Photobiohydrogen production and high-performance photobioreactor - Solar Energy Sciences and Engineering Applications - page 347

series of electron transfer via PSI and Cyt b 6 f in the thylakoid membrane and finally
reduce NADP + to NADPH. The NADPH generated by the catabolism of endogenous
glycogen is oxidized to yield the electrons required for photobiohydrogen production
using hydrogenase as the biocatalyst (Allakhverdiev et al., 2010; Appel and Schulz,
1998). Regarding the nitrogen fixing cyanobacteria, the photobiohydrogen production
enzyme is nitrogenase instead of hydrogenase, which can fix nitrogen to ammonia
accompanied by obligatory reduction of protons to hydrogen (Asada and Miyake,
1999; Benemann, 1997). Eventually, generated hydrogen is transported out of the
photobioreactor and collected.
11.2.2 Photoheterotrophic hydrogen production
Photosynthetic bacteria are prokaryotes having only one photosynthesis system, which
can utilize simple organic compounds as electron donor to produce hydrogen under
anaerobic circumstances. Gest and Kamen (1949) found the hydrogen evolution and
photosynthetic nitrogen fixation by illuminating Rhodospirilum rubrum . At present,
the mainly used photosynthetic bacteria for the photobiohydrogen production by the
photo-fermentation are: Rhodospirillum rubrum (Najafpour et al., 2004), Rhodobac-
ter sphaeroides (Koku et al., 2002), and Rhodopseudomonas palustris (Chen et al.,
2007; Suwansaard et al., 2010). As summarized in Table 11.2.1, hydrogen produc-
tion by photosynthetic bacteria offers many unique features. Firstly, photosynthetic
bacteria can use a wide range of the solar spectrum, making it more practical for
photobiohydrogen production. Secondly, photobiohydrogen production by photosyn-
thetic bacteria does not generate oxygen, avoiding the oxygen inhibition to enzymes.
Thirdly, they are able to consume many organic substrates derived from wastes, such as
acetic acid, lactic acid, butyric acid, glucose and so on. Hence, photosynthetic bacteria
have been regarded as the most promising microorganisms for both the photobiohy-
drogen production and wastewater treatment (Basak and Das, 2007; Chen et al., 2011;
Shi and Yu, 2005).
In the process of photobiohydrogen production by photosynthetic bacteria
through photo-fermentation, substrates from the bulk solution as the electron donor
and light as energy source are simultaneously transported into bacteria. Electrons are
then liberated from the electron donor and transported through the photosynthetic
apparatus driven by captured light energy. Protons are transferred through the mem-
brane and a proton gradient developed is used by the ATP-synthase enzyme to generate
ATP and reverse electron flow to produce high energy electrons. As shown in Fig-
ure 11.2.3, the bacteriochlorophyll excitation is stimulated by a photon in the reaction
centre. The generated energy leads to the release of an electron that reduces the mem-
brane quinone (Q) pool. The Q makes protons to be released to the periplasmic space
and then to reduce the cytochrome bc 1 complex (Cyt bc 1 ) that enables the reduction
of cytochrome c 2 (C 2 ). In turn, C 2 reduces the oxidized primary electron donors in the
reaction centre, forming and closing a cycle. The protons accumulated in the periplasm
form an electrochemical gradient which is used not only by the ATP-synthase to gener-
ate ATP but also to transport the electrons further to the electron acceptor Fd. When
nitrogen is present, it can be reduced by nitrogenase to ammonium using the electrons
derived from the Fd. However, under nitrogen-free circumstances, these electrons can
be used to reduce protons into hydrogen with the help of nitrogenase (Akkerman et al.,