Effect of calcite on dihydrogen consumption by microbial communities in a deep aquifer used as a UGS

The microbial and geochemical dynamics of dihydrogen (H 2 ) storage in deep aquifer underground gas storage (UGS) systems were investigated experimentally using calcite as the sole inorganic carbon source, in the absence of CO 2 . Results demonstrate that calcite sustains lithoautotrophic microbial communities capable of H 2 consumption, although rates are lower compared to CO 2 -amended systems, with inorganic carbon availability limiting microbial activity. Microbial activity causes pH alkalinization after sulfate depletion, stimulating methanogenesis. Mineral analyses reveal calcite dissolution and secondary precipitation of iron sulfides (including pyrrhotite) contribute to pH buffering. Despite CO 2 -limitation, the microbial communities remained stable and metabolically active, with sulfate-reducing bacteria and methanogens adapting over time. These findings suggest that in carbonate-rich reservoirs, geochemical and microbial processes can impact the long-term evolution of underground hydrogen storage. Understanding these interactions is essential for developing sustainable UGS solutions, minimizing environmental risks, and ensuring secure management of deep energy reserves. • The high-pressure reactor simulation with H 2 influx into a carbonated UGS showed a H 2 loss of about 3.7 mmol∙day −1 . • Even without initial CO 2 , microbial lithoautotrophic activity was seen, first producing formate and later methane. • The microbial activity led to phenomena of precipitation and dissolution of carbonates, accompanied by changes in pH.