by William H. Amos, Vermont, USA
Airborne anthrax spores, specially cultured by cruel terrorists so they will be inhaled, are much on everyone’s mind these days, yet knowledge of naturally-occurring aerial micro-specks has existed for over a century.
In 1833 soon after Charles Darwin set out on his four-year voyage around the world in the H.M.S. Beagle, he found himself bored. The trip was long and rough and he was a terrible sailor, seasick almost continuously. There was little to exercise his mind other than reading and arguments with the captain, Robert FitzRoy. They came to respect one another, but were never able to reconcile their philosophical differences.
Except for watching sea birds and occasional marine life that Darwin was able to snare as the ship wallowed along, there was little for a natural historian to study. One day near the Cape Verde Islands he found an insect on deck and began wondering what was in the air above. He suspended a loose net high in the rigging, a kind of aerial plankton net, the first of its kind. After it had billowed in the tradewinds for a day or so, he took it down and found, both to his astonishment and as he partly suspected, all kinds of tiny living things and fragments of life. All had been borne on the wind from far away. Darwin unpacked his microscope and the world’s pioneer atmospheric microbiologist went exploring.
The first subjects to capture his attention were complete insects, either alive or recently dead, and tiny ballooning spiderlings. They were easy to identify and represented many groups he was already familiar with. Bits and pieces of insects, so small the wind easily carried them, proved to be challenging puzzles he usually could solve—a mosquito wing, a beetle’s antenna, and so on.
Further down the scale were much smaller objects. Some he recognized as pollen grains, a few of which he could identify because they were from familiar European trees. Others were strange and he could only surmise that they came from very far away and had been aloft a long time. There were even smaller encysted one-celled aquatic plants, diatoms, that had been whirled aloft after their ponds had dried up. With them were equally small encysted pond animals—rotifers and protozoa among them. Although it was difficult to study specimens under high magnification because of the ship’s motion, many proved to be spores of fungi, lichens, mosses and ferns, although he had no idea from which species they came. He sketched what he saw and later a famous European biologist, C. G. Ehrenberg, put Darwin’s drawings together in a masterful engraving that was published for all to see. Charles Darwin had stumbled upon the first of his hidden worlds that no one else had ever seen before or suspected.
Years ago I was browsing in the basement of a New York bookstore and came across a fascinating technical volume, The Microbiology Of The Atmosphere by P. H. Gregory, an English plant pathologist. I had never previously thought about the subject, so I bought the book and found my curiosity immediately aroused; it opened a new door to the natural world.
Although the atmosphere has never been part of my research, every now and then I use a simple homemade gadget to see what the air brings. It consists of a tiny electric fan, a funnel through which air is drawn, and a microscope slide smeared with vaseline. After a day or so, the slide is coated with spores, tiny seeds, pollen, cysts of microscopic aquatic animals, and bits of insects.
A much more effective collecting device was made in the 1870s by British physicians studying cholera in Calcutta. Their findings shed no light on the disease, but their “aeroconiscope” made a hit and was used by other investigators all over the world. Suddenly everyone found the air filled with life never before seen. Today sophisticated instruments detect not only spores and pollen grains, but much smaller particles of molecular size that are irritants and sometimes toxic. Chemical pollution hardly existed in the 19th century.
What kinds of aerial zoos and botanic gardens envelope us? Bacteria and viruses beyond reckoning, the majority of which do not cause disease in humans. Some do, however. Influenza viruses are airborne, so it’s always wise to get your flu shot. But most airborne microbes have other destinations in all kinds of animals and plants.
The idea of disease carried through the air is very old. Perhaps you remember a grandparent closing bedroom windows “to keep out the night air.” The word “malaria” means bad air—“mal” and “aria.” Mosquitoes were the culprits, not air after dark. During the terrible plague pandemics of the Middle Ages, “plague doctors” wore long birdlike beaks filled with fragrant herbs, believed to protect them from contracting the dread disease. They used their supposed immunity to point out suspected plague carriers who were then seized and executed, often burned at the stake.
Since then, a large number of diseases at one time or another have been suspected of being transmitted through the air—often proven not to be at all. But you never know. Once on a Smithsonian expedition, I did not accompany colleagues into a bat cave on the island of Barbuda, and weeks later was the only one not to fall seriously ill with histoplasmosis, a fungal disease arising from bat guano.
The air does contain microscopic objects that disturb some of us mightily. If we respond to them, we’re allergic. Two relatives find it difficult to remain in our house for long because both react violently to cat dander. I’ve tried collecting dandruff from our cat to look at under the microscope, without success. Although she appears very clean, dander is nevertheless present and affects certain people.
Seed plants and fungi, not being mobile, must disperse their kind around the world, and do so with seeds and spores, or with pollen grains on their way to fertilize flowers far away. Forty thousand species of fungi, lichens, mosses and ferns are known to release their spores into the air where winds carry them away. Over 100,000 species of flowering plants produce airborne seeds. Some of these objects can be found in the stratosphere, although living things that high up usually die from exposure to ultraviolet rays.
An entomologist friend and former schoolmate, J. Linsley Gressitt, was a scientist with the Bishop Museum in Hawaii. He became an atmospheric biologist, a latter-day Darwin, using specially equipped aircraft to collect insects as high as the planes could fly. He found that trade winds and high jet stream winds over the Pacific carry an enormous number of living insects, demonstrating why so many very small insect species are almost cosmopolitan in their distribution.
Life in the atmosphere is affected by place, season, weather, and air mass. Because temperature inversions carry small objects aloft, there seems to be a concentrated “biological zone” at mid-altitude. We can’t see it or smell it, but it is one more component to the shell of life that surrounds our planet.
Know it or not, we sample the “living air” with every breath, although fortunately our bodies aren’t hospitable to most of these small suspended lives and they perish quickly within us. And no matter what evil people do, anthrax spores are extraordinarily rare. So breathe easily.
© 2001 William H. Amos
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