Metagenome/-transcriptome Study of Microbial Life Under Extreme CO2. Metagenome/-transcriptome Study Wetland and Mofette Soil

Underground injection of CO2 is a frequently discussed option to translocate anthropogenic CO2 emissions, but the environmental consequences of leakages are poorly understood. As a natural analogue, we studied a floodplain wetland area where localized long-term emanations of cold volcanic CO2 (mofettes) altered soil formation and led to stable hypoxia, as well as increased soil C storage (up to 47.3 w.-% C). We hypothesized that CO2 degassing is associated with functional changes in the mofette soil biome, resulting in increased C input and/or impeded mineralization. With a multi-pronged approach, involving depth-resolved C isotope geochemistry, soil activity, metatranscriptome and metagenome analysis, we identified significant differences in mofette C and energy flow compared to an unaffected floodplain wetland soil. Radiocarbon analysis revealed that high quantities of mofette soil C originated from the assimilation of Earth mantle derived CO2 (up to 67% 'dead' soil C) via plant primary production and subsurface CO2 fixation. 13CO2 labeling incubations with high p(CO2) suggested that a substantial fraction of soil C inputs could be derived from dark CO2 fixation in the mofette soil. However, this C input alone was unlikely to account for soil organic matter (SOM) accumulation in the mofette. Surprisingly, levels of activity estimators (CO2 formation and hydrolytic exoenzyme activities), as well as richness and abundance of carbohydrate active enzyme (CAZY) transcripts were similar in mofette and reference soils, indicating an unimpaired biochemical potential for mofette SOM decomposition. In contrast, we observed decreased richness of taxonomic groups and biochemical functions in the mofette. Especially the almost complete absence of Metazoa, predatory protists and putative ectomycorrhizal fungi suggested that associated functions, e.g. physical breakdown of litter, and trophic interactions, were severely impaired. We conclude that longterm exposure to high CO2 reduces soil food web complexity to levels of primary production and consumption, and therefore the efficiency of litter and OM transformation and mineralization.