Biorefinery through Algal biotechnology

Mercy Mukite Barasa1, Abubakar Sale Ahmad1, László Abrankó2, Arijit Nath1

Email mukite.barasa@gmail.com

1Department of Food Engineering, Faculty of Food Science, Szent István University, Ménesi st 44, HU-1118 Budapest, Hungary

2Department of Applied Chemistry, Faculty of Food Science, Szent István University, Villányi út 29-43, HU-1118 Budapest, Hungary

Algae are thallophytes that have chlorophyll as their primary photosynthetic pigment.

They are found in hyper-saline to freshwater environments, over a broad range of pHs,

and even relatively dry environments such as soil and rocks. Algae are classified into:

Due to stricter environmental legislation related with negative impacts of fossil fuels and

to solve the scarcity of fossil fuels, concept of “Biorefinery” has come to forefront. The

biorefinery concept refers to a network of systems/technologies that converts biomass to

useful biofuels through physical, chemical and biological routes. In the platform of

biorefinery, different types of biomass, e.g., lignocellulosic solid waste, organic

hydrocarbon, algae etc. are converted to biofuels in efficient way [1].

In this context, biorefinery through algal biotechnology came to forefront because (a)

algae can grow in wastewater, (b) adsorb and convert atmospheric or industrially

produced CO2, and (c) their metabolite, such as sterol can be converted to wide ranges of

biofuels [2]. After harvesting of algae, different technologies are used to collect lipid from

algal biomass. Algal lipids (non-polar lipids and polar lipids) can be converted to biodiesel

through transesterification reaction and hydrocarbon pool through pyrolysis process.

Furthermore, biooil can be produced by decarboxylation of algal lipid. There are three

routes for producing bioethanol from algae, such as (i) after oil extraction, algal biomass

can be considered for bioethanol production through sequential steps (pretreatment to

produce fermentable sugar and subsequently, anaerobic fermentation of fermentable

sugar with yeast), (ii) dark fermentation and (iii) photofermentation. Whole algal biomass

can be considered for pyrolysis process to obtain liquid hydrocarbon pool, known as biooil.

Furthermore, anaerobic digestion of algal biomass produces biomethane. Fermentative

production of biohydrogen by algae may consider a unique approach in biorefinery. In

biorefinery, to obtain high energy yield and ultimate utilization of algal biomass,

application secondary pyrolysis is noteworthy [3]. Therefore, it may consider as holistic

approach compare to development of petroleum-based fuels [4]. In Figure 1, concept of

biorefinery through algal biotechnology is represented.

Application of algae to produce sustainable fuels may consider as a unique approach in the

context of biorefinery. Several footsteps have been placed to produce biodiesel, bioethanol,

biooil, biomethane, and biohydrogen from algal biomass. Several challenges are bottleneck to

establish the algal-based biorefinery. The cultivation process of algae for biofuel production, and

to develop recombinant algae for higher production of lipid and biomass might need more

research. Furthermore, research about development of sustainable environmentally benign

technology for production of biofuels may reduce the limitation to establish economically

feasible algal-based biorefinery. It is expected that present lecture will make a bridge between

the blue and green biotechnology, and receive attention among different research communities.

Background art Algal Biotechnology

Conclusion

SZIEntific Meeting of Young Researchers

In industry, different types of algal cultivation system is noticed, such as(A) Open cultivation systems

(a) Non-stirred ponds(b) Stirred pond

Acknowledgements

Authors acknowledge the support from the European Union project

(grant agreement no. EFOP-3.6.3-VEKOP-16-2017-00005).

BiodieselThrough transesterification reaction, algal lipids are converted to biodiesel (pool ofesters). Algal lipids can be classified into two main categories;(a) non-polar lipids (acylglycerols, sterols, free (non-esterified) fatty acids,

hydrocarbons, wax and steryl esters)(b) polar lipids (phosphoglycerides, glycosylglycerides).After extraction of lipids from algal cell, transesterification reaction is performed withalcohol, mainly methanol. However, in transesterification reaction, both acid and alkalicatalyst can be used, alkali catalyst is more preferable because the alkali-catalyzedtransesterification process provides more yield than other one. In this cas, NaOH andKOH are commonly used as the catalyst. The resultant is a fatty acid alkylester andglycerol as a byproduct. In Figure 3, transesterification reaction of algal lipid andmethanol in presence of alkali catalyst is represented.

BioethanolBioethanol from algae can be developed in three different ways, such as:(a) After extraction of lipid from algal biomass, residual biomass can be converted to bioethanol by

sequential processes, such as (a) conversion of complex carbohydrate in biomass to simplefermentable sugar and (b) anaerobic fermentation of fermentable sugar with yeast). Residualbiomass is typically fibrous composites of microfibrillar polysaccharides embedded in matrixpolysaccharides and proteoglycans. Also, residual biomass is an abundant source of starch,produced by photosynthesis. As algae do not contain lignin, physical or chemical pretreatmentis not required. Pretreatment with enzymatic route convert complex carbohydrate tofermentable carbohydrate, which increase the efficiency of ethanol production process.

(b) Microalgae and cyanobacteria are capable of expelling ethanol through the cell wall by meansof intracellular process in the absence of light, however, it is not an efficient process for theproduction of bioethanol. The production of ethanol is favored by the accumulation ofcarbohydrates in the microalgae cells through photosynthesis, and then the microalgae areforced to synthesize ethanol through fermentative metabolism directly from their carbohydrateand lipid reserves when switching the growth to dark conditions.

BiomethaneBiomethane can be produced from algal biomass through anaerobic digestion process. Forproduction of biomethane, whole algal biomass or residual biomass after oil extraction from algalcell can be utilized. Biochemical steps for biomethane production from algal biomass are herein:

BiooilBiooil can be produced from whole algal biomass through pyrolysis process. Furthermore, biooilcan be produced by pyrolysis of algal lipid or decarboxylation of algal lipid. Pyrolysis process isperformed at high temperature, more than 300 oC. At high temperature in absence of oxygen, thephysical properties of water change such that it promotes both the degradation of themacromolecules in algal biomass and the polymerization of smaller molecules into the largercompounds that produce biooil. In pyrolysis process, char, biooil and pyrogas are produced. Biooiland pyrogas are used as a fuel. In pyrogas, concentration of CO is extreme high because in pyrolysisprocess, oxygen is not used. In some cases, after first pyrolysis, biooil again considered in secondpyrolysis process, known as secondary pyrolysis. Furthermore, non-condensable gas can be usedfor secondary pyrolysis. In Figure 6, production of biooil from algae by pyrolysis anddecarboxylation of algal lipid are represented.

Algal growth depends on several

factors, such as (a) strain, (b) supply of

inorganic nutrient (nitrogen,

phosphorous) and CO2, (c) intensity of

light and (d) type of bioreactor (open

system and closed system).

Algal growth is classified as

(a) Photoautotrophy

(b) Heterotrophy

(c) Photoheterotrophy

(d) Mixotrophy

BiohydrogenBoth green algae and cyanobacteria are capable to produce biohydrogen through diversemetabolic pathways. Green algae produces biohydrogen through direct photolysis andphotofermentation. Direct photolysis is a light-dependent process where, water splitting is

References

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It is produced through composting with cellulose degrading bacteria, earth warm and mineral solubilizing bacteria

Figure 1. Concept of biorefinery through algal biotechnology.

Figure 2. (A) Microalgae and (B) Macroalgae.

(A) (B)

Algal growth

Biofuels from algae

Figure 3. Development of biodiesel (pool of ester) from algal lipid via base-catalyzed transesterification reaction.

(B) Closed cultivation system(a) Tubular photobioreactor(b) Plate-type photobioreactor(c) Plastic bag photobioreactor

AimsDevelopment of biorefinery through algal biotechnology offers triplicate advantages in

global development agenda:

(a) Generation of renewable energy by utilization of atmospheric carbon-di oxide.

(b) Reduction of the emission of green house gasses and control the air pollution, offered

by petroleum-based fuels.

(c) Mitigate the utilization and scarcity of fossil fuel.

Beneficial aspects of algal cultivation process and its wide spectrum advantages, there is

a unique footstep has been placed to establish algal-based biorefinery. In this

presentation, concept of biorefinery through algal biotechnology is represented.

(c) Several cyanobacteria are capable to produce ethanol by photofermentation. The metabolic pathway of ethanol formation is briefly summarized herein: (a) conversion of inorganic carbon (produced from fixation of CO2) to phosphoglycerate by the Calvin cycle, and (b) its conversion to ethanol by pyruvate decarboxylase and alcohol dehydrogenase, in a sequential way. In Figure 4, production of bioethanol from algae is represented.

Figure 4. Production of ethanol from algalbiomass.

(a) conversion of complex compounds(polysaccharide and proteins) tosimple molecules by hydrolyticmicroorganisms, (b) conversion ofsimple molecules to volatile fattyacids, known as acidogenesis, (c)conversion of volatile fatty acids toacetic acid, known as acetogenesisand (d) conversion of acetic acid tomethane and CO2. In Figure 5,production of biomethane from algaeis represented.

Figure 5. Production of biomethanefrom algal biomass.

Figure 6. Production of

biooil from algae by (A)

pyrolysis and (B)

decarboxylation of algal

lipid.

(A)

(B)

performed at PSII and hydrogen isproduced during conversion of ADP to ATPin presence of ATP synthase. In thisprocess, NADPH is also produced, whichpromotes photosynthesis and formation ofstarch. Subsequently, starch is converted toAcetyl-CoA. Produced NADPH2 is convertedto NADP and H2 in presence ofhydrogenase (Figure 7A). In cyanobacteria,biohydrogen is produced by both directand indirect photolysis. Direct photolysis isperformed at vegetative cell and indirectphotolysis is performed in heterocyst cell.There, NADPH2 is produced in conversion ofstarch through oxidative pentosephosphate pathway. Subsequently, inpresence of Hox-hydrogenase, biohydrogenis produced from NADPH2 (Figure 7B).

Figure 7. Production of

biohydrogen from algae

and cyanobacteria. (A)

direct photolysis and

photofermentation are

performed in algae, (B)

direct and indirect

photolysis are performed at

cyanobacteria.