The Atmosphere: Its Developmental History and Contributions to Microbial Evolution and Habitat
There is a long-standing preconception in ecology that both landmasses and bodies of water constitute habitats for living systems whereas the atmosphere does not; this preconception, however, is now beginning to change. In recent years, there has been a resurgence of interest in the ecology of living organisms found in the atmosphere. This chapter is an account of Earth's past and present-day atmospheres as habitats for microorganisms, starting with the origin of a primary atmosphere that was completely inhospitable to any life. The developmental history of the atmosphere is considered in detail, including its co-evolution with early living systems to create the only oxygen-containing atmosphere in the Solar System and also its contributions to early microbial evolution throughout the Precambrian Supereon. The composition and physical structure of the present-day atmosphere are also considered in the context of a microbial habitat, especially as they relate to the metabolic and reproductive activities of airborne microorganisms and the movement of vast numbers of microorganisms through the troposphere by global atmospheric circulation. It is this movement that is largely responsible for the worldwide dispersal of microorganisms and connecting all microbial habitats across the Planet's surface to one another. Additionally, it is the most straightforward means of accounting for the cosmopolitan distributions reported for many microbial species and, as recently suggested, might possibly give rise to biogeographic regions in the atmosphere, currently an ongoing debate among many microbiologists.