The Fate of Selenate and Selenite Metabolized by Rhodobacter sphaeroides
Verena Van Fleet-Stalder1, Thomas
G. Chasteen2*,
Ingrid J. Pickering3,
Graham N. George3,
and Roger C. Prince4
1Department
of Biochemistry, Molecular Biology and Biophysics, University of Minnesota,
St. Paul, MN 55108-1022, USA
2Department
of Chemistry, Sam Houston State University, Huntsville, Texas, 77341-2117,
USA
3Stanford
Synchrontron Radiation Laboratory, Stanford Linear Accelerator Center,
Stanford, CA 94309, USA
4ExxonMobile
Research and Engineering Company, Annandale, NJ, 08801, USA
Summary of work
Our work with Rhodobacter sphaeroides has been broadened yet focussed
at the same time in this paper which involves work by microbiologists,
chemists, and spectroscopists. These photosynthetic bacteria were exposed
to either low (about 1 ppm Se) or high (about 100 ppm Se) amount of either
selenite or selenate and then grown photosynthetically for 14 days, far
into the stationary phase. These cultures were then assayed in the following
way:
-
Headspace above the anaerobic cultures was assayed for volatile organoselenium
species by fluorine-induced chemiluminescence;
-
after cell harvesting, separated culture supernatant was analyzed via ICP/MS,
and
-
cell were examined with X-ray absorption spectroscopy (XAS).
Cultures that had selenite (SeO32-) added showed
much higher biological conversion of that toxic selenium compared to analogous
selenate (SeO42-) experiments. This was true at both
the amendment concentrations used. Conversion here was determined by finding
Se in chemical forms different from the amended forms.
For instance, cells from low selenite amended cultures showed (via XAS)
that approximately 60% of added Se could be found in an organoselenium
species spectrally identical to selenomethionine (based on standards).
In this technique, it was decided not to differentiate between Se-methionine
and dimethyl selenide (DMSe) using the XAS spectra; however, headspace
analysis show so little DMSe in the headspace that Se-methionine is a more
probable Se-containing species in or on these cells than is DMSe.
While low selenite-amended cells yielded both Se-methionine and elemental
Se forms of selenium, selenate-amended cells showed no significant differences
in Se0 formation (13 and 15% conversion of added SeO42-).
Instead most added Se in selenate experiments ended up as organo-Se (that
is, Se-methionine).
Finally, as noted above: while volatile organo-Se species we have seen
before in phototrophic cultures of these bacteria were detected in anaerobic
culture headspace (DMSe, dimethyl selenenyl sulfide, and dimethyl diselenide),
the amount of added Se found in these forms--even taking gas phase/liquid
phase Henry's law distribution--was very small: less that 0.5% bioremediation
even for the most prolific gas phase producer, the high concentration selenate-amended
cultures.
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