Environmental Engineering Reference
In-Depth Information
able to respire a wide variety of compounds as alternate electron acceptor,
including oxygen (O
2
), nitrate (NO
3
−
), nitrite (NO
2
−
), Mn(III,IV), Fe(III),
trimethylamine-
N
-oxide (TMAO), sulfite (SO
3
2
−
), thiosulfate (S
2
O
3
2
−
), S(0),
fumarate, Cr(VI), U(VI), Tc(VII) and potentially several others [15, 53]. The
remarkable respiratory versatility displayed by
Shewanella
is thought to pro-
vide a competitive advantage in redox-stratified environments where terminal
electron acceptor type and abundance fluctuates on relatively small spatial and
temporal scales. Over 75% of the cultivatable microorganisms from the Mn-
reducing zone of the Black Sea water column are most similar to
Shewanella
[64], and nearly identical abundances are detected in the microaerobic and
anoxic zones of the water column of the Gotland Deep, the main anoxic basin
of the Central Baltic Sea [6].
The biogeochemical reactions catalyzed by
Shewanella
may influence the
aqueous geochemical and mineralogical reaction network within redox-strati-
fied environments.
Shewanella
may play an important role in carbon cycling
by catalyzing the anaerobic mineralization of low molecular weight organic
compounds in sulfate-deplete environments [1, 65, 85]. The most remarkable
activity displayed by
Shewanella
, however, is their ability to couple the oxi-
dation of organic carbon and hydrogen to either the reductive dissolution of
solid phase Fe(III)- and Mn(IV)-oxides or the reductive precipitation of toxic
metals and radionuclides such as Cr(VI), U(VI) and Tc(VII). The reductive
precipitation reactions form the basis of alternate in situ bioremediation strate-
gies since the relative solubility (and hence mobility) of Cr, U and Tc is greatly
diminished at lower oxidation states [41, 44].
Compared to the wealth of knowledge on the molecular basis of other bac-
terial respiratory processes (e.g., aerobic respiration, denitrification, sulfate
reduction, methanogenesis) [51], little is known about the molecular details
of bacterial metal reduction. Recent sequencing of the
S. oneidensis
MR-1
genome has facilitated studies on the mechanistic details of metal reduction
by
Shewanella
[29]. Genome-enabled research on
Shewanella
will be greatly
expanded in the near future with the genome sequencing of seven additional
Shewanella
strains, including
S. putrefaciens
200,
S. amazonensis
,
S. baltica
OS15,
S. frigidimarina
NCIMB 400,
S. denitrificans
OS217T,
S.
sp.PV-4and
S. putrefaciens
CN-32 (DOE-JGI 2004 Microbial Sequencing Program). The
following chapter highlights the latest findings on the molecular mechanism of
Fe(III), U(VI) and Tc(VII) reduction by
Shewanella
, with particular emphasis
on electron transport chain physiology.
