Microbial-geochemical Interactions in Underground Reservoirs: Implications for Hydrogen Storage.

Understanding microbial life in deep geological environments is essential for assessing natural biogeochemical processes and their implications for subsurface activities such as underground hydrogen storage. This study investigates the water chemistry, microbial community composition, and metabolic potential of two contrasting geological settings: porous reservoirs and salt caverns. This study is among the first to provide a large-scale comparison of microbial and geochemical processes across reservoirs. Our results show that porous reservoirs, particularly those with carbonate facies, are characterized by carbonate-rich fluids and host anaerobic microbial communities dominated by methanogenic archaea, fermentative bacteria, and acetogens. In contrast, the studied salt caverns contain salt-saturated brines and are dominated by halophilic and chemoorganotrophic microorganisms adapted to oligotrophic, high-salinity conditions. Some reservoirs showed atypical microbial profiles influenced by anthropogenic activities or hydrogeological connections, which alter local geochemistry and microbial structures. This study highlights the influence of salinity, carbon availability, temperature, and human impact on microbial community structure and metabolic functional potential. The presence of hydrogenotrophic microorganisms raises concerns about their potential to consume injected hydrogen and alter gas quality. These findings underscore the need to move beyond detection toward activity-based assessments of microbial hydrogen consumption, in order to anticipate biogeochemical dynamics and develop effective mitigation strategies. A better understanding of the interplay between reservoir chemistry, geology, and microbiology is crucial for ensuring safe and efficient underground hydrogen storage.