Bacterial cycling of sulfur in a Baltic sediment: An in situ study in closed systems

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Abstract
Bacterial cycling of sulfur at the sediment‐water interface was studied in situ at 10‐m water depth in both transparent and opaque closed‐box systems over a period of 245 days. Reducing processes dominated at Eh values below +200 mV. In the transparent systems, oxygen was produced because of photosynthesis and the redox turnover was delayed 30 days compared to the opaque systems in which reducing conditions were established in 10 days. The initial rates of bacterial sulfate reduction were 0.22 and 0.35 g SO4‐S m‐2 day‐1 for the opaque and transparent systems, respectively. The bacteria generated sulfide at rates corresponding to the depletion of sulfate. Photosynthetic sulfur bacteria developed in the transparent systems, and sulfide was oxidized at twice the rate of its formation, equal to 0.7 g sulfide‐S m‐2day‐1. A stepwise oxidation of sulfide, via elemental sulfur to sulfate, was demonstrated. Sulfate reduction dominated the opaque systems and also the transparent ones with insufficient light.