Role of subterranean microbiota in the carbon cycle and greenhouse gas dynamics.

Subterranean ecosystems play an active role in the global carbon cycle, yet only a few studies using indirect methods have focused on the role of the cave microbiota in this critical cycle. Here we present pioneering research based on in situ real-time monitoring of CO and CH diffusive fluxes and concurrent δC geochemical tracing in caves, combined with 16S microbiome analysis. Our findings show that cave sediments are promoting continuous CH consumption from cave atmosphere, resulting in a significant removal of 65% to 90%. This research reveals the most effective taxa and metabolic pathways in consumption and uptake of greenhouse gases. Methanotrophic bacteria were the most effective group involved in CH consumption, namely within the families Methylomonaceae, Methylomirabilaceae and Methylacidiphilaceae. In addition, Crossiella and Nitrosococcaceae wb1-P19 could be one of the main responsible of CO uptake, which occurs via the Calvin-Benson-Bassham cycle and reversible hydration of CO. Thus, syntrophic relationships exist between Crossiella and nitrifying bacteria that capture CO, consume inorganic N produced by heterotrophic ammonification in the surface of sediments, and induce moonmilk formation. Moonmilk is found as the most evolved phase of the microbial processes in cave sediments that fixes CO as calcite and intensifies CH oxidation. From an ecological perspective, cave sediments act qualitatively as soils, providing fundamental ecosystem services (e.g. nutrient cycling and carbon sequestration) with direct influence on greenhouse gas emissions.