Environmental Engineering Reference
In-Depth Information
nitrogen and simultaneously to generate H
2
[12]; this process is known as indirect photolysis
(Figure 18).
Figure 18. Nitrogenase reaction equation, showing electron donors, the role of ATP, and H2 production
from reduced protons [16].
The cyanobacterium Synechocystis sp. strain PCC 6803 M55, lacking genetic evidence
of both nitrogenase and uptake hydrogenases, has been found recently to exhibit direct
photolysis [13], opening the possibility that direct photolysis may be found in other
cyanobacteria as well. Cyanobacteria can also metabolize endogenous substrate anaerobically
through a number of fermentation pathways (e.g., Figure 17), several of which produce H2 by
means of bidirectional hydrogenases [14, 15].
2.1.2. Anoxygenic phototrophs
. Anoxygenic phototrophs, in contrast to oxygenic
phototrophs, are exclusively prokaryotic and often obligately anaerobic. These microbes each
possess only one photosystem: some bacterial reaction centers are analogous to PS I, and
some are analogous to PS II, but none extract electrons from water, and thus O
2
is not
produced (Figure 19). Instead of water, the reaction centers must energize electrons from
organic or inorganic substrates found in their environments. Although many photosynthetic
bacteria depend on Rubisco and the Calvin cycle for the reduction of CO
2
, some are able to
fix atmospheric CO
2
by other biochemical pathways. Despite these differences, energy
transduction is carried out by mechanisms quite similar to those found in oxygenic
phototrophs: the light- harvesting centers contain bacteriochlorophylls, analogous to the
chlorophylls but with strongest absorption in the infrared (700-1000 nm), as well as
carotenoids, and electron transport proteins are also analogous. As in oxygenic
photosynthesis, electron transfer is coupled to the generation of an electrochemical potential
that drives phosphorylation by ATP synthase, and the energy required for the reduction of
CO2 is provided by ATP and NADH, a molecule similar to NADPH [17].
Anoxygenic phototrophs fall into several categories, based on pigmentation and electron
donor. Of greatest interest to biohydrogen production are the purple non-sulfur bacteria, e.g.,
members of the genera Rhodobacter and Rhodopseudomonas, which use organic compounds
or H
2
as electron donors [17]. These microbes prefer to grow photoheterotrophically, using
simple fatty acids such as succinate and malate to supply electrons for photosynthesis as well
as carbon for biosynthesis. In these organisms, most of which are able to fix nitrogen [18], H2
production is associated primarily with nitrogenase activity [19].
2.1.3. Fermenters
. Fermentations are energy-yielding metabolic processes in which an
organic substrate is decomposed into smaller molecules, some of which are oxidized and
some of which are reduced relative to the original substrate.
