THE MICROBIOLOGY OF THE ATMOSPHERE 



In the Slimmer and autumn of 1923, Mischustin (1926) exposed Petri 

 dishes of nutrient agar in flights from Moscow. On rather slender evi- 

 dence obtained from tests in a wind-tunnel, he concluded that he was 

 sampling 20 litres of air per minute — probably a large underestimate. The 

 plane was first flown above the layer to be sampled to free it from ground 

 dust, and then lowered to the required height. At 500 metres the num- 

 bers of bacteria increased in windy weather to 7,000 or 8,000 per cubic 

 metre, from Mischustin's normal of 2,000 to 3,000 at this altitude. 

 Micrococcus and Sarcina decreased greatly in calm weather, but the 

 number of bacterial rods and of moulds increased. At 1,000-2,000 metres, 

 numbers were small. The proportion of spore-forming bacteria, of moulds, 

 and of actinomycetes, was greatest at the greater heights. The concentration 

 of organisms over the city of Moscow at 2,000 metres included an average 

 of 650 bacteria per cubic metre, and was from four to five times as great 

 as that over the surrounding countryside. 



Pollen was found at up to 5,800 metres over the Mississippi, with the 

 greatest concentrations often at from 300 to 1,100 metres (Scheppegrell, 

 1924, 1925). Craigie & Popp (1928) caught wheat-rust spores at up to 

 3,000 metres over the Canadian prairies. 



In flights from Cambridge, England, Weston (1929) found that fungi 

 and bacteria were abundant up to 3,000 metres, but relatively scarce 

 above this altitude. Air within clouds tended to contain more bacteria 

 and fungi than did air above or below clouds — a phenomenon noted by 

 other workers, including Heise & Heise (1948). 



On flights up to 2,200 metres over the arid lands of southern Arizona, 

 Browne (1930) isolated 'white and grey' bacterial colonies, Aspergillus, 

 Penicillium., Alternaria, and yeasts; but no spores of wheat rusts were 

 observed on slide spore-traps. Cotter (1931) studied dispersal of wheat 

 rust in flights near Lake Michigan, trapping on oiled microscope slides. 

 Uredospores were not more numerous during rain than in fine weather; 

 fewer were caught over Lake Michigan than over near-by land, and more 

 were caught over areas abounding in barberry (the alternate host of the 

 parasite). MacQuiddy (1935) exposed Petri dishes and slides in flights 

 up to 2,100 metres over Omaha, Nebraska. Pollen was abundant up to 

 900 metres, and bacteria and mould spores began to decrease between 

 1,200 and 1,500 metres. MacLachlan (1935) made flights in early May 

 over Massachusetts, to trace spores of the juniper rust {Gymno sporangium 

 biseptatum). Petri dishes exposed over the side of the 'plane for i minute 

 gave viable spores up to 600 metres (the maximum height tested), 

 although numbers and viability decreased steadily with height. 



(i) Aerobiological work of F. C. Meier. Fred C. Meier of the United 

 States Department of Agriculture planned an extensive investigation 

 of the upper-air spora. Unfortunately, when only preliminary abstracts 

 of his work had been published, he was lost on a flight over the Pacific 

 Ocean (Haskell & Barss, 1939). 



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