Environmental Engineering Reference
In-Depth Information
1. MECHANISM OF IRON REDUCTION
Microbial Fe(III) reduction is involved in several globally significant envi-
ronmental processes, including the biogeochemical cycling of Fe, Mn, trace
elements and phosphate, degradation of natural and contaminant organic mat-
ter, weathering of Fe(III)-containing clays and minerals and biomineralization
of Fe(II)-bearing minerals such as magnetite [19, 43, 48, 66]. The molecu-
lar mechanism of microbial Fe(III) reduction, however, is poorly understood.
Fe(III)-respiring gram-negative bacteria are presented with a unique physio-
logical problem: they are required to respire anaerobically on terminal electron
acceptors that are largely found in crystalline form or as amorphous (oxy)hy-
droxide particles presumably unable to contact inner membrane (IM)-localized
electron transport systems. To overcome this problem, Fe(III)-respiring bacte-
ria are postulated to employ a variety of novel respiratory strategies not found
in other gram-negative bacteria which respire on soluble electron acceptors
such as O
2
,NO
3
and SO
4
. These strategies include 1) direct enzymatic re-
duction of insoluble Fe(III) oxides via outer membrane (OM)-localized metal
reductases [56], 2) a two-step, electron shuttling pathway in which exogenous
electron shuttling compounds (e.g., humic acids, melanin, phenazines, antibi-
otics, anthraquinone-2,6-disulfonate; AQDS) are first microbially reduced and
subsequently chemically oxidized by the Fe(III) oxides in a second (abiotic)
electron transfer reaction [10, 11, 18, 30, 68, 86], 3) an analogous two-step
reduction pathway utilizing endogenous (quinone-like) electron shuttling com-
pounds [76, 68] and 4) a two-step, Fe(III) solubilization-reduction pathway in
which solid Fe(III) oxides are first non-reductively dissolved by bacterially-
produced organic ligands, followed by uptake and reduction of the soluble
organic Fe(III) forms by periplasmic Fe(III) reductases [47]. Several metal-
reducing members of the genus
Shewanella
are able to respire solid Fe(III)
oxides via all four mechanisms. Mechanistic details of the four pathways for
reduction of solid Fe(III) by
Shewanella
are summarized in the following sec-
tion.
1.1 Direct Enzymatic Reduction at the Outer Membrane
Shewanella
catalyzes the reduction of solid Fe(III) oxides via an electron
transport chain arranged in modular fashion. Primary dehydrogenase com-
plexes are linked to terminal reductase complexes via a menaquinone pool
[58]. Hydrogenases and flavin-containing dehydrogenases [41, 56] oxidize a
variety of electron donors (e.g., H
2
, formate, lactate) and transfer electrons via
the menaquinone pool. Lipid soluble menaquinones are required for Fe(III)
reduction by
Shewanella
membrane fractions supplied with either formate or
NADH as electron donor [76]. Menaquinol is postulated to diffuse to the quinol
oxidation site of CymA, a 21 kDa tetraheme
c
-type cytochrome which oxidizes
