The Atmospheric Microbiota

The atmosphere carries a diverse and dynamic pool of bacteria, archaea, fungi and viruses as aerosols. Once aerosolized from terrestrial and marine surfaces, air movements can transport them to high altitudes, clouds, and distances of up to thousands of kilometers from their source. During their journey, they likely interact with physical atmospheric processes in the high atmosphere, such as the nucleation of water into cloud droplets and ice crystals, and to influence chemical reactivity through complexation, binding properties and metabolic activity. They finally settle down with dust or precipitation over surface ecosystems, alive as colonizers or competitors with established communities, or dead as genetic material and organic nutrients. Despite its proximity, the atmosphere remains poorly known as an environment that transports living microorganisms and sustains their activity. In turn, the influence of the atmospheric microbiome on atmospheric processes is far from being understood. This Research Topic is intended to fill in some of these knowledge gaps, by gaining better comprehension of the atmosphere as part of the microbial biosphere. Beside the threat to cultures, livestock or human populations related with the presence of pathogens in the air, understanding the temporal and spatial distribution of airborne microbial biodiversity and biomass and their environmental drivers appears as a prerequisite. Potential sources of airborne microorganisms can be deciphered from each other using probabilistic approaches of source apportionment employing microbial tracers. Furthermore, the characterization of aerosolization mechanisms, source strengths and their spatiotemporal variability are also of particular interest, notably for modeling and predicting the aerial dispersion of microorganisms. This can relate with long distance horizontal and vertical transport and with microbial biogeography. Large knowledge gaps also exist in regards to the functioning of microorganisms in the atmosphere and clouds and their interactions with their chemical and physical environment. It is known that some species can catalyze the formation of water droplets and ice crystals, but their impacts on cloud cover and precipitation cycles still need to be examined. In addition, microorganisms can scavenge, bind, utilize or degrade chemical compounds as nutrients or as a means of detoxification, and thus interfere with chemical reactivity in the atmosphere. The associated metabolic functioning and physiological characteristic as well as the consequences for atmospheric chemistry are still largely unexplored. Finally, it is still almost totally unknown how atmospheric transport influences microbial distribution. Microbial viability is largely compromised during aerial transport. The atmosphere thus operates as a strong selective filter, which could promote the development of specific adaptations in certain microorganisms. The microorganisms able to survive their atmospheric journey are the early colonizers of new, potentially distant environments or host organisms. Knowledge concerning the impact of this continuous flow of aerial incomers as competitors or as reservoirs of genetic material on established ecosystems is sparse and requires clarification.