11.4 Algae as a Source of Bioenergy

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11.4.1

Biodiesel from Microalgae

Microalgae are potential candidate for the production of biodiesel due to their high

lipid content. The total lipid content of microalgae is an important criterion for

choosing algae for biodiesel production. It is a common attribute that microalgae

having high lipid content usually have slow growth resulting in low productivities.

Growth study has shown that most species of microalgae with high lipid productiv-

ity are not adapted to grow in wastewater. To solve this limitation microalgae were

isolated from different wastewater and optimized to grow with different organic car-

bon substrates and resulted in increased biomass production and lipid productivity

as they were already acclimatized to wastewater environment [35].

11.4.2

Bioethanol from Microalgae

Bioethanol are biofuels obtained by fermentation of sugars from various feed-

stocks. Microalgae have high amount of carbohydrates which can be fermented to

bioethanol [35]. Large amount of CO2 produced by ethanol fermentation can be

coupled with the cultivation of carbohydrate-rich microalgae which can be used as

feedstock for bioethanol production, mitigating greenhouse gases, and reutilization

[32].

11.4.3

Biomethane from Microalgae

Microalgae are used for biomethane production due to its high carbohydrate content

[33]. Anaerobic digestion is the process by which organic matter from microalgal

cells is converted to biogas through reactions catalyzed by microorganisms. The fac-

tors that determine the methane yield in anaerobic digestion of microalgae are the

composition of cell wall and its weight contributing to biomass. It has been reported

that biomethane production from C. vulgaris and Scenedesmus requires less energy

compared to biodiesel production [37]. Photoautotrophic microalgae capture CO2

during growth and oxidize H2S from biogas, generating high-quality biomethane.

11.4.4

Hydrogen Production

Fermentative metabolism results in hydrogen production using microalgae.

Biohydrogen is regarded as a sustainable and clean energy yielding high energy

(142 M J/kg). Microalgae are suitable feedstocks for hydrogen production as they

harbor hydrogenase enzyme that helps in hydrogen production. Many strains of

microalgae fix atmospheric N2 to NH3 and use sunlight as energy source to generate

biohydrogen. Some strains produce hydrogen using sunlight as electron source

and water as energy source under specific conditions. There are four different

mechanisms of hydrogen production in microalgae- direct biophotolysis, indirect

biophotolysis, dark and photo-fermentation. Several microalgae like C. pyrenoidosa,

Chlamydomonas, and Scenedesmus sp. have the capacity for producing molecular

hydrogen under anaerobic condition [38].