Microorganisms in the air through the lenses of atmospheric chemistry and microphysics

Microorganisms in the atmosphere comprise a tiny fraction (~10^-8%) of the Earth’s microbiome. A significant portion of this ‘aeromicrobiome’ consists of bacteria that typically remain airborne for a few days before being returned to the ground through wet or dry deposition. Unlike bacteria in the other Earth surface spheres (e.g., litho-, hydro-, phyllo-, cryospheres), atmospheric bacteria are aerosolized, residing in individual particles and separated by considerable distances (a few centimeters) from each other. Within these small isolated microcosms, bacteria are exposed to particular chemical and physical conditions that potentially affect their stress levels, survival and general functioning. Using fundamental chemical and microphysical concepts of atmospheric aerosol particles and cloud droplets, we examine these specific environmental conditions. In particular, we challenge the concept of clouds as microbial oases by illustrating the water amounts and time scales inside clouds. In addition, we suggest that the small volumes of cloud droplets may cause greater nutrient limitations but simultaneously reduce oxidative stress compared to other aquatic environments. Various chemical and microphysical factors may act as microbial stressors (e.g., oxidative, osmotic, and UV-induced) in the atmosphere, which may either enhance or diminish the survival and diversity of atmospheric bacteria. Based on established atmospheric chemical and microphysical principles, we discuss that observed trends of bacterial community properties and pollutant concentrations may lead to incorrect interpretations due to confounding factors. In summary, our presentation aims to motivate future experimental and modeling studies to disentangle the complex interplay of chemical and microphysical factors with the atmospheric microbiome. Such studies are important to eventually allow for a comprehensive understanding of the atmosphere’s role in affecting airborne microorganisms, a small yet rapidly evolving component of the Earth’s microbiome.

 

Ervens, B., Amato, P., Aregahegn, K., Joly, M., Khaled, A., Labed-Veydert, T., Mathonat, F., Nuñez López, L., Péguilhan, R., and Zhang, M.: Ideas and perspectives: Microorganisms in the air through the lenses of atmospheric chemistry and microphysics, Biogeosciences, 22, 243–256, https://doi.org/10.5194/bg-22-243-2025, 2025.