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Project Progress

We investigate cyanobacterial mat ecosystems that have ancestors extending back billions of years. These mats allow us to examine microbiota and ecological processes that participated in early evolution, modified the early environment and created biosignatures. Budgets of oxygen and carbon differed between marine hypersaline cyanobacterial mats growing in subtidal versus intertidal settings. This reflects, in part, differences in the relative importance of oxic versus sulfate respiration. Photosynthetically active hypersaline cyanobacterial mats produced a substantial H₂ flux at night. Fermentative processes are probably responsible for this H₂ flux. Methane and reduced sulfur gases were also produced. Intertidal mats exhibited a greater H₂ flux than subtidal mats. In the ancient low-O₂ atmosphere, this H₂ flux would have elevated rates of H₂ escape to space, contributing to the long-term increase in the oxidation state of the global environment. Bahamian stromatolites were examined for their rates of photosynthesis, aerobic respiration and sulfate reduction. These rates were much lower in lithifying stromatolites than in nonlithifying hypersaline mats. However, incubations of organic substrates with slurries from the stromatolites indicated high potential rates of both aerobic respiration and sulfate reduction. Hot spring cyanobacterial mats were both exposed to and shielded from ultraviolet (UV) radiation. Analyses of ribonucleic acid (RNA) indicated that cyanobacteria in both treatments were very similar. However, when later exposed to UV, the mats that had been UV-shielded exhibited lower rates of photosynthesis, perhaps reflecting different patterns of gene expression. A greenhouse at Ames has maintained hypersaline cyanobacterial mats to allow the effects of environmental conditions to be studied. Mats maintained for more than one year strongly resembled mats freshly collected from the field with respect to microbial populations and key process rates. A greenhouse experiment was begun to document the effects of submillimolar sulfate levels upon marine cyanobacterial mats. Sulfate levels were low in ancient marine environments.