Anaerobic sulfide oxidation and stable isotope fractionation associated with bacterial sulfur disproportionation in the presence of MnO2

The sulfur and oxygen isotope effects associated with anaerobic bacterial disproportionation of elemental sulfur by a pure culture (Desulfocapsa thiozymogenes) and an enrichment culture were investigated experimentally in the presence of synthetic Mn(IV)oxides. During bacterial disproportionation, S-34/S-32 were fractionated in dissolved sulfate compared to elemental sulfur by -0.6 to +2.0 parts per thousand (D. thiozymogenes ) and -0.2 to + 1.1 parts per thousand (enrichment culture) at cellular sulfur disproportionation rates of 10(-16) mol S-o/cell/h and 10(-17) mol S-o/cell/h. respectively. The measured sulfur isotope effects are much smaller than those observed previously for the same cultures in the presence of Fe(III) and Fe(II) compounds. indicating that microbial isotope fractionation was superimposed by the chemical re-oxidation of hydrogen sulfide by MnO2 to sulfate. Significant re-oxidation of H2S to sulfate was additionally confirmed by the oxygen isotopic composition of sulfate, which was enriched in O-18 compared to water by +8 to + 12 parts per thousand. These new experimental results imply that the overall influence of bacterial disproportionation on stable isotope partitioning in natural surface sediments depends on the proportion and relative recycling rates of reactive Fe(LII) to Mn(IV)(oxyhydr)oxides. Copyright (C) 2001 Elsevier Science Ltd.