CO2 concentration and pH alters subsurface microbial ecology at reservoir temperature and pressure

Abstract

The security of long-term CO₂ storage following geologic carbon sequestration may be impacted by biogeochemical reactions in the formation; yet little understanding exists about the impact of CO₂ gradients on microorganisms that drive biogeochemistry in the deep subsurface. The effect of CO₂ gradients on the microbial community from a brine aquifer was examined at reservoir pressure (14 MPa) and temperature (40 °C). The community was exposed to pCO₂ at 0, 0.1, 1.4 and 14 MPa for up to 56 days and was examined using 16S rRNA gene clone libraries and qPCR. Diversity indices (equitability) were also determined. In addition, the effect of lowered pH without CO₂ exposure was examined. Exposure to CO₂ resulted in a decrease in microbial diversity and a decrease in 16S rRNA gene concentrations. After 56 days, no 16S genes were recovered following exposure to 1.4 MPa pCO₂ or greater. Exposure to 0.1 MPa pCO₂ resulted in 16S gene concentrations an order of magnitude less than 0 MPa pCO₂. The equitability of 0 MPa exposure (0.54) decreased with 0.1 MPa exposure to 0.29. Microbial community with the lowered pH (pH = 4.4) and 0 MPa pCO₂ resulted in 5 orders of magnitude increased 16S gene concentration than reactors with CO₂ exposure, indicating that CO₂ was detrimental independent of pH. This suggests that even buffered reservoirs may have a decreased microbial population due to injected CO₂. Halotolerant strains Halomonas and Marinobacter appeared to be the most tolerant to CO₂ exposure and decreased pH. This is the first study to examine the initial microbial community response to a gradient of CO₂ that would follow geologic carbon sequestration.