Aeroplankton

Colourised SEM image of pollen grains
from common plants

Pollen grains observed in aeroplankton of South Europe[2]

Drawings of fungal spores found in air
Some cause asthma, such as Alternaria alternata. A drawing of a very small "dust" seed from the flower Orchis maculata is provided for comparison.[3][4]

    A = ascospore, B = basidiospore, M = mitospore

Spider ballooning structures. Black, thick points represent the spider’s body. Black lines represent ballooning threads.[5]

Distribution modes and possible geographic ranges of nematodes [6]

Dispersal is a vital component of an organism’s life-history,[7] and the potential for dispersal determines the distribution, abundance, and thus, the community dynamics of species at different sites.[8][9][10] 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 colonization efficiency of a species.[11] While larger animals can cover distances on their own and actively seek suitable habitats, small (<2 mm) organisms are often passively dispersed,[11] resulting in their more ubiquitous occurrence.[12] While active dispersal accounts for rather predictable distribution patterns, passive dispersal leads to a more randomized immigration of organisms.[8] 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.[11][13]

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).[14] Often cited as important requirement for effective wind dispersal is the presence of propagules (e.g., resting eggs, cysts, ephippia, juvenile and adult resting stages),[11][15][16] 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 temporal waters. The passively dispersed organisms are typically pioneer colonizers.[17][18][19][13]

However, wind-drifted species vary in their vagility (probability to be transported with the wind),[20] with the weight and form of the propagules, and therefore, the wind speed required for their transport,[21] determining the dispersal distance. For example, in nematodes resting eggs are less effectively transported by wind than other life stages,[22] while organisms in anhydrobiosis are lighter and thus more readily transported than hydrated forms.[23][24] 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.[17][25] The distances covered by small animals range from a few meters,[25] to several hundred meters,[17] and up to several kilometers.[22] While the wind dispersal of aquatic organisms is possible even during the wet phase of a transiently aquatic habitat,[11] during the dry stages a larger number of dormant propagules are exposed to wind and thus dispersed.[16][25][26] Freshwater organisms that must "cross the dry ocean" [11] to enter new aquatic island systems will be passively dispersed more successfully than terrestrial taxa.[11] However, 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).[17][25][26][27] While these have been qualitatively well studied, accurate estimates of their dispersal rates are lacking.[13]

Recent drilling cores from Switzerland have evidenced the oldest known fossils from flowering plants, pollen grains, which have revealed that flowering plants are 240-million-year-old.[28] Effective pollen dispersal is vital for maintenance of genetic diversity and fundamental for connectivity between spatially separated populations.[29] An efficient transfer of the pollen guarantees successful reproduction in flowering plants. No matter how pollen is dispersed, the male-female recognition is possible by mutual contact of stigma and pollen surfaces. Cytochemical reactions are responsible for pollen binding to a specific stigma.[30][2]

Allergic diseases are considered to be one of the most important contemporary public health problems affecting up to 15–35% of humans worldwide.[31] There is a body of evidence suggesting that allergic reactions induced by pollen are on the increase, particularly in highly industrial countries.[31][32][2]

A wealth of correlative evidence suggests asthma is associated with fungi and triggered by elevated numbers of fungal spores in the environment.[33] Intriguing are reports of thunderstorm asthma. 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.[34] Thunderstorms are associated with spore plumes: when spore concentrations increase dramatically over a short period of time, for example from 20,000 spores/m³ to over 170,000 spores/m³ in 2 hours.[35] However, other sources consider pollen or pollution as causes of thunderstorm asthma.[36] Transoceanic and transcontinental dust events move large numbers of spores across vast distances and have the potential to impact public health,[37] and similar correlative evidence links dust blown off the Sahara with pediatric emergency room admissions on the island of Trinidad.[38][3]

The study of fungal spores in aeroplankton is called aeromycology.

Pteridophyte spores, such as fern spores, similarly to pollen grains and fungal spores, are also components of aeroplankton.[39][40] Fungal spores usually rank first among bioaerosol constituents due to their highest concentrations (1000–10 000 m⁻³), while pollen grains and fern spores can reach a similar content (10–100 m⁻³).[41][32]

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. Aphids, for example, are frequently found at high altitudes.

Ballooning, sometimes called kiting, is a process by which spiders, and some other small invertebrates, move through the air by releasing one or more gossamer threads to catch the wind, causing them to become airborne at the mercy of air currents.[42][43] A spider (usually limited to individuals of a small species), or spiderling after hatching,[44] will climb as high as it can, stand on raised legs with its abdomen pointed upwards ("tiptoeing"),[45] and then release several silk threads from its spinnerets into the air. These automatically form a triangular shaped parachute[46] which carries the spider away on updrafts of winds where even the slightest of breezes will disperse the arachnid.[45][46] The flexibility of their silk draglines can aid the aerodynamics of their flight, causing the spiders to drift an unpredictable and sometimes long distance.[47] Even atmospheric samples collected from balloons at five kilometres altitude and ships mid-ocean have reported spider landings. Mortality is high.[48]

The Earth's static electric field may also provide lift in windless conditions.[49]

Nematodes (roundworms), the most common animal taxon, are also found among aeroplankton.[16][17][25] Nematodes are an essential trophic link between unicellular organisms like bacteria, and larger organisms such as tardigrades, copepods, flatworms, and fishes.[13] For nematodes, anhydrobiosis is a widespread strategy allowing them to survive unfavorable conditions for months and even years.[50][51] Accordingly, nematodes can be readily dispersed by wind. However, as reported by Vanschoenwinkel et al.,[25] nematodes account for only about one percent of wind-drifted animals. 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.[52][13] In addition, within a few days of forming temporary waters such as phytotelmata were shown to be colonized by numerous nematode species.[19][53][13]

A stream of airborne microorganisms circles the planet above weather systems but below commercial air lanes.[54] Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.[55][56]

Impact of microbial activity on clouds [57]

Biological processes and their targets are indicated by green arrows, while red arrows indicate abiotic processes.

EPS: Exopolysaccharide              SOA: Secondary organic aerosol
Based on coordinated metagenomics/metatranscriptomics

The outdoor atmosphere harbors diverse microbial assemblages composed of bacteria, fungi and viruses [58] whose functioning remains largely unexplored.[57] While the occasional presence of human pathogens or opportunists can cause potential hazard,[59][60] 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.[61][62] Once ripped off and aerosolized from surfaces by mechanical disturbances such as those generated by wind, raindrop impacts or water bubbling,[63][64] microbial cells are transported upward by turbulent fluxes.[65] They remain aloft for an average of ~3 days,[66] a time long enough for being transported across oceans and continents [67][68][69] 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 [70] and ice nuclei.[71][72][57]

Living airborne microorganisms may end up concretizing aerial dispersion by colonizing their new habitat,[73] provided that they survive their journey from emission to deposition. Bacterial survival is indeed naturally impaired during atmospheric transport,[74][75] but a fraction remains viable.[76][77] 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.[78][79][80] In addition, the detection of low levels of heterotrophy [81] raises questions about microbial functioning in cloud water and its potential influence on the chemical reactivity of these complex and dynamic environments.[80][82] 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.[57]

Over the last few 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. There have been studies in soils,[83] the ocean,[84][85] the human gut [86] and elsewhere.[87][88] In the atmosphere, though, microbial gene expression and metabolic functioning remain largely unexplored, in part due to low biomass and sampling difficulties.[57] So far, metagenomics has confirmed high fungal, bacterial and viral biodiversity,[89][90][91][92] and targeted genomics and transcriptomics towards ribosomal genes has supported earlier findings about the maintenance of metabolic activity in aerosols [93][94] and in clouds.[62] In atmospheric chambers airborne bacteria have been consistently demonstrated to react to the presence of a carbon substrate by regulating ribosomal gene expressions.[95][57]

• 
  Pteridophyta spores, including fern spores, in the air of Lublin
• 
  Airborne fungal spores
• 
  Detail of the formation of a dense aeroplankton cloud at sunset, over the Loire river in France

• Aeolian processes
• Aerobiology
• Biological dispersal
  • Dispersal vector
  • Seed dispersal
• Organisms at high altitude

• Living on the air