The hydrogen-producing enzymes fundamentally support
hydrogen production using the biological approach. In a
typical reaction, the enzyme catalyzes the chemical reaction
(Eq. 1). Research evidently supported that all these
hydrogen-producing
enzymes
contain
complex
metallo-clusters as active sites. Among many of such
enzymes, NiFe-hydrogenase, Fe-hydrogenase, and nitrogenase have been found responsible for the bio H 2 generations
(Hallenbeck et al. 2002). Fe-hydrogenase mainly carried out
the biophotolysis, whereas nitrogenase is involved in photo
fermentation. The following section highlights these
processes.
2H
þ
þ 2e
À
$ H 2
ð1Þ
4.1 Biophotolysis
Direct Biophotolysis
In direct biophotolysis, solar energy is directly converted to
hydrogen through photosynthetic reactions (Eq. 2). The
method has gained significant attention since it processes
solar energy to convert available substrate (water) to hydrogen and oxygen. However, hydrogen production can only be
possible under specific conditions since Fe-hydrogenase, the
enzyme used in the process is highly sensitive to oxygen.
2
2H O + ‘light energy’→
2
2
2H O
+
ð2Þ
A direct biophotolysis needs to be operated at a partial
pressure of near 1 atmosphere of O 2 . The required pressure
is almost a thousand times greater than the maximum pressure likely to be tolerated making the reaction challenging to
carry out (Hallenbeck et al. 2002). Nevertheless, researchers
have reported hydrogen production at the rate of
0.07 mmol/h/L (Kosourov et al. 2002; Melis et al. 2000).
Indirect Biophotolysis
Indirect biophotolysis involves parting of the O 2 and H 2
evolution reactions into two distinct stages, coupled via CO 2
evolution/fixation. Cyanobacteria possess an ability to use
CO 2 as a carbon source and solar power as an energy source
(Eq. 3). First, CO 2, taken up by cells, produces cellular
substances afterward used for H 2 production (Eq. 4). The
following reactions can represent the overall mechanism of
H 2 production in cyanobacteria:
2
2
6
6
H O CO
+
+ ‘light energy’→ 6 12 6
2
6
C H O
O
+
ð3Þ
6 12 6
2
6
C H O
H O
+
+ ‘light energy’→
2
2
12
6
H
CO
+
ð4Þ
Anabaena species have been explored for their potential
to generate higher rates of H 2 production (Levin et al. 2004).
Further, a mutant strain of A. variabilis was investigated for
its ability to produce hydrogen employing indirect biophotolysis. The study unveiled a hydrogen production rate of
0.355 mmol/h/L (Sveshnikov et al. 2006).
4.2 Photo Fermentation
Photo fermentation is a process to convert homogenized
simple sugars and organic acids (malic, formic, succinic,
acetic, and other acids) into hydrogen and carbon dioxide by
employing sunlight and using photosynthetic bacteria under
anaerobic conditions (Trchounian et al. 2017). Usually,
photo heterotrophic microorganisms, such as purple
non-sulfur bacteria (PNSB) and microalgae, are widely used
in photo fermentation (Ghosh et al. 2017). Since a renewable
energy source is used in photo fermentation, polluting gases
and toxic compounds are not formed in the process.
Therefore, PNSB assisted biohydrogen production using
varied biomasses has massive potential to be instigated as a
promising green technology (Trchounian et al. 2017).
Microorganisms
In the last few decades, plenteous microbial strains have
been investigated and reported to have a high rate of
photo-induced conversion of biomass into valuable hydrogen. Novel microbial strains with enhanced cellulolytic and
hemicellulolytic ability boosted the use of these strains for
the production of hydrogen from lignocellulosic biomass.
An ideal microorganism used in photo fermentation must
have high substrate conversion efficiency while able to
perform its metabolic activities under anaerobic conditions.
Moreover, it should be able to resist the adverse effect of O 2
that significantly affects the activity of Fe–Fe hydrogenase
and nitrogenase enzymes.
Pure Culture
The major categories of photosynthetic bacteria employed
for photo fermentation are photoautotrophic bacteria, facultative anaerobes, and aerobes (Hu and Wen 2008). The
critical factor of photo fermentation is the active participation of the nitrogenase enzyme from the cell membrane of
PNS bacteria. PNS bacteria contain light-harvesting complexes, reaction centers, and numerous bacteriochlorophylls
Bioconversion of Lignocellulosic Residues into Hydrogen
67
hydrogen production using the biological approach. In a
typical reaction, the enzyme catalyzes the chemical reaction
(Eq. 1). Research evidently supported that all these
hydrogen-producing
enzymes
contain
complex
metallo-clusters as active sites. Among many of such
enzymes, NiFe-hydrogenase, Fe-hydrogenase, and nitrogenase have been found responsible for the bio H 2 generations
(Hallenbeck et al. 2002). Fe-hydrogenase mainly carried out
the biophotolysis, whereas nitrogenase is involved in photo
fermentation. The following section highlights these
processes.
2H
þ
þ 2e
À
$ H 2
ð1Þ
4.1 Biophotolysis
Direct Biophotolysis
In direct biophotolysis, solar energy is directly converted to
hydrogen through photosynthetic reactions (Eq. 2). The
method has gained significant attention since it processes
solar energy to convert available substrate (water) to hydrogen and oxygen. However, hydrogen production can only be
possible under specific conditions since Fe-hydrogenase, the
enzyme used in the process is highly sensitive to oxygen.
2
2H O + ‘light energy’→
2
2
2H O
+
ð2Þ
A direct biophotolysis needs to be operated at a partial
pressure of near 1 atmosphere of O 2 . The required pressure
is almost a thousand times greater than the maximum pressure likely to be tolerated making the reaction challenging to
carry out (Hallenbeck et al. 2002). Nevertheless, researchers
have reported hydrogen production at the rate of
0.07 mmol/h/L (Kosourov et al. 2002; Melis et al. 2000).
Indirect Biophotolysis
Indirect biophotolysis involves parting of the O 2 and H 2
evolution reactions into two distinct stages, coupled via CO 2
evolution/fixation. Cyanobacteria possess an ability to use
CO 2 as a carbon source and solar power as an energy source
(Eq. 3). First, CO 2, taken up by cells, produces cellular
substances afterward used for H 2 production (Eq. 4). The
following reactions can represent the overall mechanism of
H 2 production in cyanobacteria:
2
2
6
6
H O CO
+
+ ‘light energy’→ 6 12 6
2
6
C H O
O
+
ð3Þ
6 12 6
2
6
C H O
H O
+
+ ‘light energy’→
2
2
12
6
H
CO
+
ð4Þ
Anabaena species have been explored for their potential
to generate higher rates of H 2 production (Levin et al. 2004).
Further, a mutant strain of A. variabilis was investigated for
its ability to produce hydrogen employing indirect biophotolysis. The study unveiled a hydrogen production rate of
0.355 mmol/h/L (Sveshnikov et al. 2006).
4.2 Photo Fermentation
Photo fermentation is a process to convert homogenized
simple sugars and organic acids (malic, formic, succinic,
acetic, and other acids) into hydrogen and carbon dioxide by
employing sunlight and using photosynthetic bacteria under
anaerobic conditions (Trchounian et al. 2017). Usually,
photo heterotrophic microorganisms, such as purple
non-sulfur bacteria (PNSB) and microalgae, are widely used
in photo fermentation (Ghosh et al. 2017). Since a renewable
energy source is used in photo fermentation, polluting gases
and toxic compounds are not formed in the process.
Therefore, PNSB assisted biohydrogen production using
varied biomasses has massive potential to be instigated as a
promising green technology (Trchounian et al. 2017).
Microorganisms
In the last few decades, plenteous microbial strains have
been investigated and reported to have a high rate of
photo-induced conversion of biomass into valuable hydrogen. Novel microbial strains with enhanced cellulolytic and
hemicellulolytic ability boosted the use of these strains for
the production of hydrogen from lignocellulosic biomass.
An ideal microorganism used in photo fermentation must
have high substrate conversion efficiency while able to
perform its metabolic activities under anaerobic conditions.
Moreover, it should be able to resist the adverse effect of O 2
that significantly affects the activity of Fe–Fe hydrogenase
and nitrogenase enzymes.
Pure Culture
The major categories of photosynthetic bacteria employed
for photo fermentation are photoautotrophic bacteria, facultative anaerobes, and aerobes (Hu and Wen 2008). The
critical factor of photo fermentation is the active participation of the nitrogenase enzyme from the cell membrane of
PNS bacteria. PNS bacteria contain light-harvesting complexes, reaction centers, and numerous bacteriochlorophylls
Bioconversion of Lignocellulosic Residues into Hydrogen
67