18.2 Routes of Production

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18.2

Routes of Production

18.2.1

Biophotolysis

This route of biological hydrogen production is predominant in photosynthetic

organisms such as green algae and cyanobacteria, which utilizes enzymes as cata-

lysts to split water in the presence of sunlight. Two specific hydrogenases catalyze

these reactions, viz. Fe–Fe hydrogenase in green algae and nitrogenase in cyanobac-

teria. Based on the mechanism of H2 evolution, biophotolysis is further categorized

as direct and indirect biophotolysis. During direct photolysis, the electrons gener-

ated by water splitting in the presence of light are transferred to photo-system II

(PS II), PS I, and finally to the ferredoxin (Fd) as shown in Figure 18.1a. Reduced

Fd acts as an electron carrier and reduces hydrogenase to produce molecular H2

[4]. Indirect photolysis is characterized by the conversion of light to biochemical

energy, which is stored in cells in the form of carbohydrates, which is later used for

H2 production. Nitrogenase enzyme present in the cyanobacteria catalyzes the H2

production reaction simultaneously with the reduction of N2 to ammonia.

H2O

PS II

PS I

ATPase

ATPase

H⁺

H⁺

Reducing

sugars

Pyruvate

Formate

FD(ox) FD(red)

2H⁺

Acetyl CoA

Organic acids

H⁺

H2

H2

H2

H2

e^–

e^–

e^–

e^–

2H⁺

e^–

O2 + H⁺

H2ase

H2ase

fhl

N2ase

Fd

(a) Biophotolysis

(b) Dark fermentation

(c) Photo fermentation

ATP

ATP

Reverse e^–

transport

NAD⁺ + H⁺

NADH

ADP

ADP

Bacterial

photosystem

Integrated

process

Dark

fermentation

effluent

PFOR

CO2 +

Figure 18.1

Mechanism of biological routes of hydrogen production. (a) Biophotolysis;

(b) Dark fermentation; (c) Photo-fermentation. Fd: Ferredoxin; H2ase: hydrogenase; PS:

photosystem; PFOR: pyruvate ferredoxinoxidoreductase; N2ase: nitrogenase.