ELEMENTAL SULFUR AS AN INTERMEDIATE OF SULFIDE OXIDATION WITH OXYGEN BY DESULFOBULBUS-PROPIONICUS

The sulfate-reducing bacterium Desulfobulbus propionicus oxidized sulf ide, elemental sulfur, and sulfite to sulfate with oxygen as electron acceptor. Thiosulfate was reduced and disproportionated exclusively un der anoxic conditions. When small pulses of oxygen were added to washe d cells in sulfide-containing assays, up to 3 sulfide molecules per O- 2 disappeared transiently. After complete oxygen consumption, part of the sulfide reappeared. The intermediate formed was identified as elem ental sulfur by chemical analysis and turbidity measurements. When exc ess sulfide was present, sulfur dissolved as polysulfide. This process was faster in the presence of cells than in their absence. The format ion of sulfide after complete oxygen consumption was due to a dispropo rtionation of elemental sulfur (or polysulfide) to sulfide and sulfate . The uncoupler tetrachlorosalicylanilide (TCS) and the electron trans port inhibitor myxothiazol inhibited sulfide oxidation to sulfate and caused accumulation of sulfur. In the presence of the electron transpo rt inhibitor 2-n-heptyl-4-hydroxyquinoline-N-oxide (HQNO), sulfite and thiosulfate were formed. During sulfur oxidation at low oxygen concen trations, intermediary formation of sulfide was observed, indicating d isproportionation of sulfur also under these conditions. It is conclud ed that sulfide oxidation in D. propionicus proceeds via oxidation to elemental sulfur, followed by sulfur disproportionation to sulfide and sulfate.