Environmental Engineering Reference
In-Depth Information
cally separates (sequesters in the periplasmic space) UO
2(
s
)
from extracellular
pyrolusite.
Humic acids have recently gained attention for their potential role as shut-
tles for electron transfer between anaerobically respiring
Shewanella
and solid
Fe(III)-oxides (see above). Addition of AQDS to
S. putrefaciens
CN32, how-
ever, does not enhance the reduction rate of either soluble or insoluble forms
of U(VI) [23]. AQDS actually inhibits U(VI) reduction activity, most likely by
re-directing electrons away from the U(VI) reduction pathway.
2.3 U(IV) Localization
The subcellular location of U(VI) reduction in
Shewanella
is unknown,
however, U(IV) is detected extracellularly, associated with the cell surface and
within the periplasmic space of
S. putrefaciens
reducing soluble forms of U(VI)
[39]. U(IV) is not detected in the cytoplasm. U(VI) reductases may therefore
be localized within the gram-negative cell envelope, or soluble U(VI) diffu-
sion (or transport) across the OM facilitates contact with U(VI) reductases
located in the periplasm or cytoplasmic membrane. U(VI) reduction products
are nanometer-size UO
2(
s
)
particles during U(VI) reduction by
Desulfosporos-
inus
[82]. Nanoparticles produced in the periplasm are either exported via
OM porins or other export mechanisms where they aggregate extracellularly
to form larger particles. Aggregation of U(IV) particles prior to export from
the cell may result in the periplasmic deposits detected on TEM images of
U(VI)-respiring cells [39]. U(IV) particles detected in the culture supernatant
also leads to the intriguing possibility that anaerobically-respiring
Shewanella
are able to actively excrete U(IV) particles as a means of avoiding build-up of
toxic insoluble end-products during U(VI) reduction.
S. putrefaciens
CN32 is
also capable of reducing solid forms of U(VI) [23]. The mechanism by which
Shewanella
species reduce metaschoepite is unknown, but U(VI) terminal re-
ductase localization to the outer membrane to contact solid U(VI) is possible.
2.4 Genetic Analysis
A genetic system has recently been developed to determine the molecular
mechanism of U(VI) reduction by
S. putrefaciens
[89].
S. putrefaciens
respi-
ratory (Urr) mutants unable to reduce U(VI) were isolated and tested for the
ability to respire on a suite of alternate compounds as electron acceptor, includ-
ing oxygen (O
2
), nitrate (NO
3
−
), fumarate, trimethylamine-
N
-oxide (TMAO),
dimethyl sulfoxide (DMSO), manganese oxide (MnO
2
), ferric iron (Fe(III)),
chromate (Cr(VI)), arsenate (As(V)), selenite (Se(IV)), pertechnetate (Tc(VII)),
thiosulfate (S(II)), and sulfite (S(IV)) [89]. Ethyl methane sulfonate (EMS) was
used as a chemical mutagen to generate the set of Urr mutants of
S. putrefaciens
200. Approximately 13,000 colonies arising from EMS-treated cells were trans-
