Microbially generated H2S above 90 °C in the deep biosphere

Abstract

Abstract Thermophilic microorganisms play critical roles in sulfur cycling within the deep biosphere, but knowledge of these biogeochemical reactions at temperatures >80 °C is generally restricted to marine hydrothermal systems1,2. Hydrocarbon extraction activities routinely interact with the deep subsurface, providing access to thermophilic environments where the thermal limits of life within sedimentary basins can be explored. Paramount to this, is understanding of how microbial communities in deep, hot biospheres are activated by and respond to disturbances such as the introduction of fluids from the surface. Here we show through chemical, stable isotope and microbiological analyses that microbial populations can actively catalyze sulfur cycling in subsurface hotter than 90 °C generating H2S via microbial sulfate reduction. These temperatures exceed previously reported thermal limits of sulfate reducing microorganisms3 thus narrowing the gap between temperatures permissive for biogenic and thermochemical sulfate reduction4. Our analyses demonstrate that the introduction of surface-derived fluids into nutrient-limited deep environments stimulates microbial sulfate reduction at temperatures >90 °C previously considered too high for this process with implications for underground storage activities such as CO2 and hydrogen storage.