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Though the theory of the meteorological impact of aeroplankton was originally proposed in the early 1970s, in situ studies are difficult and it has only been in the late 2000s that this has become a very active area of research.

History of Discovery
It is in the efforts of Louis Pasteur to disprove spontaneous generation that the idea of aeroplankton has its beginnings. His experiments established that microbes are born upon the air, that their abundance decreases with altitude, that dust particles facilitate their disbursement and that precipitation reduced their prevalence in the air.

For centuries sailors on the open ocean have recognized the appearance of certain plants and animals as signs of imminent landfall. These signs have included macroscopic aeroplankton. For instance, in The Voyage of the Beagle (1839) Charles Darwin makes the following observation of ballooning spiders far out at sea:

There are several accounts of insects having been blown off the Patagonian shore. Captain Cook observed it, as did more lately Captain King of the Adventure. The cause probably is due to the want of shelter, both of trees and hills, so that an insect on the wing with an offshore breeze, would be very apt to be blown out to sea. The most remarkable instance I have known of an insect being caught far from the land, was that of a large grasshopper (Acrydium), which flew on board, when the Beagle was to windward of the Cape de Verd Islands, and when the nearest point of land, not directly opposed to the trade-wind, was Cape Blanco on the coast of Africa, 370 miles distant.

On several occasions, when the Beagle has been within the mouth of the Plata, the rigging has been coated with the web of the Gossamer Spider. One day (November 1st, 1832) I paid particular attention to this subject. The weather had been fine and clear, and in the morning the air was full of patches of the flocculent web, as on an autumnal day in England. The ship was sixty miles distant from the land, in the direction of a steady, though light, breeze. Vast numbers of a small spider,about one-tenth of an inch in length, and of a dusky red colour, were attached to the webs. There must have been, I should suppose, some thousands on the ship. The little spider, when first coming in contact with the rigging, was always seated on a single thread, and not on the flocculent mass. This latter seems merely to be produced by the entanglement of the single threads. ... The little aëronaut as soon as it arrived on board was very active ... On its first arrival it appeared very thirsty, and with exserted maxillæ drank eagerly of drops of water ... may it not be in consequence of the little insect having passed through a dry and rarified atmosphere? Its stock of web seemed inexhaustible. While watching some that were suspended by a single thread, I several times observed that the slightest breath of air bore them away out of sight, in a horizontal line. On another occasion (25th) under similar circumstances, I repeatedly observed the same kind of small spider, either when placed or having crawled on some little eminence, elevate its abdomen, send forth a thread, and then sail away horizontally, but with a rapidity which was quite unaccountable...

One day, at St. Fé, I had a better opportunity of observing some similar facts. A spider which was about three-tenths of an inch in length, and which in its general appearance resembled a Citigrade (therefore quite different from the gossamer), while standing on the summit of a post, darted forth four or five threads from its spinners. These, glittering in the sunshine, might be compared to diverging rays of light; they were not, however, straight, but in undulations like films of silk blown by the wind. They were more than a yard in length, and diverged in an ascending direction from the orifices. The spider then suddenly let go its hold of the post, and was quickly borne out of sight.

Microbial Aeroplankton
Using a weather rocket in 1977, A.A. Imshenetsky, et al. were able to collect samples of pigmented conidium (fungi spores) at altitudes as high as 77 km above the Kazakh Republic. For reference, the Kármán line, widely considered the boundary between the Earth's atmosphere and outer space, is at 100 km and the International Space Station orbits between 417 and 427 km.

Despite the possibility that they vastly outnumber other types of aeroplankton, viruses as bioaerosols are almost entirely unstudied beyond the indoor, urban and planetary boundary air. Viruses are much smaller than other microbes (c. 0.02-0.30 µm versus c. 0.50-5 µm for bacteria and c. 1-100 µm for fungi) so can remain suspended in the air for much longer durations. Thus it is likely that viruses are much more numerous than other microbes in aeroplankton. It is estimated that the ratio of viral to bacterial biomass in air is somewhere between 0.01 and 10,000. Viral aeroplankton is so understudied owing to significant difficulties in conducting population studies of viruses. Bacteria, archaea, fungi and algae all have universal genes, so can be identified en masse using well-established PCR techniques. Lacking universal genes, viruses must be identified through metagenomic analysis. But it is difficult to obtain large enough viral samples, especially from air, and it is estimated that less than one percent of viruses have been sequenced (thus are available in databases for matching).

Aeroplankton and Infectious Disease
For many years upper atmospheric currents were considered a leading explanation for the seasonality of influenza outbreaks, though the current consensus is that winter months produce an environment more conducive to flu virus survival.

The spread of infectious disease is not limited to human disease only. One study has documented the upper atmospheric spread of diseases effecting corals.

Aeroplankton and Panspermia
The possibility that microorganisms in the upper reaches of the atmosphere might drift off into outer space in a process similar to that of atmospheric escape has been suggested as a possible scenario for Earth-originating panspermia.

Further General Interest Reading