Geologically produced (abiotic) molecular hydrogen and methane could be widely utilized by microbial communities in surface and subsurface environments. These microbial communities can, therefore, have a potentially significant impact on the net emissions of H2 and CH4 to Earth’s ocean and atmosphere. Abiotic H2 and CH4 could enable microbial communities to exist in rock-hosted environments and hydrothermal systems with little or no input from photosynthetic carbon fixation, making these communities potential analogs for the earliest metabolisms on Earth (or other planetary bodies). The possible dependence of rock-hosted ecosystems on H2 and CH4 should factor into current and future plans for engineering the subsurface for storage of these compounds as energy fuels.

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