Clouds: A Transient and Stressing Habitat for Microorganisms

In this chapter, we synthesized the current knowledge about clouds as ecosystems which have been discovered very recently. First, we briefly described the cloud habitat. Cloud physics chemistry and microphysics are described, showing that this environment is extreme. Microorganisms are exposed to a dynamic medium changing extremely rapidly (evaporation/condensation of the cloud droplets, quick temperature and pressure changes, freeze/thaw cycle) and also to chemical stresses (strong oxidants, acidic pHs and toxics). Then the life cycle of microorganisms in the atmosphere is detailed showing that cloud is a transient habitat: microorganisms are aerosolized, transported in the air, integrated in cloud droplets and deposited back to the ground with precipitation. Finally the cloud microbiome is described; it appears that it remains largely unknown and based mainly on culture techniques. In the second part of the chapter, the abilities of these microorganisms to survive in this stressing environment are described in details. Microbes can adapt their metabolism as it was shown that the majority of the community is metabolically active and that they metabolize organic compounds in cloud water. They have also developed general strategies that help resisting to atmospheric constraints, such as the production of extracellular polymeric substances and pigments, or the formation of spores. Finally they can respond to specific stresses such as oxidative, osmotic and temperature stresses thanks to protecting metabolites such as osmo- and thermo-protectants, anti-oxidants or by using specific enzymes.