colonization efficiency of a species. while larger animals can cover distances on their own and actively seek suitable habitats, small (<2 mm) organisms are often passively dispersed, resulting in their more ubiquitous occurrence. while active dispersal accounts for rather predictable distribution patterns, passive dispersal leads to a more randomized immigration of organisms. mechanisms for passive dispersal are the transport on (epizoochory) or in (endozoochory) larger animals (e.g., flying insects, birds, or mammals) and the erosion by wind. a propagule is any material that functions in propagating an organism to the next stage in its life cycle, such as by dispersal. the propagule is usually distinct in form from the parent organism. propagules are produced by plants (in the form of seeds or spores), fungi (in the form of spores), and bacteria (for example endospores or microbial cysts). often cited as an important requirement for effective wind dispersal is the presence of propagules (e.g., resting eggs, cysts, ephippia, juvenile and adult resting stages), which also enables organisms to survive unfavorable environmental conditions until they enter a suitable habitat. these dispersal units can be blown from surfaces such as soil, moss, and the desiccated sediments of temporary or intermittent waters. the passively dispersed organisms are typically pioneer colonizers. however, wind-drifted species vary in their vagility (probability to be transported with the wind), with the weight and form of the propagules, and therefore, the wind speed required for their transport, determining the dispersal distance. for example, in nematodes, resting eggs are less effectively transported by wind than other life stages, while organisms in anhydrobiosis are lighter and thus more readily transported than hydrated forms. because different organisms are, for the most part, not dispersed over the same distances, source habitats are also important, with the number of organisms contained in air declining with increasing distance from the source system. the distances covered by small animals range from a few meters, to hundreds, to thousands of meters. while the wind dispersal of aquatic organisms is possible even during the wet phase of a transiently aquatic habitat, during the dry stages a larger number of dormant propagules are exposed to wind and thus dispersed. freshwater organisms that must "cross the dry ocean" to enter new aquatic island systems will be passively dispersed more successfully than terrestrial taxa. numerous taxa from both soil and freshwater systems have been captured from the air (e.g., bacteria, several algae, ciliates, flagellates, rotifers, crustaceans, mites, and tardigrades). while these have been qualitatively well studied, accurate estimates of their dispersal rates are lacking. clouds impact of microbial activity on clouds biological processes and their targets are indicated by green arrows, while red arrows indicate abiotic processes. eps: exopolysaccharide soa: secondary organic aerosolbased on coordinated metagenomics/metatranscriptomics the outdoor atmosphere harbors diverse microbial assemblages composed of bacteria, fungi and viruses whose functioning remains largely unexplored. while the occasional presence of human pathogens or opportunists can cause potential hazard, in general the vast majority of airborne microbes originate from natural environments like soil or plants, with large spatial and temporal variations of biomass and biodiversity. once ripped off and aerosolized from surfaces by mechanical disturbances such as those generated by wind, raindrop impacts or water bubbling, microbial cells are transported upward by turbulent fluxes. they remain aloft for an average of ~3 days, a time long enough for being transported across oceans and continents until being finally deposited, eventually helped by water condensation and precipitation processes; microbial aerosols themselves can contribute to form clouds and trigger precipitation by serving as cloud condensation nuclei and ice nuclei. living airborne microorganisms may end up concretizing aerial dispersion by colonizing their new habitat, provided that they survive their journey from emission to deposition. bacterial survival is indeed naturally impaired during atmospheric transport, but a fraction remains viable. at high altitude, the peculiar environments offered by cloud droplets are thus regarded in some aspects as temporary microbial habitats, providing water and nutrients to airborne living cells. in addition, the detection of low levels of heterotrophy raises questions about microbial functioning in cloud water and its potential influence on the chemical reactivity of these complex and dynamic environments. the metabolic functioning of microbial cells in clouds is still albeit unknown, while fundamental for apprehending microbial life conditions during long distance aerial transport and their geochemical and ecological impacts. aerosols affect cloud formation, thereby influencing sunlight irradiation and precipitation, but the extent to which and the manner in which they influence climate remains uncertain. marine aerosols consist of a complex mixture of sea salt, non-sea-salt sulfate and organic molecules and can function as nuclei for cloud condensation, influencing the radiation balance and, hence, climate. for example, biogenic aerosols in remote marine environments (for example, the southern ocean) can increase the number and size of cloud droplets, having similar effects on climate as aerosols in highly polluted regions. specifically, phytoplankton emit dimethylsulfide, and its derivate sulfate promotes cloud condensation. understanding the ways in which marine phytoplankton contribute to aerosols will allow better predictions of how changing ocean conditions will affect clouds and feed back on climate. in addition, the atmosphere itself contains about 1022 microbial cells, and determining the ability of atmospheric microorganisms to grow and form aggregates will be valuable for assessing their influence on climate. after the tantalizing detection of phosphine (ph3) in the atmosphere of the venus planet, and in the absence of a known and plausible chemical mechanism to explain the formation of this molecule, greaves et al. speculated in 2020 that microorganisms might be present in suspension in the venus atmosphere. they have formulated the hypothesis of the microbial formation of phosphine, envisaging the possibility of a liveable window in the venusian clouds at a certain altitude with an acceptable temperature range for microbial life. however, in 2021 hallsworth et al. examined the conditions required to support the life of extremophile microorganisms in the clouds at high altitude in the venus atmosphere where favorable temperature conditions might prevail. beside the presence of sulfuric acid in the clouds which already represent a major challenge for the survival of most of microorganisms, they came to the conclusion that the venus atmosphere is too dry to host microbial life. they determined a water activity ≤ 0.004, two orders of magnitude below the 0.585 limit for known extremophiles. airborne microbiomes