N 2 þ 8e
À
þ 8H
þ
! 2NH 3 þ H 2 þ 16ADP þ 16Pi
ð3Þ
In the absence of N 2 , nitrogenase acts as an ATP-powered hydrogenase and
produces H 2 exclusively without any feedback inhibition by the following reaction
(Eq. 4) (McKinlay and Harwood 2010):
8e
À
þ 8H
þ
! þ 16ATP ! 2H 2 þ 16ADP þ 16Pi
ð4Þ
The process of heterocyst-based H 2 production by cyanobacteria suffers the
problem of low photon conversion efficiency (Benemann 2000). This is due to the
high metabolic energy requirement of the nitrogenase catalysis (2 ATP per e
−
transfer). Moreover, half of the energy metabolism of the cyanobacteria also
accounts for the biosynthesis and maintenance of heterocysts (Benemann 2000).
This energy requirement is met by the solar energy via PSI-mediated cyclic phosphorylation occurring inside the thylakoid membrane of the heterocysts. During the
N 2 fixation, nitrogenase is usually accompanied by uptake hydrogenase to reutilize
and retrieve the H 2 /e
− for minimizing the loss of energy (Tamagnini et al. 2002).
Disruption of this uptake hydrogenase activity in cyanobacteria helps in H 2 accumulation (Masukawa et al. 2002). Non-nitrogen-fixing cyanobacteria such as
Gloeobacter, Synechococcus and Synechocystis can also generate H 2 via indirect
biophotolysis. These organisms possess bidirectional [NiFe] hydrogenase which has
the capability of catalysing both the synthesis and oxidation of H 2 (Tamagnini et al.
2002). The physiological function of this enzyme is still unclear but the suggested
role includes: removal of excess reducing power during anaerobic fermentation and
allocation of electrons to the respiratory chain by H 2 oxidation (Baebprasert et al.
2010). H 2 production by this enzyme is energetically efficient as it does not require
ATP like nitrogenases. However, the rate of H 2 production by non-nitrogen-fixing
cyanobacteria is comparatively lower (0.02–0.40 µmol H 2 /mg chl a/h) than heterocystous cyanobacteria (0.17–0.42 µmol H 2 /mg chl a/h) (Levin et al. 2004).
Biohydrogen production from microalgae via photolysis is summarized in Table 1.
2.1.3 Enzymes Involved in PhotoBiological Hydrogen Production
from Microalgae
The key enzymes that can catalyse the reaction of hydrogen production in
microalgae are hydrogenase and nitrogenase.
Hydrogenases
Hydrogenases are the metalloenzymes and on the basis of the metal composition
at their catalytic sites, they can be classified as: [FeFe]-hydrogenase, [NiFe]hydrogenase and [Fe]-hydrogenase (Kim and Kim 2011). Among these, [Fe]hydrogenase is found in archaea so this will not be discussed here.
10 Biofuels from Microalgae: Biohydrogen
207
À
þ 8H
þ
! 2NH 3 þ H 2 þ 16ADP þ 16Pi
ð3Þ
In the absence of N 2 , nitrogenase acts as an ATP-powered hydrogenase and
produces H 2 exclusively without any feedback inhibition by the following reaction
(Eq. 4) (McKinlay and Harwood 2010):
8e
À
þ 8H
þ
! þ 16ATP ! 2H 2 þ 16ADP þ 16Pi
ð4Þ
The process of heterocyst-based H 2 production by cyanobacteria suffers the
problem of low photon conversion efficiency (Benemann 2000). This is due to the
high metabolic energy requirement of the nitrogenase catalysis (2 ATP per e
−
transfer). Moreover, half of the energy metabolism of the cyanobacteria also
accounts for the biosynthesis and maintenance of heterocysts (Benemann 2000).
This energy requirement is met by the solar energy via PSI-mediated cyclic phosphorylation occurring inside the thylakoid membrane of the heterocysts. During the
N 2 fixation, nitrogenase is usually accompanied by uptake hydrogenase to reutilize
and retrieve the H 2 /e
− for minimizing the loss of energy (Tamagnini et al. 2002).
Disruption of this uptake hydrogenase activity in cyanobacteria helps in H 2 accumulation (Masukawa et al. 2002). Non-nitrogen-fixing cyanobacteria such as
Gloeobacter, Synechococcus and Synechocystis can also generate H 2 via indirect
biophotolysis. These organisms possess bidirectional [NiFe] hydrogenase which has
the capability of catalysing both the synthesis and oxidation of H 2 (Tamagnini et al.
2002). The physiological function of this enzyme is still unclear but the suggested
role includes: removal of excess reducing power during anaerobic fermentation and
allocation of electrons to the respiratory chain by H 2 oxidation (Baebprasert et al.
2010). H 2 production by this enzyme is energetically efficient as it does not require
ATP like nitrogenases. However, the rate of H 2 production by non-nitrogen-fixing
cyanobacteria is comparatively lower (0.02–0.40 µmol H 2 /mg chl a/h) than heterocystous cyanobacteria (0.17–0.42 µmol H 2 /mg chl a/h) (Levin et al. 2004).
Biohydrogen production from microalgae via photolysis is summarized in Table 1.
2.1.3 Enzymes Involved in PhotoBiological Hydrogen Production
from Microalgae
The key enzymes that can catalyse the reaction of hydrogen production in
microalgae are hydrogenase and nitrogenase.
Hydrogenases
Hydrogenases are the metalloenzymes and on the basis of the metal composition
at their catalytic sites, they can be classified as: [FeFe]-hydrogenase, [NiFe]hydrogenase and [Fe]-hydrogenase (Kim and Kim 2011). Among these, [Fe]hydrogenase is found in archaea so this will not be discussed here.
10 Biofuels from Microalgae: Biohydrogen
207