Identification and characterization of bacteria in a selenium-contaminatedhypersaline evaporation pond

Solar evaporation ponds are commonly used to reduce the volume of selenifer ous agricultural drainage water in the San Joaquin Valley, Calif. These hyp ersaline ponds pose an environmental health hazard because they are heavily contaminated with selenium (Se), mainly in the form of selenate. Se in the ponds may be removed by microbial Se volatilization, a bioremediation proc ess whereby toxic, bioavailable selenate is converted to relatively nontoxi c dimethylselenide gas. In order to identify microbes that may be used for Se bioremediation, a 16S ribosomal DNA phylogenetic analysis of an aerobic hypersaline pond in the San Joaquin Valley showed that a previously unaffil iated group of uncultured bacteria (belonging to the order Cytophagales) wa s dominant, followed by a group of cultured gamma -Proteobacteria which was closely related to Halomonas species. Se K-edge X-ray absorption spectrosc opy of selenate-treated bacterial isolates showed that they accumulated a m ixture of predominantly selenate and a selenomethionine-like species, consi stent with the idea that selenate was assimilated via the S assimilation pa thway. One of these bacterial isolates (Halomonas-like strain MPD-51) was t he best candidate for the bioremediation of hypersaline evaporation ponds c ontaminated with high Se concentrations because it tolerated 2 M selenate a nd 32.5% NaCl, grew rapidly in media containing selenate, and accumulated a nd volatilized Se at high rates (1.65 mug of Se g of protein(-1) h(-1)), co mpared to other cultured bacterial isolates.