Groundwater Microbial Diversity Associated With Icelandic Basaltic Subsurface Environments.

Microbial communities in the deep basaltic aquifers of Iceland remain poorly characterized, despite their relevance for understanding subsurface biogeochemical processes, including silicate weathering. Here, we used 16S rRNA gene metabarcoding to investigate bacterial and archaeal diversity in 22 geothermal wells spanning broad gradients in temperature (30°C-110°C), pH (7.0-11.0), and bedrock age (0.01-15 Myr). We observed highly variable microbial assemblages, with several dominant taxa affiliated with known deep biosphere lineages, including hydrogenotrophs and sulfate reducers. Archaeal communities were less diverse and displayed domain-specific patterns, distinct from bacterial assemblages. Beta diversity was primarily structured by temperature and pH, and, to a lesser extent, by bedrock age. Thermodynamic and kinetic parameters derived from groundwater chemistry-including redox potential and silicate dissolution rates-also accounted for significant fractions of the variation in microbial beta diversity, although it cannot be ruled out that their influence primarily reflected underlying correlations with temperature and pH. Our results suggest that both environmental gradients and host-rock reactivity shape microbial diversity in these systems. This highlights the importance of considering geochemical context when designing subsurface microcosm experiments, and identifies candidate taxa for future studies exploring links between microbial composition and silicate weathering processes.