Microbial carbonate biomineralization in the deep subsurface biosphere: Decoding long-term carbon sequestration...

Geological carbon sequestration represents a cornerstone mitigation strategy for stabilizing atmospheric carbon dioxide (CO₂) concentrations and limiting climate impacts from anthropogenic emissions. Its long-term effectiveness depends on geochemical and biological processes within deep subsurface environments, where microbial activity regulates CO₂ transformation and stability. Among these processes, calcium carbonate biomineralization and methanogenesis are particularly important because they modify pH, alkalinity, redox conditions, and the availability of divalent cations, thereby promoting carbonate mineral precipitation and influencing the fate of injected CO₂. Within the deep subsurface biosphere, microbial communities interact with porous media where pore architecture, fluid flow, and geochemical gradients strongly influence metabolic activity and biomineralization potential. Microbial carbonate precipitation in these environments is associated with anaerobic pathways such as sulfate reduction, urea hydrolysis, and anaerobic methane oxidation, which generate alkalinity and bicarbonate and create conditions favorable for calcium carbonate (CaCO₃) formation. These microbial pathways, together with other carbon-transforming processes such as methanogenesis, acetogenesis, and fermentation, collectively shape carbonate formation within the subsurface carbon cycle and influence whether CO₂ is ultimately retained as mineral carbonates or converted into methane. However, injection of high CO₂ concentrations can shift subsurface pH, redox potential, and ionic strength, disrupting biomineralization and enhancing methanogenesis. These perturbations reshape pore-scale carbon dynamics and increase the risk of CO₂ release as methane, underscoring microbial-geochemical interactions as fundamental controls on long-term carbon sequestration outcomes. This review highlights the role of coupled microbial pathways and geolocial conditions in shaping the fate of CO₂, reinforcing their broader significance for ensuring the long-term effectiveness of carbon storage as a climate change mitigation strategy.