sea, showingtheir relative abundance during two years of dna sequencing. airborne bacteria are emitted by most earth surfaces (plants, oceans, land, and urban areas) to the atmosphere via a variety of mechanical processes such as aeolian soil erosion, sea spray production, or mechanical disturbances including anthropogenic activities. due to their relatively small size (the median aerodynamic diameter of bacteria-containing particles is around 2–4 μm), these can then be transported upward by turbulent fluxes and carried by wind to long distances. as a consequence, bacteria are present in the air up to at least the lower stratosphere. given that the atmosphere is a large conveyor belt that moves air over thousands of kilometers, microorganisms are disseminated globally. airborne transport of microbes is therefore likely pervasive at the global scale, yet there have been only a limited number of studies that have looked at the spatial distribution of microbes across different geographical regions. one of the main difficulties is linked with the low microbial biomass associated with a high diversity existing in the atmosphere outdoor (~102–105 cells/m3) thus requiring reliable sampling procedures and controls. furthermore, the site location and its environmental specificities have to be accounted for to some extent by considering chemical and meteorological variables. the environmental role of airborne cyanobacteria and microalgae is only partly understood. while present in the air, cyanobacteria and microalgae can contribute to ice nucleation and cloud droplet formation. cyanobacteria and microalgae can also impact human health. depending on their size, airborne cyanobacteria and microalgae can be inhaled by humans and settle in different parts of the respiratory system, leading to the formation or intensification of numerous diseases and ailments, e.g., allergies, dermatitis, and rhinitis. according to wiśniewska et al., these harmful microorganisms can constitute between 13% and 71% of sampled taxa. however, the interplay between microbes and atmospheric physical and chemical conditions is an open field of research that can only be fully addressed using multidisciplinary approaches. airborne microalgae and cyanobacteria are the most poorly studied organisms in aerobiology and phycology. this lack of knowledge may result from the lack of standard methods for both sampling and further analysis, especially quantitative analytical methods. few studies have been performed to determine the number of cyanobacteria and microalgae in the atmosphere however, it was shown in 2012 that the average quantity of atmospheric algae is between 100 and 1000 cells per cubic meter of air. as of 2019, about 350 taxa of cyanobacteria and microalgae have been documented in the atmosphere worldwide. cyanobacteria and microalgae end up in the air as a consequence of their emission from soil, buildings, trees, and roofs. biological particles are known to represent a significant fraction (~20–70%) of the total number of aerosols > 0.2 μm, with large spatial and temporal variations. among these, microorganisms are of particular interest in fields as diverse as epidemiology, including phytopathology, bioterrorism, forensic science, and public health, and environmental sciences, like microbial ecology, meteorology and climatology. more precisely concerning the latter, airborne microorganisms contribute to the pool of particles nucleating the condensation and crystallization of water and they are thus potentially involved in cloud formation and in the triggering of precipitation. additionally, viable microbial cells act as chemical catalyzers interfering with atmospheric chemistry. the constant flux of bacteria from the atmosphere to the earth's surface due to precipitation and dry deposition can also affect global biodiversity, but they are rarely taken into account when conducting ecological surveys. as stressed by these studies attempting to decipher and understand the spread of microbes over the planet, concerted data are needed for documenting the abundance and distribution of airborne microorganisms, including at remote and altitudes sites. bioaerosols bioaerosols, known also as primary biological aerosols, are the subset of atmospheric particles that are directly released from the biosphere into the atmosphere. they include living and dead organisms (e.g., algae, archaea, bacteria ), dispersal units (e.g., fungal spores and plant pollen ), and various fragments or excretions (e.g., plant debris and brochosomes). bioaerosol particle diameters range from nanometers up to about a tenth of a millimeter. the upper limit of the aerosol particle size range is determined by rapid sedimentation, i.e., larger particles are too heavy to remain airborne for extended periods of time. bioaerosols include living and dead organisms as well as their fragments and excrements emitted from the biosphere into the atmosphere. included are archaea, fungi, microalgae, cyanobacteria, bacteria, viruses, plant cell debris, and pollen. historically, the first investigations of the occurrence and dispersion of microorganisms and spores in the air can be traced back to the early 19th century. since then, the study of bioaerosols has come a long way, and air samples collected with aircraft, balloons, and rockets have shown that bioaerosols released from land and ocean surfaces can be transported over long distances and up to very high altitudes, i.e., between continents and beyond the troposphere. global bioaerosol cycling after emission from the biosphere, bioaerosol particles interact with other aerosol particles and trace gases in the atmosphere and can be involved in the formation of clouds and precipitation. after dry or wet deposition to the earth's surface, viable bioparticles can contribute to biological reproduction and further emission. this feedback can be particularly efficient when coupled to the water cycle (bioprecipitation). global ecosystem interactions of bioaerosol particles key aspects and areas of research required to determine and quantify the interactions and effects of biogenic aerosol particles in the earth system, including primary biological aerosols directly emitted to the atmosphere and secondary organic aerosols formed upon oxidation and gas-to-particle conversion of volatile organic compounds. bioaerosols play a key role in the dispersal of reproductive units from plants and microbes (pollen, spores, etc.), for which the atmosphere enables transport over geographic barriers and long distances. bioaerosols are thus highly relevant for the spread of organisms, allowing genetic exchange between habitats and geographic shifts of biomes. they are central elements in the development, evolution, and dynamics of ecosystems. transport and distribution factors controlling microbial communitiesof the planetary boundary layer once aerosolized, microbial cells enter the planetary boundary layer, defined as the air layer near the ground directly influenced by the planetary surface. the concentration and taxonomic diversity of airborne microbial communities in the planetary boundary layer has been recently described, though the functional potential of airborne microbial communities remains unknown. from the planetary boundary layer, the microbial community might eventually be transported upwards by air currents into the free troposphere (air layer above the planetary boundary layer) or even higher into the stratosphere. microorganisms might undergo a selection process during their way up into the troposphere and the stratosphere. subject to gravity, aerosols (or particulate matter) as well as bioaerosols become concentrated in the lower part of the troposphere that is called the planetary boundary layer. microbial concentrations thus usually show a vertical stratification from the bottom to the top of the troposphere with average estimated bacterial concentrations of 900 to 2 × 107 cells per cubic metre in the planetary boundary layer and 40 to 8 × 104 cells per cubic metre in the highest part of the troposphere called the free troposphere. the troposphere is the most dynamic layer in terms of chemistry and physics of aerosols and harbors complex chemical reactions and meteorological phenomena that lead to the coexistence of a gas phase, liquid phases (i.e., cloud, rain, and fog water) and solid phases (i.e., microscopic particulate matter, sand dust). the various atmospheric phases represent multiple biological niches. possible processes in the way atmospheric microbial communities can distribute themselves have recently been investigated in meteorology, seasons, surface conditions and global air circulation. over space and time microorganisms attached to aerosols can travel intercontinental distances, survive, and further colonize remote environments. airborne microbes are influenced by environmental and climatic patterns that are predicted to change in the near future, with unknown consequences. airborne microbial communities play significant roles in public health and meteorological processes, so it is important to understand how these communities are distributed over time and space. atmosphere layers, temperature and airborne emission sources 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. there are some metagenomic studies on airborne microbial communities over specific sites. 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. 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. airborne microbial transport is central to dispersal outcomes and several studies have demonstrated diverse microbial biosignatures are recoverable from the atmosphere. microbial transport has been shown to occur across inter-continental distances above terrestrial habitats. variation has been recorded seasonally, with underlying land use, and due to stochastic weather events such as dust storms. there is evidence specific bacterial taxa (e.g., actinomycetota and some gammaproteobacteria) are preferentially aerosolized from oceans. over urban areas dust storms as a source of aerosolized bacteria as a result of rapid industrialization and urbanization, global megacities have been impacted by extensive and intense particulate matter pollution events, which have potential human health consequences. severe particulate matter pollution is associated with chronic obstructive pulmonary disease and asthma, as well as risks for early death. while the chemical components of particulate matter pollution and their impacts on human health have been widely studied, the potential impact of pollutant-associated microbes remains unclear. airborne microbial exposure, including exposure to dust-associated organisms, has been established to both protect against and exacerbate certain diseases. understanding the temporal dynamics of the taxonomic and functional diversity of microorganisms in urban air, especially during smog events, will improve understanding of the potential microbe-associated health consequences. recent advances in airborne particle dna extraction and metagenomic library preparation have enabled low biomass environments to be subject to shotgun sequencing analysis. in 2020, qin et al. used shotgun sequencing analysis to reveal a great diversity of microbial species and antibiotic resistant genes in beijing's particulate matter, largely consistent with a recent study. the data suggest that potential pathogen and antibiotic resistance burden increases with increasing pollution levels and that severe smog events promote the exposure. in addition, the particulate matter also contained several bacteria that harbored antibiotic resistant genes flanked by mobile genetic elements, which could be associated with horizontal gene transfer. many of these bacteria were typical or putative members of the human microbiome. dispersal dispersal is a vital component of an organism's life-history, and the potential for dispersal determines the distribution, abundance, and thus, the community dynamics of species at different sites. a new habitat must first be reached before filters such as organismal abilities and adaptations, the quality of a habitat, and the established biological community determine the