SELENIUM UPTAKE BY SULFUR-ACCUMULATING BACTERIA

Selenium is a trace metal in many rock-forming minerals but is a major environmental contaminant worldwide. Uptake of selenium by S-accumula ting bacteria was examined in both pure cultures of Chromatium vinosum and in co-culture with Desulfovibrio desulfuricans. We used dual cult ures including these bacteria to concentrate selenium into intracellul ar globules. The bacterium D. desulfuricans reduces sulfate [SO42-(aq) ] to sulfide [H2S(aq)] and also reduces selenate [SeO42-(aq)] to selen ide [H2Se(aq)]. Once reduced, sulfide is enzymatically oxidized and fo rmed into intracellular globules by C. vinosum. We found that the sele nium also forms an intercellular solid but the reaction is thermodynam ically driven and proceeds by reducing S degrees(s) with H2Se(aq). Rel ative to the initial molar ratio of selenate and sulfate in the medium , selenium is concentrated 4.5 to 32-fold in the globules. Because sol id selenium is so much more stable than sulfur at growth conditions, o ther S-depositing bacteria, such as Beggiatoa and Chlorobium, should a lso concentrate selenium via this reaction, providing a strategy for e liminating contamination or for concentrating low natural levels into a usable farm.