Subsurface biosphere: what controls its composition?

Understanding the link between environmental conditions and microorganisms is essential to determine the physicochemical limits of life in the subsurface and its response to climatic and anthropogenic forcing. In this presentation, I'll use thermodynamics to show how the chemical and taxonomic composition of microbial communities is shaped by redox conditions. For microbial metabolisms to be competitive in low-energy environments, the energetic cost of biomolecule synthesis must be minimal. To test this hypothesis in the anoxic soils of peatlands, a thermodynamic model was developed to assess the link between redox conditions and the chemical composition of proteins inferred from metagenomic sequencing. Calculations show that the carbon oxidation state of proteins decreases with depth and increasing reducing conditions. The redox potential optimizing the chemical stability of proteins is comparable to in-situ measurements. Thus, the degree of protein oxidation reflects the redox conditions to which microorganisms have adapted. The correlation between protein oxidation state and microbial abundances indicates that minimizing the energetic cost of anabolism not only shapes the chemical composition of microorganisms, but also its taxonomic composition. This work sheds new light on the mechanisms that control subsurface microbial life and its evolution under changing environments.