A look into the virosphere of clouds: A world yet to be explored

Clouds are aqueous atmospheric systems hosting diverse and active microorganisms. Viruses may also persist despite harsh conditions, support active viral infections, and contribute to microbial dynamics during aerial transport. However, assessing viruses, and even more, virus-bacteria interactions in the atmospheric environment is highly challenging, and knowledge remains very limited. Here, based on current knowledge in cloud microbiology, we estimate the cloud virome at ∼10²¹ virus particles globally. One out of a million of cloud droplets is susceptible to host virus-bacteria interactions, which represents considerable volume at global scale, and we discuss potential implications for microbial ecology. We finally propose future research directions to explore further the cloud virosphere and its ecological roles.