Environmental Engineering Reference
In-Depth Information
Fe(III)-binding capability decrease (and in some cases totally inhibit) Fe(III)
reduction activity by
S. putrefaciens
[26].
Some Fe(III)-reducing bacteria generate relatively high concentrations of
soluble organic-Fe(III) in the absence of exogenous chelating compounds, an
indication that such bacteria synthesize and release organic ligands to solubilize
Fe(III) prior to reduction [67]. Soluble organic-Fe(III) is detected electrochem-
ically in
Shewanella
cultures incubated anaerobically with either ferrihydrite or
goethite [83]. Detection of soluble organic-Fe(III) prior to detection of Fe(II),
suggests that soluble organic-Fe(III) is an intermediate in the reduction of solid
Fe(III) oxides. Since lactate is the only organic ligand added to the
Shewanella
batch cultures and lactate-Fe(III) complexes do not react with Au/Hg electrodes,
electrochemical detection of soluble organic-Fe(III) suggests that
Shewanella
synthesizes and releases organic ligands to non-reductively dissolve Fe(III)
prior to reduction.
A soluble organic-Fe(III) reductase has not been definitively identified.
She-
wanella
Type II protein secretion mutants (see above) are unable to reduce
solid Fe(III) oxides, yet retain the ability to reduce all other electron acceptors,
including soluble organic-Fe(III). The inability of the Type II protein secretion
mutants to target Fe(III) reductases to the OM suggests that the Type II pro-
tein secretion mutants reduce soluble organic-Fe(III) in the periplasmic space.
When expressed in
E. coli
, the
S. oneidensis
decaheme
c
-type cytochrome MtrA
displays soluble organic-Fe(III) reductase activity [73], although confirmation
in
S. oneidensis
has yet to be reported. In
S. frigidimarina
, the transcriptional
activator IfcR is translated in the presence of soluble Fe(III) and is essential
for expression of
ifcO
and
ifcA
. IfcO is a putative OM beta-barrel protein pos-
tulated to function as a soluble Fe(III) transporter. IfcA is a flavin-containing
c-
type cytochrome with a small (10 kDa) tetraheme cytochrome domain capa-
ble of reducing soluble Fe(III) [73]. A working model of the two-step, Fe(III)
solubilization-reduction pathway in
Shewanella
is displayed in Fig. 4.
2. MECHANISM OF URANIUM REDUCTION
Members of the genera
Shewanella
[44],
Desulfovibrio
[45],
Clostridium
[22],
Geobacter
[9],
Thermus
[34],
Pyrobaculum
[32], and
Desulfosporosinus
[82] display U(VI) reduction activity.
Shewanella
and
Geobacter
enzymati-
cally reduce U(VI) to U(IV) via a dissimilatory process that supports anaerobic
growth, however the molecular mechanism of U(VI) reduction is poorly un-
derstood. Cytochrome
c
3
of U(VI)-reducing (but non-respiring)
Desulfovibrio
vulgaris
displays U(VI) reductase activity
in vitro
[45, 71]. Cytochrome
c
3
couples H
2
oxidation to U(VI) reduction. Cytochrome
c
7
of
Geobacter sul-
furreducens
also displays U(VI) reductase activity
in vitro
, however, mutants
deficient in either cytochrome
c
3
or
c
7
retain U(VI) reduction activity
in vivo
