Photobiohydrogen production and high-performance photobioreactor - Solar Energy Sciences and Engineering Applications - page 349

optimization of operating conditions is another critical issue in photobiohydrogen
production. To improve photobiohydrogen production performance, genetic modifi-
cation and metabolic engineering as well as the photobioreactor design and operating
condition optimization have become the focus in existing researches.
11.3 GENETIC AND METABOLIC ENGINEERING
The currently-employed microorganisms have many problems, such as oxygen toxicity
to enzymes, lower photosynthetic efficiency, insufficient supply of electrons and pro-
tons and presence of uptake hydrogenase, etc. Therefore various strategies, including
genetic modification of microorganism and metabolic engineering, have been adopted
for achieving a high photobiohydrogen production performance (Das et al., 2008;
Mathews and Wang, 2009).
As for green algae and cyanobacteria, the photosynthetic rate is several times
higher than the respiration rate under normal conditions (Melis and Melnicki, 2006).
Excessive oxygen produced by the photosynthetic process lowers the photobiohydro-
gen production performance due to the extreme sensitivity of enzymes to oxygen
(Manish and Banerjee, 2008; Melis, 2002). Melis (2007) attenuated the photosyn-
thesis/respiration (P/R) capacity ratio in green algae by DNA insertional mutagenesis
for the isolation and characterization of P/R aberrant mutants to reduce the oxygen
generation rate, thereby stabilizing its metabolism and hydrogen-evolution. By using
an independent approach that can lower photosynthesis and/or enhance cellular respi-
ration, anaerobic conditions can thus be established when the P/R ratio drops below
one, which increased the photobiohydrogen production performance. In addition, it
has also been shown that engineering oxygen-tolerant hydrogenase genes of, hydS
and hydL from Thiocapsa roseopersicina into sensitive organisms also helps to reduce
the oxygen sensitivity. An expression vector pEX-Tran used for Synechococcus sp.
PCC7942 transformation is readily available and suitable for other cyanobacterial
systems as well (Xu et al., 2005). Chlamydomonas reinhardtii mutants obtained by
random and oriented mutagenesis have also succeeded in alleviating the extreme oxy-
gen sensitivity of the green algal reversible hydrogenase and the oxygen tolerance was
enhanced by approximately 10-fold (Ghirardi et al., 2000).
The rate of electron transfer from PSII to PSI is about 10 times lower than the
capture rate of photon by the antenna pigments. Photons captured by the antenna
systems are not being fully used and accumulated, causing a low photosynthetic effi-
ciency. Hence, the improvement in the photosynthetic efficiency by the enhancement of
electron transfer has been studied by bioengineers (Hallenbeck and Benemann, 2002).
Truncating the chlorophyll antenna size of PSII is an effective method to boost the
photosynthetic efficiency of green algae because large antenna complexes can dissipate
excessive photons as fluorescence or heat at saturating light conditions (Polle et al.,
2003). Several genes have also been found to be able to confer a truncated antenna size
in green algae by random mutagenesis (Polle et al., 2002). The tla1 strain of chloro-
phyll deficient with a functional chlorophyll antenna size of PSI and PSII being about
50% and 65% required a higher light intensity for the saturation of photosynthesis
and thus showed larger solar conversion efficiencies and a higher photosynthetic pro-
ductivity than the wild type under mass culture conditions (Polle et al., 2003). For the