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.

地下二氧化碳（CO₂）注入是当前被广泛探讨的人为碳排放转移手段，但学界对其泄漏所引发的环境后果仍缺乏充分认知。作为天然类比研究对象，我们针对一处泛滥平原湿地展开调研：该区域长期存在局地性低温火山型二氧化碳喷溢点（mofettes），此类喷溢改变了土壤形成过程，催生了持续性低氧环境，并使土壤碳储量提升至47.3重量百分比（w.-% C）。我们提出假说：二氧化碳脱气作用与该碳酸泉土壤生物群的功能改变存在关联，进而导致碳输入增加或矿化过程受阻。我们采用多维度研究手段，涵盖深度解析碳同位素地球化学、土壤活性检测、宏转录组（metatranscriptome）与宏基因组（metagenome）分析，对比未受影响的泛滥平原湿地土壤，明确了该区域碳与能量流动存在显著差异。放射性碳分析结果显示，该碳酸泉区域土壤中的大量碳源自通过植物初级生产与地下二氧化碳固定作用同化的地幔来源二氧化碳，此类碳占比最高可达67%的‘惰性’土壤碳。针对高二氧化碳分压条件开展的碳13（¹³CO₂）标记培养实验表明，该碳酸泉土壤的碳输入有相当一部分源自暗二氧化碳固定作用。但仅依靠此类碳输入，不足以解释该区域土壤有机质（SOM）的积累现象。令人意外的是，碳酸泉区域与对照土壤的活性表征指标（二氧化碳生成量与水解胞外酶活性）、碳水化合物活性酶（CAZY）转录本的丰富度与丰度均无显著差异，这表明该区域土壤有机质分解的生化潜力未受损害。与之形成对比的是，我们发现该区域土壤的分类单元与生化功能丰富度均出现下降。尤为显著的是，该区域几乎完全缺失后生动物（Metazoa）、捕食性原生生物与疑似外生菌根真菌，这表明相关生态功能——例如凋落物物理破碎与营养级互作——已遭到严重破坏。综上，长期暴露于高浓度二氧化碳环境会使土壤食物网复杂度降至仅保留初级生产与消费的水平，进而削弱凋落物与有机质转化及矿化的效率。