Microbial growth rates captured using Raman-SIP reveal a highly active subsurface biosphere fueled by...

Microbial productivity, metabolite fluxes, nutrient cycling and biosignatures are directly linked to microbial growth rates, which remain largely unknown in subsurface environments. Here, we apply Raman-stable isotope probing with deuterated water (Raman-²H-SIP) to measure single-cell microbial growth rates in three geochemically distinct fluids obtained at 250-270 m depth in serpentinite rocks in Oman. We observe wide distributions in growth that vary with fluid geochemistry, with inferred cell-specific microbial generation times ranging from days to years. The majority of measured generation times are faster than those previously reported in subsurface rock-hosted ecosystems, showing extensive microbial growth can be supported by actively serpentinizing systems. Amendment with bicarbonate stimulates some of the fastest growth and methane production rates, highlighting microbial preference to utilizing dissolved inorganic carbon, even in [hyper]alkaline groundwaters. Overall, our cell-specific rates of growth and methanogenesis provide quantitative insights into the habitability of continental serpentinizing ecosystems and their reservoir-scale biogeochemical dynamics.