The geothermal gradient from mesophilic to thermophilic temperatures shapes microbial diversity and processes in natural gas-bearing sedimentary aquifers

Abstract. The majority of Earth’s prokaryotes live under the deep sedimentary biosphere. Geochemical processes driven by geothermal heating may play a crucial role in fueling deep subsurface microbial biomass and activities, yet their full breadth remains uncaptured. Here, we investigated the microbial community composition and metabolism in microbial natural gas-bearing aquifers at temperatures ranging from 35−80 °C, situated above nonmicrobial gas and oil-bearing sediments at temperatures exceeding 90 °C. Cultivation-based and molecular gene sequencing analyses, including radiotracer measurements, of formation water indicated variations in predominant methanogenic pathways across different temperature regimes of upper aquifers: high potential for hydrogenotrophic/methylotrophic, hydrogenotrophic and acetoclastic methanogenesis at depths with mesophilic, thermophilic and hyperthermophilic temperatures, respectively. The potential for acetoclastic methanogenesis correlated with elevated acetate concentrations with increasing depth, possibly due to the thermal decomposition of sedimentary organic matter. In addition to acetoclastic methanogenesis, in aquifers with hyperthermophilic temperatures, acetate is potentially utilized by microorganisms responsible for the dissimilatory reduction of sulfur compounds other than sulfate because of its high relative abundance at greater depths. The stable sulfur isotopic analysis of sulfur compounds in water and oil samples suggested that hydrogen sulfide generated through the thermal decomposition of sulfur compounds in oil migrates upward and is subsequently oxidized with iron oxides present in sediments, yielding elemental sulfur and thiosulfate. These compounds are consumed by sulfur-reducing microorganisms, possibly reflecting elevated microbial populations in aquifers with hyperthermophilic temperatures. These findings reveal previously overlooked geothermal heat-driven geochemical and microbiological processes involved in carbon and sulfur cycling in the deep sedimentary biosphere.