Microbial-geochemical interactions in underground reservoirs: Implications for hydrogen storage

Background 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. Results Porous reservoirs, particularly those with carbonate facies, are characterized by carbonate-rich fluids and host anaerobic microbial communities dominated by methanogenic archaea, syntrophic fermenters, and thermophiles. These communities support potential functions such as methanogenesis, hydrogen-producing fermentation, and selenate reduction. In contrast, salt caverns contain salt-saturated brines and are dominated by halophilic and generalist microorganisms adapted to oligotrophic, high-salinity conditions, mainly associated with nitrate reduction and acetate oxidation. Some reservoirs showed atypical microbial profiles influenced by anthropogenic activities or hydrogeological connections, which alter local geochemistry and microbial structures. Conclusion This study demonstrates a clear distinction between salt caverns and porous reservoirs, highlighting the influence of salinity, carbon availability, temperature, and human impact on microbial community structure and functional potential. The presence of hydrogenotrophic and spore-forming 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.