Effect of CO₂ on biogeochemical reactions and microbial community composition in bioreactors with deep groundwater and basalt

Changes in subsurface microbiology following CO₂ injection have the potential to impact carbon trapping in CO₂ storage reservoirs. However, much remains to be learned about responses of natural microbial consortia to elevated CO₂ in basaltic systems. This study asks: how will microbes from deep (700 m) groundwater change along a gradient in CO₂ (0–20 psi) in batch reactor systems containing basalt chips and groundwater amended with lactate? Reactors incubated for 87 days at 23 °C. Results for reactors with low CO₂ (0 and 3 psi) differed considerably from those with high CO₂ (10 and 20 psi). In reactors with low CO₂, pH was >6.5 and lactate started to be used within 24 days. By 40 days, lactate was completely consumed and acetate increased to ~4 mM. As lactate was consumed, sulfate decreased from 0.16 to 0 mM after 40 days. In contrast, in reactors with high CO₂, pH was <6.5, lactate and sulfate concentrations varied little and acetate was not produced. Biogeochemical modeling and community analyses indicate that differences between reactors with low and high CO₂ reflect tolerances of reactor microbes to CO₂ exposure. Communities in the low CO₂ reactors carried out syntrophic lactate oxidation coupled with methanogenesis and sulfate reduction. Bacteroidota and Firmicutes became dominant phyla after 24 days and groups capable of sulfate reduction and methanogenesis were detected. In reactors with high CO₂, however, biogeochemical activity was insignificant, no groups capable of sulfate reducion or methanogenesis were observed, and the community became less diverse during the incubation. These findings show that the response of microbial consortia can vary sharply along a CO₂ gradient, creating significant differences in community composition and biogeochemistry, and that the timescale of basalt weathering is likely not rapid enough to prevent significant stress following a rapid increase in CO₂ abundance.