Geochemical-dependent stratification of microbial chemolithoautotrophy in a high carbon dioxide subsurface aquifer

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Recent research has detected a vast diversity of novel organisms in aquifers and suggested an unexpected role for carbon dioxide fixation in the deep carbon cycle. The fraction of organisms with the capacity for chemolithoautotrophy (carbon dioxide fixation using inorganic electron donors and acceptors) and the variation of carbon dioxide fixation pathways and activity levels with geochemical environment remain essentially unknown. I propose to explore these questions by analyzing microbial communities in aquifer fluids that are sourced from different depths and geological formations by eruption of Crystal Geyser (Utah, USA) over the four-day eruption cycle. Given the geological context, it is likely that the ~800 m-deep aquifer is stratified, both geochemically and in microbial community composition. I hypothesize that the extent of chemolithoautotrophy, the pathways by which carbon dioxide fixation occurs, and activity levels will vary with availability of oxygen (for near-surface fluids), dissolved ferrous iron, hydrogen, methane, hydrogen sulfide, organic carbon and carbon dioxide concentrations (which varies by at least 30 times over the eruption cycle). I will use genome-resolved metagenomic and metatranscriptomic methods, in combination with geochemical measurements, to evaluate the metabolic potential of community members as a function of fluid composition. For organisms that use identifiable electron donors and acceptors and that are predicted to be autotrophs based on metagenomic analysis, I will use an isotopically labeled carbon dioxide microcosm experiment to test for chemolithoautotrophy. The project will provide new information about the prominence of carbon dioxide fixation in the subsurface and insight into the variety and distribution of chemolithoautotrophic activity as a function of geochemical conditions.