THE UPPER-AIR SPORA 



but Is there, in addition, a vegetative air-inhabiting plankton ? We can- 

 not yet give this apparently improbable hypothesis a decisively nega- 

 tive answer. Evidence in favour of it has been stated by R. C. McLean 

 (1935, 1943), who wTote : ' "Dust to dust" seems to be the only cycle 

 envisaged. Yet the experiments of Trillat and others show at least the 

 possibility that the air may be a vegetative habitat and the large pro- 

 portion of non-spore-formers present . . . needs more than a conven- 

 tional explanation.' Proctor & Parker (1942) noted that one third of the 

 bacteria collected from the upper air could grow at o°C., and survive 48 

 hours exposure at — 26°C. 



If there is a truly indigenous aeroplankton, its habitat must be exacting 

 in the extreme, and tolerable only by specialized bacteria, yeasts, or act- 

 inomycetes. Frequent drying must reduce the population to inactivit}% 

 though metabolism could be resumed in a cloud of water droplets when 

 gaseous nitrogen and carbon compounds could be absorbed and used. In 

 constant danger of being removed from the air by rain or snow or by 

 contact with the ground, the risk of removal would be increased by any 

 attempt to parasitize organic particles brought up by convection from 

 below. However, radioactive dust can persist for several weeks in the 

 troposphere, and this is a long period on a microbial time-scale. The aerial 

 environment is not obviously beyond the range of exploitation by micro- 

 organisms, for the rate of loss by death or deposition might not be greater 

 than for bacteria in the sea, and there would be freedom from predators. 

 If anywhere, such an aeroplankton might be expected to ride clouds on 

 the ascending side of a tropical convection 'cell' over the Equator. 



Although the origin of the upper-air spora from the soil has been 

 assumed by most investigators, the circumstantial evidence suggests a 

 wider range of sources. The bacteria are probably mainly soil forms with 

 a small proportion from sea water. But hyphal fragments, especially 

 conidiophores of Alternaria and Cladosporium, which are commonly 

 reported from the upper-air (Pady & Kapica, 1953, p. 321), evidently 

 come from the ground vegetation-layer rather than from the soil. The 

 numerous yeasts, coloured basidiospores of toadstools, and smut spores, 

 evidently originate above the soil surface. It is hard to believe that wind 

 could burrow into soil, picking out the few spores of Cladosporium and 

 Alternaria, yet leaving behind most of the far more numerous PcnicilUimi, 

 Trichoderma, Aspergillus and Mucoraceae spores — not to mention clay 

 particles ! 



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