(carbohydrate, protein and lipid) into soluble metabolite products (volatile fatty
acids and alcohols), H 2 and CO 2 by the facultative and obligate anaerobic bacteria.
The genus of bacteria typically associated with dark fermentation includes
Clostridium, Klebsiella, Enterobacter, Citrobacter, Bacillus, Lactobacillus,
Thermotoga, Anaerobiospirillum and Caldicellulosiruptor (Xia et al. 2015). The
hydrogen-producing bacteria utilizes the protons (H
+ ) as the electron acceptor and
thus disposes the excess electrons generated by the oxidation of organic substrates
in the form of H 2 (Das and Veziroglu 2001). There are two pathways for the
formation of molecular H 2 : NADH re-oxidation pathway and formate decomposition pathway. The H 2 production via NADH re-oxidation pathway is mediated by
some specific bacteria such as Clostridium sp. by the following reaction (Eq. 8)
NADH þ H
þ
! H 2 þ NAD
þ
ð8Þ
This NADH is generated due to the conversion of glucose into pyruvate during
the glycolysis pathway, which could be represented as follows (Eq. 9):
C 6 H 12 O 6 þ 2NAD
þ
! 2CH 3 COCOOH þ 2NADH þ 2H
þ
ð9Þ
Pyruvate-ferredoxin oxidoreductase catalyses the breakdown of pyruvate into
acetyl CoA which could be further metabolized either into acetate or butyrate
(Fig. 3). In both the cases, oxidation of one mole of ferredoxin by [Fe–Fe]hydrogenase yields one mole of H 2 . Maximum H 2 yield of 4 mol/mol glucose is
achieved when acetic acid is the sole metabolic end product. However, H 2
yield of only 2 mol/mol glucose is achieved when butyrate is the final product
Eqs. (10 and 11):
C 6 H 12 O 6 þ 2H 2 O ! 4H 2 þ 2CO 2 þ 2CH 3 COOH
ð10Þ
C 6 H 12 O 6 ! 2H 2 þ 2CO 2 þ CH 3 CH 2 CH 2 COOH
ð11Þ
In contrast, few facultative anaerobes such as Escherichia coli can carry out the
H 2 evolution via formate decomposition pathway. In this case, pyruvate is converted into formate and acetyl CoA by pyruvate formate lyase. Subsequently, under
the anaerobic condition the formate is cleaved into H 2 and CO 2 catalysed by
formate hydrogen lyase Eqs. (12 and 13):
CH 3 COCOOH þ HCoA ! CH 3 COCoA þ HCOO
ð12Þ
HCOOH ! H 2 þ CO 2
ð13Þ
However, when fermentation is carried out by mixed microbial consortia, glucose can undergo some other biochemical pathways which generates undesired
by-products such as lactate, propionate, succinate, 2,3 butanediol, ethanol, butanol
and isopropanol. Generation of these metabolites hampers the H 2 production and
10 Biofuels from Microalgae: Biohydrogen
211
acids and alcohols), H 2 and CO 2 by the facultative and obligate anaerobic bacteria.
The genus of bacteria typically associated with dark fermentation includes
Clostridium, Klebsiella, Enterobacter, Citrobacter, Bacillus, Lactobacillus,
Thermotoga, Anaerobiospirillum and Caldicellulosiruptor (Xia et al. 2015). The
hydrogen-producing bacteria utilizes the protons (H
+ ) as the electron acceptor and
thus disposes the excess electrons generated by the oxidation of organic substrates
in the form of H 2 (Das and Veziroglu 2001). There are two pathways for the
formation of molecular H 2 : NADH re-oxidation pathway and formate decomposition pathway. The H 2 production via NADH re-oxidation pathway is mediated by
some specific bacteria such as Clostridium sp. by the following reaction (Eq. 8)
NADH þ H
þ
! H 2 þ NAD
þ
ð8Þ
This NADH is generated due to the conversion of glucose into pyruvate during
the glycolysis pathway, which could be represented as follows (Eq. 9):
C 6 H 12 O 6 þ 2NAD
þ
! 2CH 3 COCOOH þ 2NADH þ 2H
þ
ð9Þ
Pyruvate-ferredoxin oxidoreductase catalyses the breakdown of pyruvate into
acetyl CoA which could be further metabolized either into acetate or butyrate
(Fig. 3). In both the cases, oxidation of one mole of ferredoxin by [Fe–Fe]hydrogenase yields one mole of H 2 . Maximum H 2 yield of 4 mol/mol glucose is
achieved when acetic acid is the sole metabolic end product. However, H 2
yield of only 2 mol/mol glucose is achieved when butyrate is the final product
Eqs. (10 and 11):
C 6 H 12 O 6 þ 2H 2 O ! 4H 2 þ 2CO 2 þ 2CH 3 COOH
ð10Þ
C 6 H 12 O 6 ! 2H 2 þ 2CO 2 þ CH 3 CH 2 CH 2 COOH
ð11Þ
In contrast, few facultative anaerobes such as Escherichia coli can carry out the
H 2 evolution via formate decomposition pathway. In this case, pyruvate is converted into formate and acetyl CoA by pyruvate formate lyase. Subsequently, under
the anaerobic condition the formate is cleaved into H 2 and CO 2 catalysed by
formate hydrogen lyase Eqs. (12 and 13):
CH 3 COCOOH þ HCoA ! CH 3 COCoA þ HCOO
ð12Þ
HCOOH ! H 2 þ CO 2
ð13Þ
However, when fermentation is carried out by mixed microbial consortia, glucose can undergo some other biochemical pathways which generates undesired
by-products such as lactate, propionate, succinate, 2,3 butanediol, ethanol, butanol
and isopropanol. Generation of these metabolites hampers the H 2 production and
10 Biofuels from Microalgae: Biohydrogen
211