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.