Hypersulfidic deep biosphere indicates extreme sulfur isotope fractionation during single-step microbial sulfate reduction

Coexisting dissolved sulfide and sulfate from hypersulfidic interstitial wa ters of a 380m-long sediment core show a large isotopic difference of up to 72 parts per thousand caused by in situ microbial sulfate reduction. This is considerably larger than the assumed biological maximum of 46 parts per thousand derived from laboratory studies with pure cultures of sulfate-redu cing bacteria, Similar high Fractionations inferred from sedimentary metal sulfides have been previously explained by a multistage process, involving sulfide reoxidation and disproportionation of sulfur intermediates. Our dat a show that extreme isotopic differences between sulfate and the reduced su lfur species can also be generated during microbial single-step fractionati on, This result indicates that the sulfate-reducing communities and/or thei r cellular metabolic activities in the deep biosphere may differ from those observed in near-surface sediments or the water column.