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 δ¹³C 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.