Soil microbial community changes as a result of long-term exposure to a natural CO2 vent

The capture and geological storage of CO2 can be used to reduce anthropogenic greenhouse gas emissions. To assess the environmental impact of potential CO2 leakage from deep storage reservoirs on the abundance and functional diversity of microorganisms in near-surface terrestrial environments, a natural CO2 vent (>90% CO2 in the soil gas) was studied as an analogue. The microbial communities were investigated using lipid biomarkers combined with compound-specific stable carbon isotope analyses, the determination of microbial activities, and the use of quantitative polymerase chain reactions (Q-PCR). With this complementary set of methods, significant differences between the CO2-rich vent and a reference site with a normal CO2 concentration were detected. The d13C values of the plant and microbial lipids within the CO2 vent demonstrate that substantial amounts of geothermal CO2 were incorporated into the microbial, plant, and soil carbon pools. Moreover, the numbers of Archaea and Bacteria were highest at the reference site and substantially lower at the CO2 vent. Lipid biomarker analyses, Q-PCR, and the determination of microbial activities showed the presence of CO2-utilising methanogenic Archaea, Geobacteraceae, and sulphate-reducing Bacteria (SRB) mainly at the CO2 vent, only minor quantities were found at the reference site. Stable carbon isotopic analyses revealed that the methanogenic Archaea and SRB utilised the vent-derived CO2 for assimilatory biosynthesis. Our results show a shift in the microbial community towards anaerobic and acidophilic microorganisms as a consequence of the long-term exposure of the soil environment to high CO2 concentrations.