Deep subsurface formations (for example, high-temperature oil reservoirs) are candidate sites for carbon capture and storage technology. However, very little is known about how the subsurface microbial community would respond to an increase in CO 2 pressure resulting from carbon capture and storage. Here we construct microcosms mimicking reservoir conditions (55C, 5 MPa) using high-temperature oil reservoir samples. Methanogenesis occurs under both high and low CO 2 conditions in the microcosms. However, the increase in CO 2 pressure accelerates the rate of methanogenesis to more than twice than that under low CO 2 conditions. Isotope tracer and molecular analyses show that high CO 2 conditions invoke acetoclastic methanogenesis in place of syntrophic acetate oxidation coupled with hydrogenotrophic methanogenesis that typically occurs in this environment (low CO 2 conditions). Our results present a possibility of carbon capture and storage for enhanced microbial energy production in deep subsurface environments that can mitigate global warming and energy depletion.