Fractionation of selenium isotopes during bacterial respiratory reduction of selenium oxyanions

Reduction of selenium oxyanions by microorganisms is an important process i n the biogeochemical cycling of selenium. Numerous bacteria can reduce Se o xyanions, which are used as electron accepters during the oxidation of orga nic matter in anoxic environments. In this study, we used a double spike (S e-82 and Se-74) thermal ionization mass spectrometry technique to quantify the isotopic fractionation achieved by three different species of anaerobic bacteria capable of accomplishing growth by respiratory reduction of selen ate [SeO42- or Se(VI)] or selenite [SeO32- or Se(IV)] to Se(IV) or elementa l selenium [Se(0)] coupled with the oxidation of lactate. Isotopic discrimi nation in these closed system experiments was evaluated by Rayleigh fractio nation equations and numerical models. Growing cultures of Bacillus selenit ireducens, a haloalkaliphile capable of growth using Se(IV) as an electron acceptor, induced a Se-80/Se-76 fractionation of -8.0 +/- 0.4 parts per tho usand (instantaneous epsilon value) during reduction of Se(IV) to Se(0). Wi th Bacillus arsenicoselenatis, a haloalkaliphile capable of growth using Se (VI) as an electron acceptor, fractionations of -5.0 +/- 0.5 parts per thou sand and -6.0 +/- 1.0 parts per thousand were observed for reduction of Se( VI) to Se(IV) and reduction of Se(IV) to Se(0), respectively. In growing cu ltures of Sulfurospirillum barnesii, a freshwater species capable of growth using Se(VI), fractionation was small initially, but near the end of the l og growth phase, it increased to -4.0 +/- 1.0 parts per thousand and -8.4 /- 0.4 parts per thousand for reduction of Se(VI) to Se(IV) and reduction o f Se(IV) to Se(0), respectively. Washed cell suspensions of S. barnesii ind uced fractionations of -1.1 +/- 0.4 parts per thousand during Se(VI) reduct ion, and -9.1 +/- 0.5% for Se(IV) reduction, with some evidence for smaller values (e.g., -1.7 parts per thousand) in the earliest-formed Se(0) result s. These results demonstrate that dissimilatory reduction of selenate or se lenite induces significant isotopic fractionation, and suggest that signifi cant Se isotope ratio variation will be found in nature. Copyright (C) 2000 Elsevier Science Ltd.