The sulfur cycle of freshwater sediments: Role of thiosulfate

The formation and pathways of thiosulfate (S 2 O 3 2− ) in anoxic sediment were studied in the Odder River and Brabrand Lake, Denmark. Time‐course experiments were done in slurries with four 35 S tracers: SO 4 2− , H 2 S, and S 2 O 3 2− with either the inner (oxidized) or the outer (reduced) S atom labeled. The two sediments gave similar results. Of all the S 2 O 3 2− consumed in Brabrand Lake sediment, 6% was oxidized, 50% was reduced, and 44% was disproportionated. S 2 O 3 2− disproportionation is an inorganic fermentation in certain SO 4 2− ‐reducing bacteria by which the inner and outer S atoms are simultaneously transformed into SO 4 2 − and sulfide, respectively. Altogether, 28% of the S 2 O 3 2− ‐S was converted into SO 4 2− and 72% was converted into sulfide. S 2 O 3 2− inhibited SO 4 2− reduction by 75%. The immediate products of anoxic sulfide oxidation were 34% SO 4 2− and 66% S 2 O 3 2− . Half of the oxidized sulfide was ultimately converted into SO 4 2− and half was recycled back to sulfide via S 2 O 3 2− . Two‐thirds of the S in the sulfide‐thiosulfate “minicycle” remained at an oxidation state of −2. S 2 O 3 2− 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.