The capture and geological storage of CO
2 can be used to reduce anthropogenic greenhouse gas emissions. To assess the environmental impact of potential CO
2 leakage from deep storage reservoirs on the abundance and functional diversity of microorganisms in near-surface terrestrial environments, a natural CO
2 vent (>90% CO
2 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 CO
2-rich vent and a reference site with a normal CO
2 concentration were detected. The δ
13C values of the plant and microbial lipids within the CO
2 vent demonstrate that substantial amounts of geothermal CO
2 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 CO
2 vent. Lipid biomarker analyses, Q-PCR, and the determination of microbial activities showed the presence of CO
2-utilising methanogenic
Archaea,
Geobacteraceae, and sulphate-reducing
Bacteria (SRB) mainly at the CO
2 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 CO
2 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 CO
2 concentrations.
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