Microbial sulfur cycling in an anoxic, sulfide bearing spring was studied as a model of processes occurring in the Archean biosphere. During the Archean and early Proterozoic periods, all microbial processes occurred anaerobically, but currently understood mesophilic surficial microbial communities are typically oxic or exist with oxic waters overlying anoxic sediments or bottom layers. A spring in Oklahoma discharges chemically anomalous springwater, containing abundant sulfide and no detectable oxygen. Sulfate concentration increases with distance from the source and apparently has its origins in the sulfide emerging from the spring. Microbial activity studies, in-situ experiments, molecular ecology and stable isotope studies suggest that as sulfide-rich water flows from the spring down a nearby stream, anaerobic phototrophic bacteria play a critical role in oxidizing sulfide to polysulfide and sulfate. Fine scale measurements of sulfate reduction within sediment cores show that sulfur cycling occurs within the microbial mats, as sulfate-reducing bacteria are closely associated with sulfide-oxidizing communities. Our characterization of this ecosystem suggests that the sulfide emerging from the spring supports a diverse phototrophic, primary producing community that then provides substrates (in the form of electron donor, electron acceptor, and organic carbon) for sulfate reducing and sulfur disproportionating bacteria. Within this rich and microbially diverse system, we find a modern analog for an Archaean aquatic environment.