THE SULFUR CYCLE OF FRESH-WATER SEDIMENTS - ROLE OF THIOSULFATE

The formation and pathways of thiosulfate (S2O3(2-)) in anoxic sediment were studied in the Odder River and Brabrand Lake, Denmark. Time-course experiments were done in slurries with four S-35 tracers: SO4(2-), H2S, and S2O3(2-) with either the inner (oxidized) or the outer (reduced) S atom labeled. The two sediments gave similar results. Of all the S2O3(2-) consumed in Brabrand Lake sediment, 6% was oxidized, 50% was reduced, and 44% was disproportionated. S2O3(2-) disproportionation is an inorganic fermentation in certain SO4(2-) reducing bacteria by which the inner and outer S atoms are simultaneously transformed into SO4(2-) and sulfide, respectively. Altogether, 28% of the S2O3(2-) -S was converted into SO4(2-) and 72% was converted into sulfide. S2O3(2-) inhibited SO4(2-) reduction by 75%. The immediate products of anoxic sulfide oxidation were 34% SO4(2-) and 66% S2O3(2-). Half of the oxidized sulfide was ultimately converte into SO4(2-) and half was recycled back to sulfide via S2O3(2-). Two-thirds of the S in the sulfide-thiosulfate "minicycle" remained at an oxidation state of -2. S2O3(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.