REDUCTION OF SELENIUM OXYANIONS BY ENTEROBACTER-CLOACAE STRAIN SLD1A-1 - REDUCTION OF SELENATE TO SELENITE

Reduction of selenate (SeO42-) to selenite (SeO32-) by Enterobacter cl oacae strain SLD1a-1 (ATCC 700258), a facultative anaerobe isolated fr om agricultural drainage water, was studied in microcosm experiments u sing washed cell suspensions. The washed cell suspension removed 92% o f added SeO42-(at 127 mu M) from solution over 10.5 h using glucose as the electron donor. A method was developed that allowed for the asses sment of factors affecting only reduction of SeO42- to SeO32-, the fir st step in to Se-0 and/or Se2-. The method consisted of treating the c ell suspensions with 2,4-dinitrophenol (DNP), after which SeO42- reduc tion proceeds relatively unimpeded but SeO32- reduction is inhibited a nd SeO32- accumulates in solution and is quantified. Optimum pH was fo und to be 6.5-7.0, and an electrical conductivity of 10 dS m(-1) and g reater inhibited the reaction. Reduction of SeO42- at 127 mu M by SLD1 a-1 was unaffected by NO3-, NO2-, SO42-, AsO43-, and Fe3+ at equimolar concentrations but was inhibited by NO3- and SO42- at levels 5 and 23 6 times greater, respectively, than the SeO42- concentration and by Cr O42- and SO32- at concentrations equimolar with that of SeO42-. It was found that the more easily an electron donor enters the glycolytic pa thway, the greater its propensity to promote SeO42- reduction. In addi tion, initially growing the organism in the presence of lactose and ma ltose enhanced the organism's subsequent use of these electron donors in reduction of SeO42-. Optimum temperature for the reaction was 30 de grees C, and lowering the oxygen level enhanced SeO42- reduction and r emoval of Se from solution. Our results provide useful information for optimization of a bioreactor using E. cloacae SLD1a-1 to treat Se-con taminated water.