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23Mantle degassing provides a substantial amount of reduced and oxidized gases shaping microbial 24 metabolism at volcanic sites across the globe, yet little is known about its impact on microbial life 25 under non-thermal conditions. Here, we characterized deep subsurface fluids from a cold-water 26 geyser driven by mantle degassing using genome-resolved metagenomics to investigate how the 27 gases impact the metabolism and activity of indigenous microbes compared to non-impacted sites. 28While species-specific analyses of Altiarchaeota suggest site-specific adaptations and a particular 29 biogeographic pattern, chemolithoautotrophic core features of the communities appeared to be 30 conserved across 17 groundwater ecosystems between 5 and 3200 m depth. We identified a 31 significant negative correlation between ecosystem depth and bacterial replication, except for 32 samples impacted by high amounts of subsurface gases, which exhibited near-surface activity. Our 33 results suggest that geological degassing leads to higher nutrient flows and microbial activity in 34 the deep subsurface than previously estimated.

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