This efficiency can be improved by modifying the antenna complex through genetic
engineering. Reduction in the antenna size can minimize the energy wastage and
improve the penetration of light inside the reactor. Polle et al. (2002) developed
C. reinhardtii strain having truncated Chl antenna size of PSII, which showed better
H 2 production and cellular productivity than wild strain. In other study, a truncated
antenna mutant of C. reinhardtii under high light conditions showed 8.5-fold higher
solar-to-hydrogen conversion efficiency than parent strain (Kosourov et al. 2011).
To reduce the energy losses, recently simultaneous down regulation of entire LHC
gene family in C. reinhardtii Stm3LR3 was carried out by applying RNAi technology. The mutant exhibited high photosynthetic efficiency under elevated light
intensity (Mussgnug et al. 2007). Cyanobacteria (Synechocystis PCC 6803) lacking
phycocyanin or whole phycobilisome expected to produce H 2 efficiently under
photoautotrophic condition (Bernát et al. 2009).
2.5 Elimination of Uptake Hydrogenases
Another main concern to obtain adequate amount of hydrogen is the elimination of
uptake hydrogenase present in the heterocyst of the nitrogen-fixing cyanobacteria.
These hydrogenases catalyse the oxidation of H 2 to recover the energy lost during
nitrogen fixation. In several studies, mutants developed by knockout of the uptake
hydrogenase genes (hupL or hupS) resulted in higher H 2 yield. Significantly, higher
amount of H 2 was obtained by the mutants of Anabaena variabilis developed by
disruption of hup genes (Mikheeva et al. 1995; Happe et al. 2000). Khetkorn et al.
(2012) demonstrated fourfold increase in hydrogen production of A. siamensis
TISTR 8012 by the disruption of hupS gene. Although deletion of uptake hydrogenase helps in improving the H 2 production, hox-encoded [NiFe]-bidirectional
hydrogenases may still reabsorb the H 2 produced by the nitrogenase due to its small
Km value for H 2 . In this regard, Masukawa et al. (2002) studied the effect of hupL,
hoxH and hupL/hoxH deletions on photobiological H 2 production by Anabaena
sp. PCC 7120. Compared to wild strain, the hupL
− mutant produced H 2 at
4–7 times high rate. However, the hoxH
− mutant did not show any improvement in
H 2 production.
2.6 Substrate Utilization
In indirect biophotolysis, for H 2 production the e
− is supplied via external substrate.
Strategies for improving the utilization of different substrate (sugars) by microalgae
might be helpful in enhancing the biomass and biohydrogen production. In this
view, modification in the transporter protein may assist the efficient transfer of
external substrate inside the cell. Recently, hexose symporter (HUP1) gene from
C. kessleri was heterologously expressed in C. reinhardtii stm6 cells, lacking the
220
H. Singh and D. Das
engineering. Reduction in the antenna size can minimize the energy wastage and
improve the penetration of light inside the reactor. Polle et al. (2002) developed
C. reinhardtii strain having truncated Chl antenna size of PSII, which showed better
H 2 production and cellular productivity than wild strain. In other study, a truncated
antenna mutant of C. reinhardtii under high light conditions showed 8.5-fold higher
solar-to-hydrogen conversion efficiency than parent strain (Kosourov et al. 2011).
To reduce the energy losses, recently simultaneous down regulation of entire LHC
gene family in C. reinhardtii Stm3LR3 was carried out by applying RNAi technology. The mutant exhibited high photosynthetic efficiency under elevated light
intensity (Mussgnug et al. 2007). Cyanobacteria (Synechocystis PCC 6803) lacking
phycocyanin or whole phycobilisome expected to produce H 2 efficiently under
photoautotrophic condition (Bernát et al. 2009).
2.5 Elimination of Uptake Hydrogenases
Another main concern to obtain adequate amount of hydrogen is the elimination of
uptake hydrogenase present in the heterocyst of the nitrogen-fixing cyanobacteria.
These hydrogenases catalyse the oxidation of H 2 to recover the energy lost during
nitrogen fixation. In several studies, mutants developed by knockout of the uptake
hydrogenase genes (hupL or hupS) resulted in higher H 2 yield. Significantly, higher
amount of H 2 was obtained by the mutants of Anabaena variabilis developed by
disruption of hup genes (Mikheeva et al. 1995; Happe et al. 2000). Khetkorn et al.
(2012) demonstrated fourfold increase in hydrogen production of A. siamensis
TISTR 8012 by the disruption of hupS gene. Although deletion of uptake hydrogenase helps in improving the H 2 production, hox-encoded [NiFe]-bidirectional
hydrogenases may still reabsorb the H 2 produced by the nitrogenase due to its small
Km value for H 2 . In this regard, Masukawa et al. (2002) studied the effect of hupL,
hoxH and hupL/hoxH deletions on photobiological H 2 production by Anabaena
sp. PCC 7120. Compared to wild strain, the hupL
− mutant produced H 2 at
4–7 times high rate. However, the hoxH
− mutant did not show any improvement in
H 2 production.
2.6 Substrate Utilization
In indirect biophotolysis, for H 2 production the e
− is supplied via external substrate.
Strategies for improving the utilization of different substrate (sugars) by microalgae
might be helpful in enhancing the biomass and biohydrogen production. In this
view, modification in the transporter protein may assist the efficient transfer of
external substrate inside the cell. Recently, hexose symporter (HUP1) gene from
C. kessleri was heterologously expressed in C. reinhardtii stm6 cells, lacking the
220
H. Singh and D. Das