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Project Progress

We investigate photosynthetic microbial mat ecosystems and their fossils. These mats allow us to examine microbiota and ecological processes that participated in early evolution, modified the early environment, and created biosignatures. Photosynthetically active cyanobacterial mats were studied in a hypersaline pond near Guerrero Negro, Mexico. These mats produced substantial quantities of volatile fatty acids (VFA), most of which derived specifically from fermentative processes. We investigated the mechanisms and conditions that favor the production of dimethylsulfide (DMS) and methanethiol (MT). DMS and MT are probably formed principally by reactions between low molecular weight organic carbon compounds and biogenic hydrogen sulfide in mat porewaters. DMS and MT can escape to the atmosphere and become biosignatures for these microbial communities. We compared H₂ concentrations in anoxic open marine shelf sediments with those in hypersaline phototrophic cyanobacterial mats. In contrast with the anoxic shelf sediments, the phototrophic mats sustain high H₂ levels that facilitate methanogenesis even in the presence of elevated sulfate concentrations. We demonstrated that hypersaline mats can be maintained in a greenhouse for more than a year without sustaining major changes, as indicated by microscopic, genetic, biomarker and biogeochemical process observations. Because sulfate levels were low in ancient seas, we recently conducted a greenhouse experiment for more than one year to document the effects of submillimolar sulfate levels upon hypersaline cyanobacterial mats. We investigated the diversity, distribution and phenotypes of uncultivated green nonsulfur bacteria (GNS) in photosynthetic microbial mats in the hot, alkaline Mushroom Spring, Yellowstone National Park. We found a large diversity of GNS. Some GNS occurred in close association with cyanobacteria. Diverse GNS assimilated acetate. We determined carbon isotope discrimination during both heterotrophic and lithotrophic growth of pure cultures of sulfate reducing bacteria (SRB). SRB grown lithoautotrophically exhibited substantially larger discrimination, relative to their carbon sources, than SRB grown heterotrophically.