Environmental Engineering Reference
In-Depth Information
conditions, which form the next priority. Genetic modifications of antennae to limit self-
shading and to allow utilization of a greater portion of the solar spectrum remain a high
priority for all photobiological H
2
-production techniques.
2.6. Water-gas Shift Mediated H2 Production
The water-gas shift reaction describes the oxidation of CO to CO
2
with the release of H
2
:
CO(g) + H
2
O(l)
CO
2
(g) + H2(g)
This process is a light-independent reaction carried out by certain photoheterotrophic
bacteria within the Rhodospirillaceae such as Rhodospirillum rubrum and Rubrivivax
gelatinosus CBS, using an enzyme known as carbon monoxide dehydrogenase (CODH) in
combination with a hydrogenase [89, 90]. Although light does not affect the CO oxidation
rate, in light an uptake hydrogenase is able to oxidize the H2 to support light-dependent CO2
fixation, whereas H2 accumulates during incubation in darkness [90].
During this metabolism, the bacteria are able to use CO as the sole source of carbon and
energy for ATP production. This has two important implications. First, it enables the
microbes to produce additional H
2
from synthesis gas, which is a mixture composed primarily
of CO and H
2
that results from the gasification of biomass. As a result, H2 production from
biomass that is not readily fermented, such as lignin, becomes possible. Second, the water-gas
shift process is highly thermodynamically favorable, with the result that enzymatic catalysis
allows CO partial pressures to diminish rapidly and to very low levels (approaching 100
percent removal from synthesis gas). This process thus offers the possibility of a low-cost
purification step for H2 before use in fuel cells, in which catalysts are rapidly poisoned by CO
[5, 90].
2.6.1. Genetics
. The regulation and structure of the CO dehydrogenase system of
Rhodospirillum rubrum is well-characterized and is presumably similar to that of Rubrivivax
gelatinosus. In R. rubrum, in the presence of CO, the CooA regulatory protein binds to the
promoters of the cooFSCTJ and cooMKLXUH operons, initiating expression of the CO
oxidation system. CODH, which oxidizes CO to CO
2
, is encoded by the cooS gene. Electrons
released by the oxidation of CO are transferred to a ferredoxin-like protein, CooF, and then
through an undefined path to the CO-tolerant hydrogenase, CooH, which uses them to reduce
H+ to yield H2. Functions of the other genes in the two operons have been studied as well
[91, 92]. While the CODH-associated hydrogenase of R. rubrum is highly O
2
-sensitive, that
of R. gelatinosus is surprisingly O
2
-tolerant, exhibiting a half-life near 20 hours when whole
cells were stirred in air [90].
2.6.2. Rate and efficiency
. H2 production by the water-gas shift reaction is one of the
most rapid processes known, achieving 96.0 millimoles H2 per liter culture per hour under
laboratory conditions (Table 5); estimated substrate conversion efficiencies have not yet been
reported [5].
2.6.3. Research priorities
. A major challenge for the water-gas shift reaction is the
achievement of efficient transfer of synthesis gas into aqueous solution, as the CO must be
available to the bacteria at sufficient concentrations to allow efficient metabolism [5].
Genetically, in addition, greater understanding of the interaction between CODH and the
hydrogenase might indicate features amenable to optimization, and investigation of the O2-
