Figure 2.

Defining the atmospheric ecological niche. (A) The atmosphere comprises a series of vertically delineated concentric layers that are primarily defined by thermal properties. The troposphere contains 75% of the molecular and gaseous mass of the atmosphere and almost all of the water vapour, clouds and aerosolized particles including microorganisms. (B) Four boundaries that occur at varying altitude within the troposphere are important in terms of atmospheric microbial ecology: The atmospheric boundary layer delineates the region of air closest to the surface where the bulk of surface–atmosphere interactions occur, and this includes exchange of microorganisms with terrestrial and marine ecosystems. The lifted condensation level (LCL) delineates the layer above which cloud formation occurs. It is variable with temperature and humidity and delineates the thermal limit for condensation of saturated water vapour on particulate surfaces. The freezing isotherm delineates the layer above which freezing occurs and is also the limit for active non-psychrophilic microbial metabolism. We note that the relationship between the atmospheric boundary layer, the LCL and the freezing isotherm, which are highly dynamic, is significantly more complex than presented in the figure, e.g. both the LCL and the freezing isotherm can sit at ground level well within the boundary layer. The boundary between the LCL and freezing isotherm represents the theoretical range for mesophilic microbial metabolic activity, with potential for psychrophilic and xerophilic activity to extend beyond this zone. The tropopause marks the upper limit of the troposphere and mixing with upper layers is limited by the thermal inversion in the stratosphere. The bulk of atmospheric ozone is located in the lower stratosphere and this likely delineates the altitudinal limit of survival for many atmospheric microorganisms. Blue shapes denote biotic attributes and orange boxes denote physical attributes.