Microbial responses to CO₂ during carbon sequestration : insights into an unexplored extreme environment
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When CO₂ is sequestered into deep saline aquifers, significant changes to the biogeochemistry of the system are inevitable and will affect native microbial populations both directly and indirectly. These communities are important as they catalyze many geochemical reactions in these reservoirs. We present evidence that the injection of CO₂ will cause a large scale disturbance to subsurface microbial populations which will ultimately affect the solution and mineral trapping of CO₂ as well as the movement of CO₂ charged water through the subsurface. Representative subsurface microorganisms including a Gram negative bacterium (G⁻), two Gram positive bacteria (G⁺), and an archaeon were tested for CO₂ survival at pressures up to 50 bar and exposure times up to 24 hours. CO₂ tolerance varied but shows effects on microbes is more complex than just decreasing pH and is not significantly dependent on cell wall structure. Imaging reveals that CO₂ disrupts the cytoplasm possibly from changes to intracellular pH. The geochemical effect of CO₂ stress is a decrease in metabolic activity such as Fe reduction and methanogenesis. Subsurface microbial populations interact with the surrounding reservoir minerals which likely influence their ability to survive under CO₂ stress. When the G⁻ organism was grown in the presence of a mineral substrate, survival depended on the mineral type. Quartz sandstone provided a good substrate for survival while kaolinite provided a poor substrate for survival. Biofilms on quartz sandstone were rich in extracellular polymeric substances (EPS) that likely act as a barrier to slow the penetration of CO₂ into the cell. The release of toxic metals from mineral dissolution at high PCO₂ enhanced cell death. To understand the long term effects of CO₂ on microbial communities, water samples were taken from CO₂ springs in the western United States and compared to unaffected springs. Community 16S rRNA sequence data suggests that CO₂ exposed environments exhibit lower microbial diversity, suggesting environmentally stressed communities. However, differences among diversity in the springs surveyed also indicates other environmental factors that affect diversity beyond CO₂. Furthermore, the isolation of a novel fermentative Lactobacillus strain from a CO₂ spring, indicates viable microbial communities can exist at high PCO₂.