The purple sulfur bacterium Thiocapsa roseopersicina, being the dominant anoxygenic phototroph in microbial mats, was tested for growth on polysulfide as the electron donor for carbon dioxide fixation. Data collected in continuous cultures revealed μmax to be 0.065 h-1 and the saturation affinity constant K s to be 6.7 μM. The value of the inhibition constant K i was estimated in batch cultures and was found to be approximately 1100 μM. When grown on monosulfide, the organism was capable of trisulfide utilization without lag. Monosulfide-limited growth was established to have a μmax of 0.091 h-1 and K s of 8.0 μM. Field observations revealed polysulfide, present at supra-optimal concentrations, as a major pool of reduced sulfur in a laminated marine sediment ecosystem.