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

Applied and Environmental Microbiology, 2000, 66(11), 4849-4853. AEM's abstract

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: 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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