ISOTOPE FRACTIONATION AND SULFUR METABOLISM BY PURE AND ENRICHMENT CULTURES OF ELEMENTAL SULFUR-DISPROPORTIONATING BACTERIA

We have explored the sulfur metabolism and accompanying fractionation of sulfur isotopes during the disproportionation of elemental sulfur b y seven different enrichments and three pure bacterial cultures. Cultu res were obtained from both marine and freshwater environments. In all cases appreciable fractionation accompanied elemental sulfur dispropo rtionation, with two ranges of fractionation observed. All cultures ex cept Desulfobulbus propionicus produced sulfide depleted in S-34 by be tween 5.5 and 6.9 per mil (avg of 6.3 per mi) and sulfate enriched in S-34 by between 17.1 and 20.2 per mil (avg of 18.8 per ml). The narrow range of fractionations suggests a conserved biochemistry for the dis proportionation of elemental sulfur by many different marine and fresh water bacteria. Fractionations accompanying elemental sulfur dispropor tionation by Db. propionicus were nearly twice as great as the others, suggesting a different cellular level pathway of sulfur processing by this organism. In nearly every case pyrite formation accompanied the disproportionation of elemental sulfur. By using sulfur isotopes as a tracer of sulfur source, we could identify that pyrite formed both by the addition of elemental sulfur to FeS and from reaction between FeS and H2S. Both processes were equally fast and up to 10(4)-10(5) times faster than expected from the reported kinetics of inorganic pyrite-fo rmation reactions. We speculate that bacteria may have enhanced rates of pyrite formation in our experimental systems. The organisms explore d here have different strategies for growth and survival, and they may be active in environments ranging from dissolved sulfide-poor suboxic sediments to interfaces supporting steep opposing gradients of oxygen and sulfide. A large environmental range, combined with high bacteria l numbers, significant isotope fractionations, and a possible role in pyrite formation, make elemental sulfur-disproportionating bacteria po tentially significant actors in the sedimentary cycling of sulfur comp ounds.