Carbon flow from volcanic CO2 into soil microbial communities of a wetland mofette

Effects of extremely high carbon dioxide (CO₂) concentrations on soil microbial communities and associated processes are largely unknown. We studied a wetland area affected by spots of subcrustal CO₂degassing (mofettes) with focus on anaerobic autotrophic methanogenesis and acetogenesis because the pore gas phase was largely hypoxic. Compared with a reference soil, the mofette was more acidic (ΔpH ∼0.8), strongly enriched in organic carbon (up to 10 times), and exhibited lower prokaryotic diversity. It was dominated by methanogens and subdivision 1Acidobacteria, which likely thrived under stable hypoxia and acidic pH. Anoxic incubations revealed enhanced formation of acetate and methane (CH₄) from hydrogen (H₂) and CO₂consistent with elevated CH₄and acetate levels in the mofette soil.¹³CO₂mofette soil incubations showed high label incorporations with ∼512 ng¹³C g (dry weight (dw)) soil⁻¹ d⁻¹into the bulk soil and up to 10.7 ng¹³C g (dw) soil⁻¹ d⁻¹into almost all analyzed bacterial lipids. Incorporation of CO₂-derived carbon into archaeal lipids was much lower and restricted to the first 10 cm of the soil. DNA-SIP analysis revealed that acidophilic methanogens affiliated withMethanoregulaceaeand hitherto unknown acetogens appeared to be involved in the chemolithoautotrophic utilization of¹³CO₂. Subdivision 1Acidobacteriaceaeassimilated¹³CO₂likely via anaplerotic reactions becauseAcidobacteriaceaeare not known to harbor enzymatic pathways for autotrophic CO₂assimilation. We conclude that CO₂-induced geochemical changes promoted anaerobic and acidophilic organisms and altered carbon turnover in affected soils.