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Seasonal change in microbial sulfur cycling in monomictic Lake Fukami‐ike, Japan

Sulfur budgets and seasonal progression of sulfur cycling processes were investigated in Lake Fukami‐ike, a small monomictic lake in Japan. The sulfur isotopic mass balance shows that the main sulfur source is groundwater inflow (δ34S = ‐15.0‰). The sulfate influx is approximately 20% reduced by microbial sulfate reduction and deposited as sulfide (δ34S = ‐22.2‰; 34 kmol yr−1); the remaining exits from the outflow channel (δ34S = ‐12.6‰). In the water column, microbiological analyses reveal that green sulfur bacteria dominate more than 90% of the population in the anoxic layer. A monthly decrease in sulfate concentration at each depth indicates that sulfate reduction proceeds within the water column as a first‐order reaction because of low sulfate concentration (< 600 µmol L−1). The isotopic fractionation factor of sulfate reduction is estimated to be 0.980 by the isotopic and concentration data of sulfate and sulfide from April to August. Numerical simulation indicates that the relative contribution of phototrophic sulfide oxidation is over 50 times slower than sulfate reduction from April to August but over two times higher from October to November because of lower sulfate conditions (< 300 µmol L−1). Sulfate concentration is the factor for controlling the relative contribution of sulfate reduction and phototrophic sulfide oxidation in monomictic lakes, which contrasts with that observed in permanently stratified lakes.