It is difficult to establish which species is the most abundant in the world, although it seems obvious that microorganisms are placed at the top of the podium. This includes viruses, bacteria, protozoa and fungi, among others. The number of individuals that make up these forms of life is incalculable, as is perhaps their variety. Today we are going to talk about some microbes that have the peculiarity of living in the air, floating at the mercy of the currents and serving as food for some birds, which is why they have been compared to marine plankton and, consequently, they receive the name of aeroplankton.
The RAE defines plankton as a set of animal and vegetable organisms, generally tiny, that float and are passively displaced in salty or fresh waters. Its etymology, of Greek origin, alludes precisely to that wandering character as opposed to the nektonwhich is formed by beings that swim.
In an intermediate phase between water and air would be the neuston and, more specifically, the epineuston groups those that are already in the aerial phase (that is, on the film that forms the water) in front of the hyponeuston, which is still in the watery (below). Going up a step we would already be talking about aeroplankton.
Various species of plankton/Image: Christian Sardet-CNRS-Tara Expeditions on Wikimedia Commons
These are invertebrates that also have passive movement and are so abundant that sometimes, depending on air currents and concentration, they can even be seen with the naked eye or with a magnifying glass. In fact, its diversity is such that it includes those that require a microscope -optical or electronic- to see them but also others of a larger size -although tiny in any case-, so taxonomically they cover a fairly wide range that goes from the smallest viruses a thousand species of bacteria, forty thousand fungi and hundreds of protists, algae, mosses and liverworts (spores, pollen and seeds).
Breaking down, one can name unicellular algae, fungal and pteridophytic spores, angiosperm and gymnosperm pollen, protozoan cysts, rotifers, nematodes and tardigrades, arachnids and small insects, to which can be added the so-called aeronecton, which form macroinsects, ucelids (apodidos and hirundins) and even chiropterans (which are small mammals, bats). There is no clear limit between aeroplankton and aeronecton, so some establish the difference in length (greater or less than one centimeter) and others in flight capacity.
Scheme proposed by PF Balboa for raising the flight of spiders using silk “sails” or “paragliders” / Image: Pozitron in Wikimedia Commons
In addition, the composition varies greatly geographically and seasonally, intervening factors such as time of day, rainfall, temperature, humidity, season, altitude or biogeographic zone. Also the pressure exerted by Man through pesticides and insecticides or their modification of the environment; An example: the development of cultivated areas means that, after the harvest, the air is too dry for certain bioindicator species of good environmental quality (let alone if it is polluted air typical of urban environments). Still, humans breathe thousands of these organisms daily.
As can be seen, it also includes higher animals, although most are invertebrates. This is the case of arthropods such as small spiders that take advantage of the currents to propel themselves using the silk threads they produce in the manner of sails, in what is known as arachnid flight. Another example could be also tiny insects that sometimes move driven by the wind or float in the air, even at several thousand meters of altitude; In the latter case, aphids stand out, phytopathogens that usually constitute pests for crops, such as the famous phylloxera or aphids.
There are more, such as certain Diptera, Coleoptera and Hymenoptera whose dimensions and weight are so small, even in the adult state, that they are often moved hundreds of kilometres. They constitute one of the bases of the food chain as they are a food source for some species of birds, including some as familiar as swallows and swifts, which are capable of eating tens of thousands of insects daily (about fifty grams per individual). , so in passing they exercise control over the aeroplankton. However, the members of the aeroplankton themselves can cannibalize it, such as spiders and bats.
For some, moving around powered by the wind is an efficient transportation system; but for others the thing acquires a very special quality: the air is not a means but an end in itself and they live permanently in it. There we would already be talking about minor beings but very resistant to extreme conditions (temperature, pressure, humidity, radiation, etc.) that exist in records up to seventy-seven thousand meters above sea level, feeding on organic substances in suspension, acid formic, alcohols, etc.
This is the case of some species of prokaryotes (bacteria) such as Micrococcus albus Y Mycobacterium lutandumor mushrooms like Papulaspora anomala, Circinella muscae, Aspergillus niger Y Penicillium notatum. These last two names may be familiar to the reader, since the first is the cause of a common mold in vegetables, fruits and vegetables while the second belongs to the genus from which penicillin is manufactured. However, they are not the only ones that can be found among the clouds, other genera having also been identified: Micrococcus, Bacillus, Streptomyces, Flavobacterium…
Furthermore, the bacteria themselves -at least certain types of them- intervene in the formation of the clouds, facilitating condensation and collaborating in the generation of other bacteria, the biosurfactants, which have a role in reducing the surface tension of the water.
It is even debated whether aeroplankton participate in the formation of rain, snow and hail, perhaps determining the ordering of water molecules thanks to the proteins in their membranes. All this supposes a parallelism with the role that it is believed that phylloplankton could have in the formation of clouds and precipitation.
Pollen grains photographed with a scanning electron microscope/Image: public domain on Wikimedia Commons
Something similar to what we previously told about invertebrates happens with other varieties of aeroplankton, since many plants release pollen so that the wind spreads it and that is the basis of cross fertilization (that which occurs outside the body). Cereals tend to use this system, as do conifers, unintentionally causing an effect on human beings: allergies, which also appear when spores and seeds are transported in suspension in the air. We already know who to blame.
Climate and environmental quality. Biomonitoring of air quality (VI Annual Meeting of Climatology Association of Spanish Geographers. Alberto Martí, ed.)/Studies in the distribution of insects by aerial currents (AC Hardy and PS Milne)/Aeroplankton. living in the clouds (Cnidus in Science and its Demons)/Declines of aerial insectivores in North America follow a geographic gradient (Silke Nebel, Alex Mills, John D. McCracken, and Philip D. Taylor)/Wikipedia