Photobiohydrogen production and high-performance photobioreactor - Solar Energy Sciences and Engineering Applications - page 350

photosynthetic bacteria, a stable mutant of the photosynthetic bacterium Rhodobacter
sphaeroides with an altered light-harvesting system by UV irradiation was found to be
able to decrease the core antennal content by 2.7-fold and increase peripheral antennal
content by 1.6-fold as compared to the wild-type strain. The resulting photobiohydro-
gen production rate in this mutant under the light of 800- and 850-nm corresponding
to the absorption maxima of peripheral antennal was 1.5 times higher than the wild-
type strain (Vasilyeva et al., 1999). In another novel mutant of MTP4 created from
the wild-type strain Rhodobacter sphaeroides RV by UV irradiation, the contents of
chlorophylls and carotenoids of the chromatophores were reduced to 41 and 49%,
respectively. With this mutant, the photobiohydrogen production rate was increased
50% higher than the wild-type strain RV (Kondo et al., 2002).
According to the discussion in preceding section, it can be understood that the
insufficient supply of electrons and protons can also seriously affect the performance
of microorganisms for photobiohydrogen production. Lee and Greenbaum (2003)
proposed a genetic insertion of a polypeptide protein channel with a hydrogenase
promoter to reduce the proton gradient across the thylakoid membrane by avoiding
the ATP-synthase channel to produce ATP. Kruse et al. (2005) isolated a strain Stm6
with modified respiratory metabolism in C. reinhardtii to increase the availability of
electrons and protons for hydrogen evolution under anaerobic conditions. The results
showed that the strain could accumulate a large amount of starch in the cells and pro-
vide low dissolved oxygen concentration, both of which contributed to the 5-13 times
increase of the photobiohydrogen production rate (Kruse et al., 2005). Since excess
reducing equivalents generated by organic acid oxidation in Rhodobacter capsulatus
can be consumed to reduce protons into hydrogen by nitrogenase, the elimination of the
cyt cbb 3 oxidase that serves as a redox signal to the RegB/RegA regulatory system can
also enhance nitrogenase expression, significantly increasing the photobiohydrogen
production rate (Ozturk et al., 2006).
Besides, produced hydrogen can be consumed by the uptake hydrogenase
(NiFe-hydrogenase) in the process of the photobiohydrogen production. Therefore,
the hydrogen consumption needs to be prevented by inactivating uptake [NiFe]-
hydrogenase. Masukawa et al. (2002) constructed a hydrogenase mutants, hupL(-)
mutant (deficient in the uptake hydrogenase) from Anabaena sp . PCC 7120, by which
4-7 times higher photobiohydrogen production rate than that of the wild strain under
optimal conditions was achieved, indicating that the removal of the hupL gene is an
effective way for the improvement of the photobiohydrogen production performance
in a nitrogenase-based system. Other researchers have also obtained a hypF-deficient
mutant to dramatically increase the hydrogen evolution capacity of Thiocapsa roseop-
ersicina BBS under nitrogen-fixing conditions because of the inactivation of hydrogen
uptake activity (Fodor et al. 2001). A suicide vector containing a gentamicin cassette in
the hupSL genes into Rhodobacter sphaeroides O.U.001 has been introduced and the
results showed that the uptake hydrogenase genes were destroyed by site directed muta-
genesis. The wild type and the mutant cells showed similar growth patterns but the total
volume of hydrogen gas evolved by the mutant was 20% higher than that of the wild
type strain (Kars et al., 2008). In addition, ammonium ion can inhibit the synthesis of
nitrogenase and reduce the energy level for photosynthetic bacteria, thereby affecting
the photobiohydrogen production performance. In this case, the glnB and glnK play
key roles in repressing the nitrogenase expression in the presence of ammonium ion.