related to its capacity to plentifully photosynthesize [ 26 ]. Lipids, polysaccharides, unsaturated fats, pigmentary compounds, cancer prevention agents, and minerals are among the naturally dynamic mixtures found in algal concentrates (Fig.  1 ). By way of levels of oils among 20 and 50%, such as Chlorella sp., Tetraselmis sp., Dunaliella sp., Isochrysis sp., Nannochloris sp., and Nannochloropsis sp., greater developments are reported. For the manufacture of biofuels, it is essential to produce lipids at high growth rates since high biomass productivity increases yield per harvest volume and high lipid content lowers the cost of extraction per unit product. Therefore, metabolic engineering of microalgae is required to enable the constitutive production of large amounts of lipids without compromising growth [ 27 ]. Moreover, lipids encompass the fatty acids which are essential for certain biofuels production [ 28 ]. Table ​ Table1 1 represents the lipid content of different algae species [ 29 ]. Notwithstanding biofuels, algae have been viewed as expected makers of synthetics that protect against viral, bacterial, and fungal infections and are also used for the production of antioxidants [ 30 ]. In a few viewpoints, microalgae feedstock is desirable to produce biofuels as microalgae does not need cultivable land and new water for development, they are not eatable hence no impact on food chain, can be developed to a few overlays regardless of occasional circumstances, alleviation of barometrical CO 2 and waste water treatment [ 31 ].

Cultivation of algae for biofuel production

Lipid content and fatty acid content of different algal strains

Biodiesel, bioethanol, biohydrogen, and biobutanol are the basic biofuels derived from algal biomass.

Biologically prepared butanol is termed biobutanol which resembles gasoline and exhibits several promising applications. The most efficient method that has been used in past for the production of biobutanol is acetone–butanol ethanol (ABE) fermentation. Because of its extraordinary enactment and benefits, this valuable sustainable biofuel can be used along with the currently available fuels [ 32 ]. Because of the higher productivity rate and the presence of carbohydrates, microalgae are assumed to be the most promising feedstock for biofuel production. A large number of algal carbohydrates can be converted into simpler compounds, called monosaccharides, and then can be utilized in the fuel production process. Nowadays, biobutanol production is carried out by some of the world’s famous producers which include Gevo, Butamax, Green Biologics, and US Technology Corporation [ 33 ]. The production of butanol under the biological methods is done in the presence of an anaerobic environment and is considered a phase of ABE fermentation. For the very first time in 1862, Louis Pasteur was the first scientist who reported the microbial manufacturing of biobutanol [ 34 ]. Acetone–butanol–ethanol fermentation is a biphasic system in which during the acidogenesis phase, butyric acid and acetic acid are formed. After the production of acids, the re-assimilation of these acids results in the yielding of solvents including ethanol, butanol, and acetone [ 35 ]. However, this process of butanol production synthesizes the other solvents simultaneously, thus the selectivity rate of our desired product is decreased [ 36 ]. It has been described that with the inclusion of enzymes such as cellulases and xylanases, scientists engaged C. saccharoperbutylacetonicum to use algal biomass, which was obtained from the wastewater for the production of biobutanol. Green seaweed has also been employed for biobutanol production using the strains of strains C. acetobutylicum and C. beijerinckii along with the metabolization of xylose and glucose. Macroalgae obtained from a marine ecosystem have also been shown to be a promising candidate for butanol production such as Ceylon moss, a marine macroalgae feedstock was used and Clostridial strains were employed for the extraction of biobutanol from it [ 37 ].

Biodiesel refers to the biofuel which is comprised of mono-alkyl esters. These esters are obtained from organic oils, algae, plants, or animals by the method of transesterification [ 38 ]. The process of transesterification is an equilibrium method that employs the presence of a catalyst and processes the algae oil into biodiesel in the presence of potassium hydroxide-like alkali [ 39 ]. To produce the algae-derived biodiesel, a huge amount of algal biomass is required. Algae that are typically employed in biodiesel production are unicellular ones that are mostly found in the aquatic environment. These algal strains are usually characterized as eukaryotes that have immense potential to photosynthesize and have higher rates of growth and greater density of population. In the presence of optimal conditions, even in less than 24 h, a green alga has the potential of doubling its biomass [ 40 ]. For effective production of biodiesel, the algal strains need to be effectively cultivated and then the biomass is harvested from the reactor. The most important methods that are currently under use for microalgal harvesting include sedimentation, flocculation, filtration, electrophoresis, and centrifugation [ 41 ]. After harvesting, normally the dry weight of the biomass needs to be increased but if the aim is to develop the production system for biodiesel or biogas, then the necessitation of this step can be removed as the production of biodiesel and biogas can accept the moisture content of high amount and we can easily proceed the process directly after the wet extraction of lipids [ 42 ].

The US Department of Energy's Aquatic Species Program concentrated on the production of biodiesel from microalgae and provided the final report according to which, it was suggested that biodiesel could be the only feasible approach to provide us with sufficient fuel to substitute for the existing world utilization of biodiesel [ 43 ]. It has been estimated that if we use biodiesel which is obtained from algae as a substitute for the 1.1bn tons of conventional diesel which is globally produced per year then it would require only 57.3 million hectares of land, which would be immensely promising in contrast to other biofuels [ 44 ]. Although biodiesel has great potential, it cannot easily compete with other petroleum fuels due to certain limitations. Its high cost and the requirement for a huge supply of organic oils is a big hurdle in the competition. It has been predicted that when the cost of petroleum fuels will become high and when the supplies will diminish gradually, only then, the alternative biofuels will become more approachable to investors and purchasers [ 45 ]. For biodiesel to turn out to be the substitute fuel of choice, it necessitates a vast amount of inexpensive biomass. Utilizing novel and advanced cultivation methods, algae might permit the production of biodiesel to accomplish the rate and scale of manufacture required to race with, or even substitute, petroleum fuels [ 45 ].

Biohydrogen

A diverse range of biochemical reactions derived from the microbes leads to the production of biohydrogen as a by-product. Biohydrogen exhibits a regular and short-lived nature. Biohydrogen can be referred to as the production of hydrogen by two mechanisms; thermochemical treatment or employing biological methods [ 46 ]. Biohydrogen is considered one of the most capable sustainable energy sources and can decrease the pressure which is being caused by a very limited supply of other resources. Moreover, it promotes the usage of the environmentally approachable technique. Biophotolysis of water, photo fermentation by photosynthetic bacteria, and anaerobic fermentation of biomass are some of the methods for producing biohydrogen [ 47 ]. A promising method of biohydrogen production is the biophotolysis of water using microalgae [ 48 ]. Another method of developing sustainable fuel production is the anaerobic fermentation of carbohydrate-rich biomass. Carbohydrates are efficient compounds where monomers are obtained and are employed in biohydrogen production. One such example of a monosaccharide is mannitol which is the probable substrate to produce biohydrogen using macroalgae [ 49 ]. Macroalgae is considered the most significant feed of biomass for biohydrogen production [ 50 ].

The process of direct photolysis has only been described in the microalgae species. This method employs the ability of green algae or cyanobacteria to photosynthesize. The process follows by the absorption of light leading to the splitting of water. The resulting electrons are then get transferred to enzymes such as nitrogenase or hydrogenase which will lead to the production of biohydrogen [ 51 ]. Indirect photolysis is a two-step method. In the initial step, highly photosynthetic biomass is prepared and in the next step, the process is followed by the anaerobic dark fermentation for the production of hydrogen. The step during which hydrogen is released is highly sensitive to oxygen. This is the reason the evolution steps of oxygen and hydrogen need to be separated in minimum time. Multiple models have been established for the process of indirect photolysis to be carried out. Most of these systems use algae and aim to use their capacity to photoautotrophically produce a huge amount of biomass per surface area [ 52 ].

As monomers are greatly employed in biohydrogen production, so the conversion of polymeric carbohydrates into monomers is considered a limiting step of the production process. To enhance hydrogen production using algae, a diverse range of pretreatments are employed to carry out the de-polymerization of polymeric sugars. The process of dark fermentation results in the negative net energy balance of differences between the energy which is evolved as hydrogen and the one that is used to produce biohydrogen. To make this whole procedure an economical one, the process of algal dark fermentation must be incorporated with a biorefinery method, where the outlets are commercialized into valuable biomolecules [ 53 ].

As it is known that the activity of hydrogenase is strongly inhibited by the presence of oxygen, researchers have utilized several methods to avoid this inhibition by avoiding the evolution of oxygen in the photosystem. One such method is to regulate the oxygen release by the usage of butyric acids and acetic acids [ 54 ]. Another limitation in commercializing the practical process is the higher costs of PBRs and the photon conversion efficiencies [ 55 ].

A more theoretical method needs to be developed to overcome the challenges that oxygen sensitivity is causing to encourage the studies and research on functional systems based on algae for producing biohydrogen. To improve hydrogen production, novel techniques must be introduced for the separation of oxygen from other biochemical reactions. It has been suggested that using the genetic engineering approach, such algal strains can be developed that can withstand and easily tolerate oxygen. The forthcoming role of biohydrogen as an unpolluted energy source for fuel cells manufacturing approximately zero emanations, and as a transitional energy carrier for storing and transporting sustainable energy, is progressively renowned globally [ 56 ].

In the present world, huge attention is being given to bioethanol due to its ecological advantages. Bioethanol can be obtained by all three generations of feedstocks including plants, lignocellulosic, and algal biomass [ 57 ]. To make bioethanol using algal biomass, the cultivated algae are harvested and then dried to remove nearly 50 percent of the moisture so that a solid material can be obtained and handled with ease. For this purpose, the harvested biomass of algae is made to undergo an appropriate process of dehydration to reduce the quantity of water before the oil is extracted. The moisture content is normally removed by a feasible drying procedure. Algal biomass can be dried by various approaches such as freeze drying, sun drying, or spring drying [ 58 – 60 ]. The process which is typically employed for the production of bioethanol from the algal biomass is fermentation. This process is used for the conversion of starch, sugars, or cellulose existing in algal biomass into bioethanol. This process is carried out by crushing biomass followed by the transformation of starch into sugars. Water and yeast are then mixed with it in the bioreactors which are called fermenters [ 61 ]. Yeast is used because it is involved in the breaking of sugar and transformation into bioethanol. Distillation is then carried out as a cleansing method for the removal of water and other contaminants in the thinned alcohol leading to the production of concentrated ethanol. The desired concentrated bioethanol is drained and converted into fluid form. This bioethanol is used for the supplementation or substitution of fuel in cars [ 62 ]. The production of bioethanol using microalgae can be used to surmount those environmental issues problems where producing bioethanol from conventional feedstock is found to be emitting more greenhouse gases than fossil fuels as a consequence of the feedstock manufacture and applications throughout the procedure [ 63 ].

Algal cultivation techniques

In addition to nutrients such as phosphorus, nitrogen, potassium, zinc, and calcium, algal cultivation necessitates water, carbon dioxide, and sunlight to yield biomass through photosynthesis, by the conversion of solar energy into chemical energy stored in the microalgal cells. Four major types of algal cultivation methods have been recognized based on certain conditions required for growth. These cultivation techniques are photoautotrophic, heterotrophic, photoheterotrophic, and mixotrophic cultures [ 64 , 65 ]. Under the mixotrophic mode of cultivation, microalgae can drive both photoautotrophy and heterotrophy and can exploit equally the inorganic and organic sources of carbon [ 66 ]. Under photoautotrophic mode, chemical energy is made by the process of photosynthesis, during which processed microalgae exploit light as the energy source and inorganic carbon as the carbon source. A CO 2 -rich environment could enhance biomass productivity to a certain extent [ 67 ].

Photoheterotrophic cultivation which is also called photo-metabolism besides photo-assimilation is the mode of cultivation that requires light and this light is needed to utilize the organic compounds as a source of carbon [ 68 , 69 ]. Heterotrophic cultivation exploits organic carbon materials as a source of energy and carbon to promote algal growth [ 70 ]. Among the above-mentioned approaches, photoautotrophic production is most extensively used because it is appropriate for large-scale algal biomass production [ 71 ]. The cultivation systems for algal production consist of three simple choices—open, closed, and hybrid systems. While the open systems are cost-effective and the closed systems are more efficient in nutrient removal, the hybrid systems are a culmination of open and closed systems specifically meant for high productivity in terms of biomass generation [ 72 ].

Open pond systems

Around the globe, open pond systems with the usual depth of 1530 cm, are commonly used to cultivate algae. Carbon dioxide which is readily present in the atmosphere can be alleviated by algae. The most generally practiced schemes for research and industrialized algal cultivation include the raceway pond, the closed pond, the shallow big pond, and the circular pond tank [ 73 ]. The cultures that are developed in open ponds can stand protection from adversarial ecological circumstances (rainfall, temperature, and luminosity) by the usage of a greenhouse. Microalgae that cultivate in adverse conditions, such as a basic medium or highly saline one, should be approved in command to attain axenic cultures [ 74 ]. Open ponds built in a wastewater treatment plant can be circular or driven by gravity flow [ 72 ].

The location of a pond is the most basic criterion for open systems. The location should be chosen based on maximum sunlight provision and the availability of all the requirements needed by the algal strains. The stirring unit is mostly absent in these kinds of systems as a result of which there is poor mixing but overall, these culture systems allow the culture process to be handled and monitored most simply and economically. The natural pond is typically not as much of a half meter deep as a consequence of which light breaches the water and a large number of algal cells can absorb it. An earlier report suggested that plastic films can also be exploited by layering them over the water surface for improved temperature control. Several algal strains, mostly for example Dunaliella salina , can be cultivated in these types of open systems for profitable motives [ 75 ]. An elongated spinning arm is set in the center of the pond which actions like a clock dial and executes a function of a paddlewheel which is conversant in the structure to that of a raceway pond. Mixing of algae cells and culture media is extra effective as compared to an unstirred pool, but as the algae get exposed to the environment, the contamination becomes unavoidable. As per the research literature, the efficiencies in circular ponds range between 8.5 and 21 g/(m 2  d) [ 76 ].

The raceway ponds for algal cultivation have been used since the 1950s. Initially, they were used for the Spirulina cultures. They can be consisting of either a racetrack channel or an oval channel. They are normally constructed utilizing a concrete solid [ 77 ]. Raceway ponds offer a continuous supply of nutrients and carbon dioxide along with the recirculation of algal culture. They are armed with a paddle wheel to be responsible for mild mixing to inhibit sedimentation. An aerator can be utilized for the intensification of air flow rate and hence carbon dioxide utilization [ 78 ].

A sustainable process involves using wastewater for algal production, which would provide the combined advantages of bioremediation of nutrients like phosphorus and nitrogen along with the production of biofuels. Considering brackish water, wastewater, and marine waters as a growth medium for algae can help to overcome important ecological challenges, but it will necessitate substantial research [ 79 ]. The productivity of algae in large-scale ponds is firmly governed by pH and dissolved oxygen concentration. Principle component analysis is a powerful instrument to perceive the restricted reduction in productivity made by unsuitable processing of the cultures [ 80 ]. To become a feasible option for scaling the production of algal biomass, open systems have to become cheaper to build and operate while sustaining robust and productive growth [ 81 ].

The efficiency of open ponds is questionable, even though their construction and operation costs are modest. An open system is difficult to monitor as there is a higher need for land, and there is a higher risk of contamination, as well as constraints due to weather and light intensity. The main drawbacks of an open-air system are its sensitivity to weather, season, and time of day. Some drawbacks of open pond systems prevent their use such as the inability to provide us with monocultures because several native algae and algae graze contribute to contamination. Pond temperature is not usually under control and the intensity of light is reliant on arriving solar insolation. Hence, the effectiveness of the open ponds is reliant on the native daytime deviations in temperature and solar insolation. While the cooling generated from the evaporative process somewhat controls the temperatures of open ponds, it as well points to a substantial loss of water [ 82 , 83 ]

Photobioreactors

Horizontal tubular photobioreactor.

At the commercial level, algal growth is widely done using horizontal tubular photobioreactors (HTB). This kind of bioreactor consists of a long arrangement of tubes that can be made of either transparent silicone, glass, or plastic material. These tubes are placed horizontally, and their diameter is kept small so that the area for the penetration of light is enhanced. This large area for illumination makes the tubular photobioreactor a good choice for the cultivation of algae. The circulation of algal cells in tubular PBR can be done by airlift technology quantum fracturing or using a centrifugal pump. Contemporary methods have been discovered in scheming tubular PBRs to guarantee a thin layer of culture suspension that is free from contamination along with extraordinary exposure to light and decreased energy requirement. However, the rise in the diameter of the tube can lead to a decreased surface-area-to-volume ratio hence less illumination. The increased diameter of the tube can also lead to the unequal distribution of solar energy to algal cells present at different levels in the tube. The longer tubes can cause the accumulation of oxygen which plays an inhibitory role in the photosynthesis of algal strains. These difficulties can limit the scale-up of the tubular photobioreactor. Another limitation of tubular photobioreactor is that temperature control in tubular PBR is not an easy task. Although, thermostats and cooling tubes can be used they are quite expensive to install. HTB can be scaled up by placing the tubes either above one another or using the coiled tubes. The rise of pH of the cultures in these kinds of bioreactors requires recurrent carbonation as a result of which the cost of algal production would be increased. It also requires a large land area to be operational as compared to the vertical ones [ 84 ]. Recently a new HTB which is named ‘Biocoil’ has been designed in the UK. The material that is exploited for its manufacturing is Teflon or low-density polyethylene and tested effectively at an experimental scale (2000 L) for growing several strains of algae. The poor gas exchange and the large gas gradient along the tubes, which is brought on by the majority of the gas exchange taking place in a separate chamber, are drawbacks of employing these types of reactors. High energy input and occasionally an accumulation of biomass in the tubes are additional drawbacks [ 85 ].

Vertical column photobioreactor

This kind of bioreactor is made up of glass or acrylic tubing which is placed vertically and allows the light to penetrate them. A gas sparger system is used for introducing the tiny bubbles of inlet gas into the reactor and allows the efficient mixing, mass transfer of carbon dioxide, and removal of oxygen. Usually, there is no incorporation of a physical agitation system in a vertical column photobioreactor. Vertical PBRs can be classified into airlift reactors and bubble columns based on arrays of liquid flow [ 86 ].

Bubble column reactors

The height of a bubble column reactor is larger than twice that of the vessel's diameter. These are cost-effective and are made of a large surface area for illumination. No moving parts are required in these kinds of reactors as efficient mixing and mass transfer are carried out using a sparger. The design of the sparger plays a key role in the enactment of the photobioreactor. Perforated plates are normally utilized as spargers for shattering and redistribution of the coalesced bubbles. Light is provided by an external source. By moving from the central dark zone to the upper light zone, this liquid circulation develops a differential gas flow rate which is crucial for photosynthetic efficiency. Bubble size, though, seemingly is as well vital for diminishing shear damage to cells [ 87 ]. Because of the high mass transfer, low energy costs, and exceptionally low physical stress, some bubble column PBRs are armed with a rubber membrane diffuser or double spargers to expand the mass transfer of gases: availability of carbon dioxide and removal of oxygen. If the dual spargers are used, the efficacy of CO 2 transfer is amplified fivefold compared to that of conventional sparging. According to the membrane diffuser's performance, the membrane's slits are more like holes with elastic lids that serve as valves to stop bubbles from entering the gas stream. Thus, the membrane diffuser serves as a one-way valve to stop liquid backflow [ 88 ].

Airlift reactors consist of a vessel having two interrelating zones. The gas blend streams up to the surface from the sparger in one cylinder, called the gas riser. The further area named the down comer, is the place where the medium streams down in the direction of the base and flows inside the riser and the down comer. The time gas residence in diverse zones impacts gas–liquid mass transfer, heat transfer, mixing, and turbulence and, therefore, is important to control the operation. A rectangular airlift photobioreactor has presented improved mixing features and improved photosynthetic competence, though its design is intricate and, therefore, is not easy to scale-up. The main limitation of using these kinds of vertical bubble column PBRs is that some of the algal strains such as S. costatum and C. muelleri have been reported to experience sheer stress in algal tubes and because of the pressure provided by the pumps, some algal cells cannot survive [ 89 ]. Recently, the effectiveness of a unique zigzag-flow column photobioreactor (ZZ-flow PBR) created to cultivate A. platensis with high biomass has been assessed. Four optimized zigzag baffle structures were put over the outer (riser) segment of the ZZ-flow PBR during installation. In comparison to traditional column PBR, the rate of intracellular photosynthesis and electron transport was improved, increasing biomass output and CO 2 fixation [ 90 ].

Helical type photobioreactor

Helical PBRs are made up of a coiled transparent and flexible tube with a tiny diameter and a degassing unit that can be both detached and joined. The culture is driven over a lengthy tube to the degassing device using a centrifugal pump. The energy needed by the centrifugal pump recirculating the culture and accompanying shear stress bounds the marketable application of this type of photobioreactor, which may be scaled up by simply adding a light-harvesting device. Another downside of the technology is polluting the inside of the reactor [ 91 ]. The helical PBR was also given a cone shape with a cone angle of 60°, resulting in a conical helical reactor. For the conical helical system, the angle and height are carefully determined. Polyvinyl chloride tubing was coiled in a conical framework to create the conical helical reactor. The liquid was recirculated using an air pump. This system also includes a degassing system and a heat exchanger for temperature regulation. The photoreceiving area and thus photosynthetic productivities rise by a factor of two when employing a 60°. Among all other cone angles examined for this reactor, the photosynthetic efficiency of 6.84 percent was the highest. The key benefit of the cone shape is the increased light-harvesting efficiency while maintaining the same basal area. Another benefit of this type of reactor is that it requires less energy and places less mechanical stress on algae cells. Because of its defined angle and size, increasing the number of light collecting units is the only method to scale-up, but it results in more energy loss in the intricate branches of flow networks [ 92 ]. Low gas exchange, high shear stress, the buildup of biomass in the tubes, and the high energy input are all drawbacks of this type of reactor [ 85 ].

Flat-panel PBR

A flat panel photobioreactor usually consists of a transparent vessel that is made up of glass, plexiglass, or polyethylene film, and its thickness lies between 56 cm. The surface-area-to-volume ratio of these bioreactors is greater as compared to the tubular bioreactors. How flat-panel PBRs are designed entails suitable alignment to capture the solar potential for algal growth. The panels are ordered in head-to-head or parallel plates to prevent self-shading which is the main cause of photosynthetic inhibition leading to reduced algal growth. The provision of light might be achieved using light-emitting diode lights or optical fibers that achieve effective radiance to encourage the thriving growth of algae. Water is sprayed over the surface of panels for controlling the temperature. Heat exchangers are also used for this purpose, but they are not as cost-effective. Flat panel bioreactors frameworks when working at indoor surroundings, the elements, for example, space among light sources and panels, temperature impacts, enlightenment of one or both panel sides, light way are pivotal. Expanding volume results in the expansion of hydrostatic tension, in this way making scale-up troublesome. In addition, the hydrodynamic pressure might influence microalgae development.

In the aim to mass cultivate green algae, the flat panel PBR is incredibly suggested attributable to an extraordinary proficiency of photosynthesis and lower measure of disintegrated oxygen amassing, however, there are challenges connected with sterilization. However, these kinds of PBRs are exceptionally useful but the difficulty of scaling up and high operating costs sometimes limit the use of flat panel PBR. A less expensive design of flat panel PBR has been suggested where they utilized plastic packs inside the rectangular casing [ 93 ]. However, flat plate systems may also suffer from certain limitations. For example, few glitches such as greater space necessitation, a huge amount of solar energy, chances of hydrodynamic stress to some algal strains, problematic maintenance, low effectiveness in terms of mass production per unit of space, and fouling up of the channels [ 94 ].

Stirred tank photobioreactor (STR)

The most convenient type of reactor is the stirred tank reactor, which uses impellers of various sizes and shapes to generate mechanical agitation. Baffles are used to minimize the vortex. At the bottom, CO 2 -enriched air is bubbled for the provision of a carbon source for algae development. This sort of bioreactor ought to be converted to a photobioreactor by externally illuminating it with fluorescent lights or optical fibers. The unemployed sparged gas and created oxygen throughout photosynthesis are separated from the gassed liquid to the gas phase by a large disengagement zone. Stirred tank reactors were first presented as a way to produce microalgae photo autotrophically utilizing artificial light or sunlight since they were an industry and laboratory standard. To develop Selenastrum capricornutum , a Hydraulically Integrated Serial Turbidostat Algal Reactor (HISTAR) system with an entire volume of 3.6 cubic meters was utilized. Two sealed turbidostats and a sequence of open hydraulically coupled continuous flow stirred-tank reactors comprised HISTAR (CFSTRs). The inoculated culture's biomass was amplified by the CFSTRs. This technology has been lately exploited in the direction of creating a deterministic system to anticipate microalgal yield to determine practical viability for far-reaching application; though, no reports of future deployment have been made [ 95 ]. However, the chief drawback of this system is the small surface-area-to-volume ratio that reduces light collecting effectiveness. The usage of optical fibers for illumination has also been attempted, although this has the issue of obstructing the mixing pattern [ 96 ].

Advanced systems

The hybrid frameworks are exceptionally planned frameworks which are a summit of the two sorts of development framework, conservative being used and implied for huge scope green growth development. These frameworks defeat the constraints of open ponds and the high starting working expense related to closed systems. For this situation, algal growths are first refined in a PBR, to accomplish high-density inoculants and afterward moved to an open framework, accordingly, working with the fulfillment of ideal biomass creation. The possibilities of defilement in open frameworks are significantly diminished when moved to the open framework, with algae getting predominant and contending successfully with different microorganisms [ 73 ]. Hybrid systems, on the other hand, involve substantial infrastructure, costly maintenance, and ongoing supervision [ 97 ].

A modified cultivation technique known as an Algal Turf Scrubber (ATS) was first presented by Professor Walter Adey at the beginning of the 1980s [ 98 ]. A downward-sloping surface that allows water or influent to flow across it intermittently or continuously is provided by an ATS culture system, which encourages the growth of macroalgae [ 99 ]. Microalgae in the ATS system grow effectively as a result of proper inorganic compound intake and photosynthesis-based dissolved oxygen release. The procedure has some downsides, including the need for enough acreage, a lesser capacity for processing wastewater, and significant infrastructure [ 97 ].

Technologies for biomass to biofuel conversion

Available processes that carry out the transformation of algae-derived biomass to diverse energy resources are categorized as thermochemical conversion, biochemical conversion, chemical pathways, and direct combustion (Fig.  2 ). Algal biomass can be converted into biofuels with the help of thermochemical methods including pyrolysis, gasification, and superficial liquid extraction, or by carrying out hydrothermal liquefaction. Apart from the sugars and lipids, all of the algae-derived biomass can be converted into biofuels using these techniques. After cultivation, if the further processing of algal biomass involves the use of the thermochemical method, there is no need of employing special surroundings such as nitrogen scarcity during the cultivation process in hope of achieving maximum content of lipid [ 100 ]. The process of gasification involves the algae-based biomass reaction in a gasifier under the partial oxidation of air. This process is carried out with any kind of combustion and in the presence of oxygen, air, or steam. A number of other downstream methods also accompany this traditional method. The gasification process along with some other downstream processing techniques eventually results in the production of carbon monoxide, methane, and hydrogen, combined with definite undesired by-products that are solid [ 101 ].

Different configurations for photobioreactors: A tubular photobioreactor, B bubble column photobioreactor, C flat panel photobioreactor, D helical tubular photobioreactor, E a simply stirred tank photobioreactor

Another method of thermochemical conversion is pyrolysis during which thermal decomposition of algae is done without using any air or oxygen. The procedure is usually operated under atmospheric pressure and the temperature employed for heating usually ranges from 400 to 600 °C for conventional pyrolysis. However, 800 °C and a least 300 °C temperature are employed for microwave pyrolysis and catalytic pyrolysis, respectively [ 102 ]. One main advantage of pyrolysis is that Pyrolysis greater yields of bio-oil are conceivable (approximately 57.5% w/w). However, this process is limited by its need for dried biomass having least of the moisture content [ 9 ] . During the hydrothermal liquefaction, usually the algae make approximately five to fifty percent of the slurry feed. Extremely high temperatures nearly 250–500 °C to physically and chemically convert the biomass into fuels. Auto thermal water is used under 5–20 bar pressure and the process is carried out either in the presence or absence of a catalyst [ 101 , 102 ]. Liquefaction results in the production of bio-oil along with some other by-products of methane that exists in a gaseous state. Although the reactions are highly complicated greater yields can be obtained using this reaction [ 9 ]. Microalgae can be converted in a biochemical process using either microbes or enzymes by the action of which algae are broken down into fuels. In contrast to the thermochemical conversion methods, biochemical conversions usually occur at a slower rate and are not much energy-intensive. Fermentation, photo-biological H2 production, and anaerobic digestion are some of the biochemical conversion strategies and repeatedly necessitate pretreatment of the biomass, particularly before carrying out the fermentation and anaerobic digestion [ 103 ].

Biomethane is generally produced by the anaerobic digestion during which carbon dioxide and traces of hydrogen sulfide are also manufactured along with biomethane from algal biomass from the enzyme or microbe catalyzed conversion of organic matter. Anaerobic digestion is usually appropriate for the organic matter that exhibits a large amount of moisture such as algal biomass. The three steps of this process are carried out in sequential order: hydrolysis, acetogenesis, and methanogenesis. Since hydrolysis acts as the rate-limiting step in the process of anaerobic digestion, the algal liability to the attack by an enzyme is a significant aspect that might be accomplished through pretreatment. Even though the typical power-driven pretreatment is the common custom designed for microalgae, they remain to be extra suitable to be used for thermal pretreatment [ 104 ]. Microalgae that are comprised of greater starch-based content are typically employed for the process of fermentation. This process involves the conversion of the principal components like sugars and starch into bioethanol by carrying out hydrolysis. The process is further followed by the fermentation by yeast leading to the production of bioethanol. The algal cell wall is then exposed to the pretreatment processes for the releasing of carbohydrates either by sonication or enzymatic mechanisms [ 105 ]. However, this process is limited to the transformation of lipids only and does not exploit starch and protein portions of feedstock [ 9 ].

Factors affecting algal cultivation and biofuel production

Both biotic and abiotic factors may influence the biomass yield and thus the production of biofuels. Light, temperature, pH, nutrients, and carbon dioxide are among the abiotic factors while the specie of algae used for cultivation is a biotic factor that can have an impact on the yield and productivity.

It has been observed that the growth and the biomass accumulation of the algal species during cultivation are highly reliant on the wavelength as well as the intensity of light. In some cases, it has been shown that as the intensity of light is increased, the lipid content also increased [ 106 ]. It is known that the choice of algae as a feedstock is typically made by keeping in mind its ability to highly photosynthesize. As light is required for the process of photosynthesis and growth, it acts as one of the most important factors involved in controlling lipid accumulation and lipid enhancement. The shading effect of the light is also observed by some researchers leading to the understanding that the shedding light inhibits the growth of some algal strains and as soon as the shading light materials were removed, the agar continues to grow with ease and in a quicker manner [ 96 ]. It has been found that growth achieved using the fluorescent light source is comparatively enhanced as compared to the other sources of light [ 107 ].

Temperature

Temperature is among several significant aspects that contribute to the growth of algae, lipid accumulation, and biofuel production. Most of the algae species grow optimally in the range of 20–35 degrees centigrade. But some species fall in the mesophilic category and approximately 40 degrees is still a bearable temperature for them. Overheating or heating at a temperature that is less than the required one can lead to the reduction of the yield and cell damage, respectively [ 108 ]. It has also been observed that the number of lipids in the algal biomass was shown to be decreased both by extremely high and extremely low temperatures [ 96 ].

Carbon dioxide

Atmosphere, gas emissions from the exhausts of industries, and soluble carbonates are some of the important sources by which carbon dioxide can be obtained [ 109 ]. A greater amount of carbon dioxide is a necessity for the production of higher algal lipid content [ 110 ]. Different works of literature provide different evidence for the impact of carbon dioxide on lipid accumulation. It has been observed that the growth of algae is enhanced, and the process of fatty acid synthesis is diminished by decreasing the concentration of carbon dioxide, on the other hand, fatty acid synthesis is enhanced by a higher amount of carbon dioxide. However, increasing the concentration of carbon dioxide will highly affect the carbon chain. One of the studies done on the Chlorella pyrenoidosa SJTU-2 and Scenedesmus obliquus SJTU-3 has shown that at 10 percent of carbon dioxide, the growth was enhanced. However, the accumulation of lipids and the synthesis of fatty acids increased when 30–50% of CO 2 was employed [ 111 ].

Lipid accumulation, oil synthesis, and the enzymatic activity for the growth of algae are highly influenced by pH which is among the most important factors. The growth of algae is enhanced by the environment which is acidic to some extent or has a neutral pH. But the nutrient medium has the presence of carbonic acids that can result in a lower pH as a consequence of which, the circumstances become opposing to the algal growth. Moreover, as the concentration of bicarbonates at low pH is less than that at high pH, this causes a negative influence on the carbon Integration for lipids formation [ 110 , 112 ]. The change in pH has an impact on the microalgal biological reactions in a variable manner. It has been observed that the augmented pH eagerly repressed the cell division of Chlorella , activated the discharge of autospores, and as a final point give rise to TAG exploitation (Fig.  2 ) [ 113 ].

If the limitations of various nutrients are assumed, then diversification of biochemical compounds can be detected in algae. However, it depends upon the type of nutrient which is limited and the extent of the limitation. If the pH and temperature are maintained at the optimal range, then the rate at which algae grows is directly proportional to the rate at which the extreme limiting nutrient is uptaken. The most vital macronutrients that are mandatory for the proper growth and development of algae are phosphate and nitrogen. Moreover, the nutrients that are non-minerals in nature and are essential for algal growth are carbon, oxygen, and hydrogen. However, the growth and metabolic pathways of algae are not challenged by the profusion of oxygen and hydrogen [ 9 ]. It has been shown by some scientists that the nutrients which are released from the algal biomass can be recycled and again used in the nutrient media. Such nutrients are first converted into a co-product that exists in the liquid state and is then made available to be used again. An example was shown by growing a bi-culture of two algal strains and recycling and using the nutrients that were released during the process of carbonization as the feedstock for the production of biodiesel [ 114 ].

Algae species

The biochemical makeup of the algal community has an impact on its biomass's capacity for producing low-cost biofuels. The colonial species, which often dominate in high-rate algal ponds and have the advantage of being readily harvested by simple, low-cost gravity settling, have received little attention. Five wastewater colonial algae species that are frequently found in high-rate algal ponds were examined for their efficacy in wastewater treatment and their potential value for producing biofuels in terms of their biochemical composition and biomass energy yield: Coelastrum  species, Desmodesmus species, Pediastrum boryanum , Micractinium pusillum , Mucidosphaerium pulchellum , and others. Summer has higher algal biomass output, lipid, and energy content than winter, depending on the species. Under both summer and winter circumstances, the Mucidosphaerium pulchellum and Micractinium pusillum cultures produced the most biomass and had the highest lipid and energy contents. Micractinium pusillum , however, settles far more readily than Mucidosphaerium pulchellum , indicating that of the colonial algae species examined, Micractinium pusillum offers the highest promise for both wastewater treatment and low-cost biofuel generation [ 115 ]. Although no polyculture is shown to produce more biomass than the most prolific monoculture, greater species diversity significantly can boost the output in comparison to the average across monocultures. However, there are data that suggest polycultures may be less likely to make undesirable crop function trade-offs, supporting the idea that variety can support several functions simultaneously [ 116 ].

Limitations and benefits

In contrast to the first and second-era biofuels, algal biofuels do not compete much with fossil fuels [ 117 ]. Algae exhibit a considerable energy need of the numerous equipment and capital contributions of the augmented agronomy rotations in contrast to the terrestrial feedstock [ 118 ]. This is the reason for a comparatively lower return of net energy and diminished ability to compete [ 119 , 120 ]. This significant need for energy can hypothetically lead to a loss of net energy for algae-based biodiesel, or at best a bordering improvement assumed the present expertise [ 121 ]. It is assumed that PBRs show greater costs for cultivation and eventually a decreased ratio of energy. One main disadvantage of using PBRs for algal cultivation is that the construction of photobioreactor and the need to circulate the culture demands for most of the energy cost [ 120 , 122 ]. It has been discovered that key drivers of financial risk include both weather and pricing changes. This is the first probabilistic assessment of weather-related production consequences for algae growers, which is important given the industry's global expansion and the fact that the US 2018 Farm Bill now recognizes algae as a new commodity eligible for crop insurance [ 123 ].

Nonetheless, where the manufacturing industry is somewhat novel, there is the possibility of enhancements in the strains of algae and manufacturing tools that can guarantee an advanced likelihood of a net energy balance that will be positive, however, yet there is no certainty [ 124 ].

Algae-derived biofuels might be feasible as probable air travel petroleum if assumed for their dense energy characteristics and also shows great potential in research for the companies associated with air travel [ 125 ]. Some of the advantages of using algae as a potential source of biofuel are also discussed in Fig.  3 . A large number of by-products that can be extensively used on a commercial level can be prepared by microalgae [ 126 ]. The forthcoming marketable feasibility of microalgae as a feedstock for the production of biofuel may also be determined by a suitable commercial usage of these by-products [ 126 , 127 ].

If open pond systems are compared with the PBRs, it is found that the PBRs exhibit a more effective ratio of energy [ 119 ]. Open-lakes were likewise shown to have a lesser amount of energy-demanding development, with more critical energy expenses being brought about by reaping and drying stages, tallying as much as multiple times the energy proportion [ 118 , 127 , 128 ].

Just like terrestrial cultivation, the biomass is prepared via photosynthesis with the help of algae [ 39 ]. Although this process occurs with more affectivity in algae regarding the farmed are but the conversion process is comparatively still not economical [ 129 , 130 ]. In literature, there is a great emphasis on the importance of developing commercially feasible productivities to lesser net costs [ 131 ]. Production of microalgae at the commercial level is additionally expected to have positive net fossil fuel by-products, in contrast to its earthbound partners, because of the precise manufacturing climate and associated apparatus that need fossil-inferred power [ 128 , 132 ]. Furthermore, the utilization of petroleum products in the downstream handling of the biomass can likewise check the greenhouse gas sequestration paybacks accomplished in the upstream development, as with regular biofuels (Fig.  4 ) [ 75 , 133 ].

Advantages of algal fuel

It has been suggested that during the process of cultivation, recycling the outlet gas can result in a net decrease in carbon emissions. Advantages of effective carbon fixation can also be achieved if the flue gas is introduced as carbon dioxide input into the medium for algal growth [ 134 , 135 ]. The growth of biomass is not affected by this technique [ 124 ]. Some exploratory and applied research on the proficiency of a microalgal strain to utilize a highly concentrated vent gas supply exhibited the attainability and productivity of this application past earthbound horticulture [ 130 , 136 – 138 ]. Notwithstanding this sequestration advantage, the net carbon dioxide help from microalgae is reliant upon the discharges from the ensuing utilization of the biomass as a fuel. Expecting the carbon dioxide acclimatized to be transmitted on burning, the remaining emanations will rely upon the amount of energy of the biomass handling that might utilize petroleum products [ 135 ].

As it is known that inorganic compounds are also required in the nutrient media for algal cultivation. Nitrogen is chiefly used in the growth medium, so there is a possibility that by the use of microalgae, we can make it possible to remove the huge amount of nitrate compounds that are present in the wastewater and cause eutrophication [ 139 ].

The intrinsic advantage of microalgae is that they do not immediately compete with food by vying for a valuable agricultural area with established terrestrial crops. Accepting patterns for expanded strategy sustenance for transportation biofuels, microalgae as a feedstock can reduce more or less of the threats that first and second-age biofuels are causing to food security. Even though there is the most ideal probability for some microalgae strains as an additive in human weight control plans [ 135 ], it presently does not frame boundless dietary decisions. Cultivation of algae likewise decreases the contest for water assumed that it is ideally grown in wastewater [ 140 ], even though as recently referenced, the high supplement immersion can be significant to the practicality of its production for important results [ 141 , 142 ].

In addition, where there is an emphasis to cultivate the feedstock away from the agronomic area, there is the benefit of using macroalgae and microalgae in cost reduction that is related to the scarcity of land resources. The cultivation of algae does not necessitate the same need for land as is needed for the land-dwelling biomass [ 143 ]. Generally, the cultivation of algae for the production of biofuels can most probably have the least effect on the security of food. It competes for the fuel versus food debate. The use of algal feedstock significantly reduces the stress on the first- and second-generation feedstock-related effects on the foodstuff and agronomical resources. Moreover, the diminished need for farmable land refutes the requirement for inescapable change of timberlands and forests. This lessens expected impacts on carbon sink and loss of biodiversity [ 144 , 145 ].

Developing a biofuel industry based on algal cultivation can provide us with a lot of socio-economic advantages contributing to a publically maintainable result. Social sustainability includes, among further features, the possibility for an extra unbiased circulation of financial assistance through the public, comprising local and municipal societies, and enhancements in the life worth [ 146 ]. One of the most certain advantages is the fabrication of such an energy industry that can maintain and meet the long-term needs of the fuel along with more chances of employment. Such an industry can also be led to the growth of the economy in the local societies. Compared to this, the industries that are based on fossils are reliant on a limited number of resources [ 6 ]. As durable maintainable engineering, the production of biofuel using the microalgae can, moreover, offer openings for the progression of associated employments [ 147 ]. Industries based on the use of algae also provide chances for economical enhancement. Microalgae-centered manufacturers also provide a chance for economic growth in rural and topical areas [ 148 ]. Although the use of algae accounts for the higher costs of energy and production, still the production of algae-based biofuels prevents several limitations that were caused by the first and second-generation biofuels [ 124 ].

Future prospects

Algal fuels might be superior to fossil fuels when considering the life-cycle evaluation, however, this field is still in its infancy. Despite the conflicting views, the concept of algae-based biofuels makes sense both philosophically and practically [ 39 , 149 – 151 ]. Algal fuels manage a net positive energy recovery despite the currently underdeveloped manufacturing techniques, however, the precise amount is still up for debate. Algal biodiesel appears to have a reduced water footprint than biodiesel made from other crops [ 68 , 69 , 133 , 149 , 151 ]. Additionally, compared to open ponds, photobioreactors create an algal broth that is much more concentrated, which significantly lowers the dewatering expenses. It might be able to create dewatered algal biomass using tubular photobioreactors for about $4 per kilogram dry weight [ 152 ]. Moreover, there are probable enhancements in the cultivation and there is an aim to decrease the capital cost using the machinery of low cost for the further processing of algal biomass [ 117 , 127 ]. Proper provisions of nutrients, CO 2 , and water in specific are supposed to be a limiting factor in the practicable cultivation of algae [ 153 ]. Another approach to significantly reducing the operational cost is to make possible the recycling of nutrients, water, and carbon dioxide during the production process [ 154 ]. Improvements are also required to effectively minimize the energy and cost required for the microalgae processing methods to be applicable at a commercial scale. By improving the techniques used at the harvesting stage, the costs associated with further processing steps to produce microalgae-based bioproducts and biofuels could be reduced [ 155 ].

Similar findings have been reached by additional independent research. As the production facility's scale is raised, the cost per unit of manufacturing algal biomass will further decline. The practicality of algae biofuels will probably be most impacted in the long run by genetic engineering. The possibilities for algal oil can be improved by improvements in methods for isolating the algae biomass from the water and extracting the oil from the biomass [ 150 ]. For instance, certain photosynthetic microorganisms' cells have been genetically modified to secrete oil that would typically be retained within the cell, making the process of recovering oil easier [ 156 ]. It will be a huge improvement if algal species can be developed to utilize atmospheric nitrogen instead of the nitrogen fertilizers that are currently needed. The manufacture of nitrogen fertilizers is highly dependent on petroleum [ 157 ].

Furthermore, putting in place the right regulatory frameworks to reflect the most affordable price can increase production viability as a long-term and sustainable replacement for fossil fuels. As evidenced by the comparatively rapid expansion of terrestrial feedstock, producers and consumers respond to the incentives provided by such policies (for example, in Brazil). Although the same regulations apply to the production of microalgae, the higher start-up costs and risks function as an additional barrier to investment compared to the less expensive agricultural-based production. Finding a policy mix that provides suitable incentives for third-generation biofuels while transitioning away from conventional approaches and managing the associated risks is likely to be as challenging given the technological advancements necessary to justify these incentives and the fuel's viability. Considering the potential of microalgae as a biofuel feedstock, accepting these challenges would appear to be founded on long-term optimism rather than utopian assumptions [ 152 ].

Moreover, recently more attention is being given to the co-culturing technique. Microalgae grow symbiotically with other heterotrophic microorganisms, including bacteria, yeast, fungi, and other algae/microalgae, in a co-cultivation method. They trade nutrients and metabolites, which boost productivity and make it easier to commercialize microalgal-based fuel. Co-cultivation makes it easier to gather biomass and value waste, which contributes to the development of an algae biorefinery platform for the generation of bioenergy [ 158 ].

Algae-based fuels seem quite promising. If the complete environmental impact of the latter forms of fuels is taken into account, they might already be seen as being competitive with petroleum-based fuels. We may be forced to abandon petroleum long before it runs out by climate change-related issues.

Conclusions

The creation of third-generation biofuels, a superior type of biofuel, is the most promising application of the biomass obtained from algae species. Algae are adaptable plants that may flourish in a variety of aquatic environments, including water that contains a lot of salt or waste. Algae-producing facilities can be found in areas that are not suited for the growth of forests or agroecosystems. As a result, the production of algae does not compete with that of food, fiber, or fuel. Algae have been extensively exploited in industrial applications with the most intensive usage in the production of biofuels such as biobutanol, biodiesel, biohydrogen, or bioethanol. It has been reviewed that for the production of algal fuels, algae can be cultivated in all sorts of systems that can be closed, open, or hybrid. The production of biofuels depends upon several factors which influence the cultivation of algae. The practicality of algae biofuels will probably be most impacted in the long run by genetic engineering. Algal oil's prospects will be improved by improvements in methods for separating algal biomass from water and extracting oil from biomass. In the next 7–10 years, algae-based fuel production might be cost-effective, widely adaptable, and operational, but only if we continue improving our awareness of these magnificent species while also improving our capacity to tailor them for the specific aim of growing new energy industry. In the coming years, algae biomass could play a key role in resolving the conflict between food production and biofuel production.

Author contributions

TM, NH: conceptualization, data analysis and curation, project administration, supervision, validation, writing—original draft, review and editing. AS, SIM: data analysis and curation, validation, writing—review and editing. KG: data analysis, writing—review and editing. HMNI: formal analysis, writing—review and editing. MB: conceptualization, data analysis and curation, project administration, co-supervision, validation, writing—original draft, review and editing.

Consejo Nacional de Ciencia y Tecnología (CONACyT) Mexico is thankfully acknowledged for partially supporting this work under Sistema Nacional de Investigadores (SNI) program awarded to Hafiz M.N. Iqbal (CVU: 735340). This work was partially funded by the National Science Centre, Poland under the research Grant number 2020/37/K/ST8/03805.

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Applications of the microalgae chlamydomonas and its bacterial consortia in detoxification and bioproduction.

1. Introduction

2. microalgae cultivation methods, 3. wastewater types, composition and treatment methods, 4. main mechanisms and molecules detoxified by chlamydomonas, 5. chlamydomonas –bacterial consortia for bioremediation, 6. chlamydomonas –bacterial consortia for biomass and bio-product generation, 6.1. biomass, 6.2. biofuels, 6.2.1. biohydrogen, 6.2.2. lipids, 6.3. biofertilizers, 7. conclusions and future perspectives, author contributions, data availability statement, acknowledgments, conflicts of interest.

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Torres, M.J.; Bellido-Pedraza, C.M.; Llamas, A. Applications of the Microalgae Chlamydomonas and Its Bacterial Consortia in Detoxification and Bioproduction. Life 2024 , 14 , 940. https://doi.org/10.3390/life14080940

Torres MJ, Bellido-Pedraza CM, Llamas A. Applications of the Microalgae Chlamydomonas and Its Bacterial Consortia in Detoxification and Bioproduction. Life . 2024; 14(8):940. https://doi.org/10.3390/life14080940

Torres, María J., Carmen M. Bellido-Pedraza, and Angel Llamas. 2024. "Applications of the Microalgae Chlamydomonas and Its Bacterial Consortia in Detoxification and Bioproduction" Life 14, no. 8: 940. https://doi.org/10.3390/life14080940

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