Carbon uptake, microbial community structure, and mineralization of layered mats from Imperial Geyser, Yellowstone National Park

Layered microbial mats provide an analog for early microbial communities, and remain one of the few microbiological structures consistently preserved in the geologic record. Despite this, growth rates, metabolic capabilities, and methods of mineralization in modern communities are poorly understood. Imperial Geyser, an alkaline siliceous hot spring in Yellowstone National Park, provides a useful setting to study these parameters. Mat and water samples (T = 64-40 °C) were collected for 13C analysis and 13C-spiked bicarbonate and acetate incubation experiments. Carbon isotopes were measured for the stream water, pore water and biomass. We experimentally determined rates of bicarbonate uptake, acetate uptake and mineral content. Bicarbonate uptake rates ranged from 0 - 0.4% per day, while acetate uptake rates ranged from 0 - 2.0% per day. These results indicate that the mat biomass is capable of turnover in about 300 days resulting in potential growth rates of 1-2 cm/year. Organic carbon content (% dry weight) ranged from 2 to 16%, and decreased with depth in the mat. The mineral content of these mats is predominantly amorphous SiO2. An inverse correlation between mineral percent and bicarbonate uptake rate was observed, suggesting that there may be a link between metabolism and the prevention of mineralization. Comparing the 13C and carbon uptake rates with 16S rDNA pyrosequencing data we were able to hypothesize the carbon fixation pathways and heterotrophic interactions occurring in this environment. In general, two patterns of 13C values were observed. The first pattern was characterized by increased heterotrophy with depth. In the other, preliminary evidence supporting a photoheterotrophic lifestyle for Roseiflexus spp. was found.