Microbial Life Under Extreme CO2

Effects of extremely high CO2 concentrations on soil microbial communities and associated processes are largely unknown. We studied a wetland area affected by spots of subcrustal CO2 degassing (mofettes) with focus on anaerobic autotrophic methanogenesis and acetogenesis since pore gas phase was largely hypoxic. Compared with a reference soil, the mofette was more acidic (?pH ~0.8), strongly enriched in organic carbon (up to 10 times), and exhibited lower prokaryotic diversity. It was dominated by methanogens and subdivision 1 Acidobacteria, which likely thrived under stable hypoxia and acidic pH. Anoxic incubations revealed enhanced formation of acetate and methane from hydrogen and CO2 consistent with elevated methane and acetate levels in the mofette soil. 13CO2 mofette soil incubations showed high label incorporations with ~512 ng 13C g dry soil-1 d-1 into the bulk soil and up to 10.7 ng 13C g dry soil-1 d-1 into 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 with Methanoregulaceae and hitherto unknown acetogens appeared to be involved in the chemolithoautotrophic utilization of 13CO2. Subdivision 1 Acidobacteriaceae assimilated 13CO2 likely via anaplerotic reactions since Acidobacteriaceaea are not known to harbor enzymatic pathways for autotrophic CO2 assimilation. We conclude that CO2-induced geochemical changes promoted anaerobic and acidophilic organisms and altered carbon turnover in affected soils.