Environmental Engineering Reference
In-Depth Information
itself [8, 16, 84, 89]. Several mutants, however, retained the ability to respire
all alternate electron acceptors except Tc(VII). In one specific mutant, Tc(VII)
reduction was severely impaired with either lactate or H
2
as electron donor, yet
was not affected with formate as electron donor [72].
The similarities in Tc(VII) reduction between
S. oneidensis
MR-1 and
S. pu-
trefaciens
CN32 indicate that the H
2
- and lactate-dependent Tc(VII) reduction
pathways may share electron transport chain components, although the Tc(VII)
terminal reductases are different.
Figure 6.
Working model for lactate-dependent Tc(VII) reduction pathway in S. oneidensis.
The different forms of Tc(IV) oxides are deposited according to the location
of the terminal reductase within the cell (Figs. 6 and 7). The lactate-dependent
Tc(VII) reduction pathway is predicted to terminate with a Tc(VII) reductase
located in the OM and the presence of hydrous Tc(IV) oxides in the surrounding
media (Fig. 6). H
2
-dependent Tc(VII) reduction is predicted to be linked to
both a periplasmic- and OM-associated reductase accounting for accumulation
of amorphous Tc(IV) oxides at both locations in H
2
-oxidizing cells (Fig. 7)
[91]. Neither the Tc(VII) reduction end product nor the subcellular location of
these products have been determined in the case of formate-dependent Tc(VII)
reduction. Formate-dependent Tc(VII) reduction appears to proceed through
a unique pathway sharing few components with either the H
2
- or lactate-
dependent Tc(VII) reduction pathways [72].
4. CONCLUSIONS
Metal-reducing gram-negative bacteria such as
Shewanella
are presented
with a unique physiological challenge: they are required to respire anaerobically
on terminal electron acceptors which are either highly insoluble (e.g., Fe(III)-
and Mn(IV)-oxides) and reduced to soluble end-products or highly soluble
(e.g., U(VI) and Tc(VII)) and reduced to insoluble end-products. To overcome
physiological problems associated with metal solubility, metal-respiring
She-
wanella
have been found to localize terminal reductase complexes to the outer
membrane, deliver electrons to extracellular electron acceptors via exogenous
or endogenous electron shuttles or produce metal-chelating compounds that
dramatically increase the solubility of solid terminal electron acceptors. Fur-
ther research on the molecular mechanism of metal reduction by
Shewanella
