Environmental Engineering Reference
In-Depth Information
2.2 Electron Donors and Competing Electron Acceptors
U(VI) reduction by
Shewanella
is coupled to oxidation of hydrogen, lactate,
formate or pyruvate [44]. U(VI) reduction rates are highest with H
2
as electron
donor [38] . Two explanations may account for the increased rate of U(VI)
reduction coupled to H
2
oxidation [3, 38]. First, electron flow through the
electron transport chain may be more rapid when coupled to H
2
rather than
lactate oxidation. Periplasmic H
2
hydrogenases may pass electrons through the
electron transport chain more rapidly than those generated from cytoplasmic
membrane-localized lactate dehydrogenase. Secondly, mass flux of neutrally
charged H
2
to the enzymatic site of oxidation may be faster than negatively
charged lactate. The negative charge of the lactate ion inhibits diffusion across
the cell surface to the cytoplasmic membrane, thereby requiring an active
transport system.
The presence of competing terminal electron acceptors also interferes with
microbial U(VI) reduction. Thermodynamic predictions indicate that electron
acceptors are utilized in order of highest free energy yield, a likely explanation
for the inhibition of U(VI) reduction in the presence of nitrate [21]. Thermody-
namic considerations alone, however, do not explain the observed preference
for some Fe(III) species over U(VI). Although the reduction of U(VI) coupled
to the oxidation of organic compounds should yield greater free energy than
Fe(III) [12], and despite the high solubility of U(VI), ferrihydrite inhibits U(VI)
reduction by
S. alga
BrY [90]. Inhibition by hematite or goethite, however, is
not observed. Amorphous Fe(III) hydroxides such as ferrihydrite are able to
compete with U(VI) as electron acceptor, while U(VI) is favored over more
crystalline forms of Fe(III). Kinetic factors, such as organism-Fe(III) oxide
attachment rate or enzyme-mediated electron transfer rates may enhance the
Fe(III) reduction rates relative to U(VI).
Electron transport to Mn(IV) provides a greater free energy yield than elec-
tron transport to U(VI), and is therefore predicted to be a preferred electron
acceptor [12, 36]. Bioavailable Mn(IV)-oxides such as birnessite and bixbyite
follow this prediction, however, U(VI) is reduced concurrently with less sol-
uble forms of Mn(IV) [24]. To determine if this finding is due to electron
acceptor competition or abiotic oxidation of U(IV) by Mn(IV),
S. putrefaciens
CN32 was incubated with U(VI) and pyrolusite (β-MnO
2(
s
)
) [39]. Extracellu-
lar, cell surface-associated, and periplasmic UO
2(
s
)
aggregates were detected
by Transmission Electron Microscopy (TEM) when cells were incubated only
with U(VI). Upon addition of pyrolusite, extracellular UO
2(
s
)
was depleted but
periplasmic and cell surface-associated UO
2(
s
)
remained. These results suggest
that U(IV) functions as an electron shuttle and is oxidized by the extracellular
pyrolusite. U(VI) is completely reduced provided the OM of intact cells physi-
