Environmental Engineering Reference
In-Depth Information
commercial water-gas-shift process, and by 'wiring' the two redox enzymes together the
bacteria harnesses the small exothermic energy output (
28 kJ (mol CO)
1
)
ʔ
G
298
¼
to adenosine triphosphate (ATP) synthesis [
21
].
2H
þ
þ
2e
CO
þ
H
2
O
!
CO
2
þ
ð
4
Þ
2H
þ
þ
2e
!
H
2
ð
5
Þ
CO
þ
H
2
O
!
CO
2
þ
H
2
ð
6
Þ
Alternatively, the acetogenic bacterium
Acetobacterium woodii
can grow using
aH
2
and CO
2
mixture as the sole growth substrate because of the action of a
bifunctional enzyme that contains both a hydrogenase and a formate dehydrogenase
active site. Overall this H
2
-dependent CO
2
reductase complex catalyzes the con-
version of H
2
and CO
2
to formic acid, reaction (
7
). From an energy technology
viewpoint this can be seen as a microbial method for converting H
2
into an easily
storable liquid fuel via CO
2
sequestration [
22
].
H
2
þ
CO
2
!
HCOOH
ð
7
Þ
Although most hydrogenases are not expressed or functional in oxygenic con-
ditions, it is not the case that oxygenic hydrogenotrophic (H
2
-converting) growth is
impossible. For example,
Ralstonia eutropha
(
R. eutropha
) is a well-studied
'Knallgas' bacterium which relies on H
2
uptake (equation
8
) in air when growing
under autotrophic conditions, whereby H
2
is used as a source of energy via the
activity of a membrane-bound [NiFe] hydrogenase (Figure
1
)[
23
].
In terms of respiratory energy output hydrogenase O
2
functionality is very
beneficial; H
2
is one of the strongest reducing agents (equation
5
) with a standard
reduction potential (
E
298
(H
+
/H
2
)) of 0 V and O
2
is one of the strongest oxidizing
agents (equation
9
)(
E
298
(O
2
/H
2
O)
+ 1.23 V). Coupling these processes across the
cytoplasmic membrane therefore gives the cell access to a high membrane-potential
and 'wiring' H
2
oxidation to O
2
reduction in such a way means that the bacterial
respiration is analogous to a H
2
/O
2
fuel cell.
¼
2e
þ
2H
þ
H
2
!
ð
8
Þ
2H
þ
þ
2e
!
=
2
O
2
þ
H
2
O
ð
9
Þ
1
H
2
þ
=
2
O
2
!
H
2
O
ð
10
Þ
1
Dihydrogen is often such a valuable microbial resource that in mixed populations
like the gut microbiota H
2
-producing and H
2
-uptake organisms will symbiotically
complement one another, with the gaseous, fast-diffusing nature of H
2
promoting
such
intra
cellular exchanges. The balance between the uptake and production parts of
the gut H
2
cycle is important because fermentative bacteria generate H
2
as a method
for disposing of reducing equivalents. Any H
2
buildup forces these microbes to
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