In their report "Abiotic selenium redox
transformations in the presence of Fe(II,III) oxides," S. C. B. Myneni et al. state (1, p. 1109) that
the reduction of selenium (Se)(VI) by iron (Fe)(II,III) oxides
("green rust") in an artificially constituted system "provides
direct evidence for the formation of reduced Se species in anoxic
sediments." By extrapolating their experiments, done under highly
specialized laboratory conditions, to encompass suboxic sediments in
general, Myneni et al. appear to challenge a body
of evidence which demonstrates that the bulk of Se(VI)
reduction in such natural systems is directly mediated by bacteria (for
example, through dissimilatory selenate reduction).
The evidence presented by Myneni et al. to support their
contention for abiotic reduction in sediments was a selective
comparison of their rate constants to some of those in the literature.
However, agreement of rate constant data provides only circumstantial
evidence and does not constitute a rigorous proof for the underlying
mechanism. We concede that some amount of abiotic reduction of Se(VI)
can theoretically occur in natural systems. However, Myneni et
al. have yet to show conclusively that abiotic Se(VI) reduction
actually occurs in recent sediments. Furthermore, they also would have to demonstrate that its quantitative significance is of a magnitude comparable to that achieved by the direct bacterial reduction of this
element in such environments.
Ronald S. Oremland
U.S. Geological Survey,
Menlo Park, CA 94025, USA
John Stolz
Department of Biological Sciences,
Duquesne University,
Pittsburgh, PA 15282, USA
Derek Lovley
Department of Microbiology,
University of Massachusetts,
Amherst, MA 01003, USA
REFERENCES
-
S. C. B. Myneni,
T. K. Tokunaga,
G. E. Brown Jr.,
Science
278,
1106
(1997)
[Abstract/Free Full Text]
.
1 December 1997; revised 19 June 1998; accepted 28 July
1998
Response: We thank Oremland et
al. for reiterating the importance of microbial reactions
involving Se(VI) in soils and sediments, which several researchers have
shown for many species of microorganisms. Our report neither
contradicts these previous observations nor suggests that bacterial
processes are not important for Se(VI) reduction in nature. Instead, we
showed that Se(VI) can be reduced abiotically under various laboratory
conditions by green rust, an Fe(II,III) oxide that can exist in some
types of anoxic sediments.
By oxidizing organic matter and depleting oxygen, microorganisms create
anoxic conditions in soils and sediments, which may further lead to the
formation of several reductants, such as NH4+,
various organic acids, and Fe-oxyhydroxides. Thus the redox transformations of major and trace elements (including Se) and organic
compounds in sediments may be facilitated directly by the
microorganisms (for example, by dissimilatory selenate reduction) and
by the inorganic and organic reductants formed in the sediments as a
result of microbial processes. Our report discusses Se(VI) reactions
with green rust, a potential reductant formed as a result of
biogeochemical processes in sediments. While the spectroscopic and
microscopic data in our report provide direct evidence for such
reactions, the thermodynamic and kinetic information that we discussed
supports the possibility that green rust plays a role in Se(VI)
reduction in these samples.
Procedures that have been used to distinguish biotic and abiotic
reactions--for example, autoclaving (2) or the addition of
chemicals--not only change the activity of microorganisms, but also
affect the rate at which oxygen is consumed and organic matter decays
in soils and sediments. This further affects sediment redox conditions
(1) and modifies the concentration of natural reductants.
Thus, such procedures provide only indirect evidence for microbial
reduction of Se(VI) and do not rule out other mechanisms.
In summary, Se(VI) reduction in a sediment can occur through different
pathways, and the magnitude of Se(VI) reduction through any of these
pathways is determined by Se(VI) reduction rates, the reductant's
concentration, and the activity of microorganisms. In addition, the
rate at which anoxic conditions are imposed in the sediment is also
critical. All these factors have to be evaluated when a contaminant
transformation in sediments is evaluated.
S. C. B. Myneni
T. K. Tokunaga
Earth Sciences Division,
Lawrence Berkeley National Laboratory,
University of California,
Berkeley, CA 94720, USA
G. E. Brown, Jr.
Department of Geological and Environmental Sciences and Stanford
Synchrotron Radiation Laboratory,
Stanford University,
Stanford, CA 94305, USA
REFERENCES
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P. G. Tratneyek and
N. L. Wolfe,
J. Environ. Qual.
22,
375
(1993)
[Abstract/Free Full Text].
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R. S. Oremland,
N. A. Steinberg,
A. S. Maest,
L. G. Miller,
J. T. Hollibaugh,
Environ. Sci. Technol.
24,
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(1990)
[CrossRef];
R. S. Oremland
et al.,
Appl. Environ. Microbiol.
55,
2333
(1989)
[Abstract/Free Full Text]
.
23 February 1998; accepted 28 July 1998
