Rates of sulfate reduction and thiosulfate consumption in a marine microbial mat

The sulfur cycle in a microbial mat was studied by determining viable counts of sulfate-reducing bacteria, chemolitboautotrophic sulfur bacteria and anoxygenic phototrophic bacteria. All three functional groups of sulfur bacteria revealed a maximum population density in the uppermost 5 mm of the mat: 1.1 × l0 s cells of sulfate reducers cm -3 sediment, 2.0 × 109 cells of chemolithoau- totrophs cm -3 sediment, and 4.0 x 107 cells of anoxygenic phototrophs cm -3 sediment. Bacterial dynamics were studied by sulfate reduction rate measurements, both under anoxic conditions (dark incubation)and oxic conditions (incubation in the light), and determination of the vertical distribution of the potential rate of thiosulfate consumption under oxic conditions. Sulfate re-duction rates in the top 5 mm of the sediment were 566 nmol cm -3 d -t in the absence of oxygen, and 123 nmol cm -3 d -t in the presence of oxygen. In the latter case, the maximum rate was found in the 5-10-ram depth horizon (361 nmol cm -3 d-~). Biological consumption of amended thiosulfate was rapid and decreased with depth, while in the presence of molybdate, thiosulfate consumption decreased to 10-30 of the original rate.