Energetic constraints on chemolithoautotrophy and habitability in the deep continental subsurface: Kidd Creek Deep Fluid and Deep Life Observatory

In the absence of sunlight-derived energy, water–rock interactions in the continental crust can generate energy for extant chemolithoautotrophs in Earth’s deep biosphere. Saline fracture fluids with residence times between hundreds of millions to a billion years at Kidd Creek Observatory in Ontario, Canada, provide a prime opportunity to investigate the potential for life in such remote and relatively inaccessible habitats. Estimates of the thermodynamic yield from energy-generating metabolic reactions are often used to evaluate potential microbial metabolisms in a variety of ecosystems. Here we report the energy densities from nine potential chemolithoautotrophic reactions related to sulfate reduction, methanogenesis, and acetogenesis, and in combination with previous isotopic and microbiological data, investigate the contributions of microbial activities and abiotic processes to the geochemical signatures of Kidd Creek fracture fluids. Autotrophic sulfate reduction consistently yielded the highest energy densities compared to sulfate reduction by short chain alkanes and simple organic acids —a finding consistent with a previous cell culture study that demonstrated extant microbial communities based on cell counts and Most Probable Number Analysis. Estimates of power supply based on energetic yields from sulfate reduction suggest that the Kidd Creek fracture waters can theoretically support ∼10 2 to 10 3 cells/L. The results also indicate the potential for microbial methanogenesis, a process also supported by recent clumped methane isotopologue studies. However, isotopic and geochemical studies suggest only very low rates of biological methane production, as abiotic production remains the dominant methane production process in the Kidd Creek system. High thermodynamic energy yields from autotrophic methanogenesis and microbial acetogenesis via CO 2 reduction are likely due to very high concentrations of dissolved hydrogen gas in the fracture waters, accumulating from the overall slow rates of microbial utilization in such a low biomass ecosystem.