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 d isproportionation of elemental sulfur by a pure culture (Desulfocapsa thioz ymogenes) 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 sulfu r 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 disproportionati on 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) comp ounds. indicating that microbial isotope fractionation was superimposed by the chemical re-oxidation of hydrogen sulfide by MnO2 to sulfate. Significa nt re-oxidation of H2S to sulfate was additionally confirmed by the oxygen isotopic composition of sulfate, which was enriched in O-18 compared to wat er by +8 to + 12 parts per thousand. These new experimental results imply t hat the overall influence of bacterial disproportionation on stable isotope partitioning in natural surface sediments depends on the proportion and re lative recycling rates of reactive Fe(LII) to Mn(IV)(oxyhydr)oxides. Copyri ght (C) 2001 Elsevier Science Ltd.