Aeroplankton

Air mass circulation globally disperses vast numbers of the floating aerial organisms, which travel across and between continents, creating biogeographic patterns by surviving and settling in remote environments.

[10] The circulation of atmospheric microorganisms results in global health concerns and ecological processes such as widespread dispersal of both pathogens [11] and antibiotic resistances,[12] cloud formation and precipitation,[8] and colonization of pristine environments.

[20] In recent years, next generation DNA sequencing technologies, such as metabarcoding as well as coordinated metagenomics and metatranscriptomics studies, have been providing new insights into microbial ecosystem functioning, and the relationships that microorganisms maintain with their environment.

[27] So far, metagenomics has confirmed high fungal, bacterial, and viral biodiversity,[29][30][31][32] and targeted genomics and transcriptomics towards ribosomal genes has supported earlier findings about the maintenance of metabolic activity in aerosols [33][34] and in clouds.

In a now classic study from the United Kingdom, an outbreak of acute asthma was linked to increases in Didymella exitialis ascospores and Sporobolomyces basidiospores associated with a severe weather event.

[41][62] Many small animals, mainly arthropods (such as insects and spiders), are also carried upwards into the atmosphere by air currents and may be found floating several thousand feet up.

[64][65] A spider (usually limited to individuals of a small species), or spiderling after hatching,[66] will climb as high as it can, stand on raised legs with its abdomen pointed upwards ("tiptoeing"),[67] and then release several silk threads from its spinnerets into the air.

Among the habitats colonized by nematodes are those that are strongly exposed to wind erosion as e.g., the shorelines of permanent waters, soils, mosses, dead wood, and tree bark.

[92][93] Due to their relatively small size (the median aerodynamic diameter of bacteria-containing particles is around 2–4 μm),[62] these can then be transported upward by turbulent fluxes [94] and carried by wind to long distances.

[10][100] 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)[101][102][35] thus requiring reliable sampling procedures and controls.

[62][105][106][107][108][109] 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.

[126][127] 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.

[130] 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.

[84][131][132][133] As stressed by these studies attempting to decipher and understand the spread of microbes over the planet,[134][102][135] concerted data are needed for documenting the abundance and distribution of airborne microorganisms, including at remote and altitudes sites.

[164] While larger animals can cover distances on their own and actively seek suitable habitats, small (<2 mm) organisms are often passively dispersed,[164] resulting in their more ubiquitous occurrence.

[164] 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).

[176] Possible processes in the way atmospheric microbial communities can distribute themselves have recently been investigated in meteorology,[3][4][10][178][189] seasons,[178][190][191][102][192] surface conditions [189][190][191][192] and global air circulation.

[199][200][201][202][203] Given that cultivable organisms represent about 1% of the entire microbial community,[204] 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.

[222][6] As a result of rapid industrialization and urbanization, global megacities have been impacted by extensive and intense particulate matter pollution events,[223] which have potential human health consequences.

[235][236][237] Recent advances in airborne particle DNA extraction and metagenomic library preparation have enabled low biomass environments to be subject to shotgun sequencing analysis.

[236][237] 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.

[27] While the occasional presence of human pathogens or opportunists can cause potential hazard,[241][242] 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.

[190][35] Once ripped off and aerosolized from surfaces by mechanical disturbances such as those generated by wind, raindrop impacts or water bubbling,[243][92] microbial cells are transported upward by turbulent fluxes.

[250][251] 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.

[252][253][199] In addition, the detection of low levels of heterotrophy [254] raises questions about microbial functioning in cloud water and its potential influence on the chemical reactivity of these complex and dynamic environments.

[255] 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.

[256][261] 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.

[265] 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 Venusian atmosphere is too dry to host microbial life.

[16] In 2020, Archer et al. reported evidence for a dynamic microbial presence at the ocean–atmosphere interface at the Great Barrier Reef, and identified air mass trajectories over oceanic and continental surfaces associated with observed shifts in airborne bacterial and fungal diversity.

Here we test the hypothesis that persistent microbial inputs to the ocean–atmosphere interface of the Great Barrier Reef ecosystem vary according to surface cover (i.e. land vs. ocean) during transit of the air-mass.