The sulfur cycle of freshwater sediments: Role of thiosulfate

Abstract

The formation and pathways of thiosulfate (S2O32−) in anoxic sediment were studied in the Odder River and Brabrand Lake, Denmark. Time‐course experiments were done in slurries with four 35S tracers: SO42−, H2S, and S2O32− with either the inner (oxidized) or the outer (reduced) S atom labeled. The two sediments gave similar results. Of all the S2O32− consumed in Brabrand Lake sediment, 6% was oxidized, 50% was reduced, and 44% was disproportionated. S2O32− disproportionation is an inorganic fermentation in certain SO42−‐reducing bacteria by which the inner and outer S atoms are simultaneously transformed into SO42− and sulfide, respectively. Altogether, 28% of the S2O32−‐S was converted into SO42− and 72% was converted into sulfide. S2O32− inhibited SO42− reduction by 75%. The immediate products of anoxic sulfide oxidation were 34% SO42− and 66% S2O32−. Half of the oxidized sulfide was ultimately converted into SO42− and half was recycled back to sulfide via S2O32−. Two‐thirds of the S in the sulfide‐thiosulfate “minicycle” remained at an oxidation state of −2. S2O32− is thus a key intermediate in the S cycle, both as a main product of anoxic sulfide oxidation and as a shunt between oxidative and reductive pathways.