Reduction of selenium oxyanions by Enterobacter cloacae strain SLD1a‐1: Reduction of selenate to selenite

Abstract

AbstractReduction of selenate ( SeO2−4) to selenite ( SeO2−3) by Enterobacter cloacae strain SLD1a‐1 (ATCC 700258), a facultative anaerobe isolated from agricultural drainage water, was studied in microcosm experiments using washed cell suspensions. The washed cell suspension removed 92% of added SeO2−4 (at 127 μM) from solution over 10.5 h using glucose as the electron donor. A method was developed that allowed for the assessment of factors affecting only reduction of SeO2−4 to SeO2−3, the first step in the reduction of SeO2−4 to Se0 and/or Se2−. The method consisted of treating the cell suspensions with 2,4‐dinitrophenol (DNP), after which SeO2−4 reduction proceeds relatively unimpeded but SeO2−3 reduction is inhibited and SeO2−3 accumulates in solution and is quantified. Optimum pH was found to be 6.5–7.0, and an electrical conductivity of 10 dS m−1 and greater inhibited the reaction. Reduction of SeO2−4 at 127 μM by SLD1a‐1 was unaffected by NO3−, NO2−, SO2−4, AsO3−4, and Fe3+ at equimolar concentrations but was inhibited by NO3− and SO2−4 at levels 5 and 236 times greater, respectively, than the SeO2−4 concentration and by CrO2−4 and SO2−3 at concentrations equimolar with that of SeO2−4. It was found that the more easily an electron donor enters the glycolytic pathway, the greater its propensity to promote SeO2−4 reduction. In addition, initially growing the organism in the presence of lactose and maltose enhanced the organism's subsequent use of these electron donors in reduction of SeO2−4. Optimum temperature for the reaction was 30°C, and lowering the oxygen level enhanced SeO2−4 reduction and removal of Se from solution. Our results provide useful information for optimization of a bioreactor using E. cloacae SLD1a‐1 to treat Se‐contaminated water.