Simultaneous reduction of nitrate and selenate by cell suspensions of selenium-respiring bacteria

Washed-cell suspensions of Sulfurospirillum barnesii reduced selenate [Se(V I)] when cells were cultured with nitrate, thiosulfate, arsenate, or fumara te as the electron acceptor. When the concentration of the electron donor w as limiting, Se(VI) reduction in whole cells was approximately fourfold gre ater in Se(VI)-grown cells than was observed in nitrate-grown cells; corres pondingly, nitrate reduction was similar to 11-fold higher in nitrate-grown cells than in Se(VI)-grown cells. However, a simultaneous reduction of nit rate and Se(VI) was observed in both cases. At nonlimiting electron donor c oncentrations, nitrate-grown cells suspended with equimolar nitrate and sel enate achieved a complete reductive removal of nitrogen and selenium oxyani ons, with the bulk of nitrate reduction preceding that of selenate reductio n. Chloramphenicol did not inhibit these reductions. The Se(VI)-respiring h aloalkaliphile Bacillus arsenicoselenatis gave similar results, but its Se( VI) reductase was not constitutive in nitrate-grown cells. No reduction of Se(VI) was noted for Bacillus selenitireducens, which respires selenite. Th e results of kinetic experiments with cell membrane preparations of S. barn esii suggest the presence of constitutive selenate and nitrate reduction, a s well as an inducible, high-affinity nitrate reductase in nitrate-grown ce lls which also has a low affinity for selenate. The simultaneous reduction of micromolar Se(VI) in the presence of millimolar nitrate indicates that t hese organisms may have a functional use in bioremediating nitrate-rich, se leniferous agricultural wastewaters. Results with Se-75-selenate tracer sho w that these organisms can lower ambient Se(VI) concentrations to levels in compliance with new regulations proposed for release of selenium oxyanions into the environment.