Energetic constraints to the survival and activity of microbial life in Earth’s atmosphere

The atmosphere could be one of Earth’s largest and most interconnected ecosystems. It is an environment characterized by low temperatures, low nutrient availability, aridity, and high ultraviolet radiation. Nevertheless, research in cold, arid and oligotrophic extreme environments have demonstrated that the conditions of the atmosphere are within the boundaries currently considered to support microbial life. Moreover, certain microorganisms are capable of gaining energy by oxidizing atmospheric gasses (hydrogen (H₂), carbon monoxide (CO) and methane (CH₄)) at trace concentrations. Measurement and experimental investigations of the atmospheric microbiome (the aeromicrobiome) are extremely challenging due to the compounding difficulties of low biomass samples, contamination issues, and lack of standardized sampling procedures. Numerical modelling can advance aeromicrobiology research by providing a complementary means to evaluate the habitability of the atmosphere and the potential activity of atmospheric microorganisms. We developed a theoretical framework combining the state-of-the-art knowledge of potential atmospheric-dwelling microorganisms, thermodynamic principles, and global climate and atmospheric gas composition data from MERRA2 (Modern-Era Retrospective analysis for Research and Applications, v2) and CAMS (Copernicus Atmosphere Monitoring Service). Our modelling analysis demonstrates that hydrogen oxidation, carbon monoxide oxidation, and methane oxidation are energy yielding catabolisms (ΔG_r < 0, i.e. thermodynamically feasible) under atmospheric conditions, throughout the entire troposphere, all year round. It is therefore possible that these catabolisms are a viable source of energy to microorganisms in the atmosphere.  We also reveal spatially and temporal energetic ‘hot spots’ where catabolic energy yield is greater, due to localized atmospheric gas concentrations and temperatures. In addition to supplying energy, atmospheric methane oxidation and hydrogen oxidation generate water as a catabolic byproduct, potentially alleviating limitations to microbial survival and activity that are imposed by the extreme aridity of the atmosphere. Theoretical modelling can accelerate aerobiology research by generating theory-informed hypotheses about which microbial cohorts are more probable to be metabolically active in the Earth’s atmosphere and guiding experimental research to where and how we may find and study them.