Evolution of Hydrogen-Based Microbial Communities in Deep Hydrothermal Aquifers

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The hydrothermal systems underlying the Snake River plain in Idaho present an opportunity to directly observe how microbial communities in deep subsurface hydrothermal systems change over time. The Snake River plain was formed as the North American plate has moved west over the Yellowstone Hotspot producing a time-transgressive series of volcanic fields and associated hydrothermal systems that range in age from the 2.0 million year old (Ma) Yellowstone volcanic field to the 16.1 Ma McDermitt volcanic field. Recent studies show the presence of a predominantly autotrophic, hydrogen-based microbial community at Lidy Hot Spring, associated with the 6.6 Ma Heise volcanic field. Preliminary studies suggest the presence of heterotrophic microorganisms in spring waters associated with older volcanic fields on the Snake River plain. It is proposed to systematically characterize changes in the microbial ecology and organic geochemistry of progressively older hydrothermal systems underlying the Snake River plain of Idaho. Understanding how autotrophic, hydrogen-based microbial communities in subsurface hydrothermal systems change over time, and understanding the accompanying changes in the reactivity and composition of organic matter in spring waters could yield insight to understanding the progression of life on the early Earth. Specifically, the relative proportion of Archaea and Bacteria present in spring waters will be determined, and the proportion of Bacteria capable of specific types of heterotrophic metabolism (i.e. sulfate-, iron- and nitrate reduction) will be assessed with quantitative PCR using primer sets specific for appropriate phylogenetic or functional genes. Parallel to this effort, the chemical and isotopic composition of dissolved organic matter (DOM) in the spring waters will be documented to assess changes in the bioavailability of substrates and to determine how this material reflects the composition of the resident microflora. The apparent transition from autotrophic to heterotrophic microbial communities provides an opportunity to investigate the roles of specific biological processes in the diagenesis of DOM and the production of refractory organic matter that is preserved in the environment. These data will be used to describe the variability and ecological progression of deep hydrothermal systems over a timeframe of approximately ten million years.