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Distribution 2
Microorganisms are ubiquitous in the atmosphere and reach concentrations of up to 106 microbial cells per cubic meter of air. Due to their important roles in public health and meteorological processes,    understanding how airborne microbial communities are distributed over time and space is critical. While the concentration and taxonomic diversity of airborne microbial communities in the planetary boundary layer have recently been described, the functional potential of airborne microbial communities remains unknown. Most studies have focused on laboratory cultivation to identify possible metabolic functions of microbial strains of atmospheric origin, mainly from cloud water. Given that cultivable organisms represent about 1 % of the entire microbial community, culture-independent techniques and especially metagenomic studies applied to atmospheric microbiology have the potential to provide additional information on the selection and genetic adaptation of airborne microorganisms. However, to our knowledge, only five metagenomic studies on airborne microbial communities at one or two specific sites per study exist. Metagenomic investigations of complex microbial communities in many ecosystems (for example, soil, seawater, lakes, feces and sludge) have provided evidence that microorganism functional signatures reflect the abiotic conditions of their environment, with different relative abundances of specific microbial functional classes. This observed correlation of microbial-community functional potential and the physical and chemical characteristics of their environments could have resulted from genetic modifications (microbial adaptation   ) and/or physical selection. The latter refers to the death of sensitive cells and the survival of resistant or previously adapted cells. This physical selection can occur when microorganisms are exposed to physiologically adverse conditions.
 * understanding how airborne microbial communities are distributed over time and space is critical

The presence of a specific microbial functional signature in the atmosphere has not been investigated yet. Microbial strains of airborne origin have been shown to survive and develop under conditions typically found in cloud water (i.e., high concentrations of H2O2, typical cloud carbonaceous sources, ultraviolet – UV – radiation etc. While atmospheric chemicals might lead to some microbial adaptation, physical and unfavorable conditions of the atmosphere such as UV radiation, low water content and cold temperatures might select which microorganisms can survive in the atmosphere. From the pool of microbial cells being aerosolized from Earth's surfaces, these adverse conditions might act as a filter in selecting cells already resistant to unfavorable physical conditions. Fungal cells and especially fungal spores might be particularly adapted to survive in the atmosphere due to their innate resistance and might behave differently than bacterial cells. Still, the proportion and nature (i.e., fungi versus bacteria) of microbial cells that are resistant to the harsh atmospheric conditions within airborne microbial communities are unknown.