Oxidation and reduction of radiolabeled inorganic sulfur compounds in an estuarine sediment, Kysing Fjord, Denmark
35S-labeled SO 42-, S 2O 32-, S 0, ΣHS - (=H 2S + HS - + S 2-), and FeS 2 were used to trace the oxidative and reductive pathways of the sulfur cycle. We studied the transformation of 35SO 42-, 35S 2O 3-, 35S 0, AV 35S ( =ΣH 35S - + Fe 35S), and Fe 35S 2 in 8-cm long undisturbed estuarine sediment cores in time course experiments of up to 24 h. All the tracers, except for pyrite, were oxidized and/or reduced at all depths. More than 60% of the 35S from 35SO 22- reduction was recovered as 35S 0g in the top two cm of the sediment. At >2 cm, nearly all of the reduced 35SO 42- was recovered as AV 35S plus Fe 35S 2. Thiosulfate was not detected in the sediment. From the combined data of outer- and inner-labeled 35S 2O 32- injections, concurrent oxidation, reduction, and disproportionation of S 2O 32- were demonstrated. In oxidized surface sediment the three processes comprised 10, 24, and 66%, respectively, of the metabolized 35S 2O 32-. In reduced sediment these percentages were 16, 45, and 39%. Injections of SH 35S - into S 2O 32- spiked sediment cores produced 71% 35SO 42- and 29% 35S 2O 32- (% of the total 35SO 42- + 35S 2O 32- in the oxidized zone and 8% 35SO 42- and 92% 35S 2O 32- in the reduced zone. A similar experiment with 35S 0 produced 62% 35SO 42- and 38% 35S 2O 32- in the oxidized zone and 22 and 78% in the reduced zone. We calculated that more than half of the 35S 0 and ΣH 35S - oxidation to SO 42- proceeded through S 2O 32- in the reduced sediment. In the oxidized sediment these percentages were 15 and 21% for SH 35S - and 35S 0, respectively. Thiosulfate was thus a key intermediate in the S cycle.