THE MICROBIOLOGY OF THE ATMOSPHERE 



For the extreme limit of dispersal we have as yet no direct quantitative 

 microbial evidence, but recent nuclear test explosions throw light on the 

 problem (Libby, 1956; Stewart & Crooks, 1958). Clouds of radioactive 

 dust, thrown up from bombs of less than one megaton, rise to 10-12 km., 

 but tend to stay in the troposphere without penetrating to the stratosphere. 

 In the troposphere the dust-cloud, consisting of particles mostly less than 

 I ft in equivalent diameter, diffuses vertically and horizontally as it travels 

 eastwards on the prevailing winds of temperate latitudes. The cloud circles 

 the Earth every 4 or 5 weeks, but most of the particles are removed 

 from the atmosphere in a month or two. A region of stable air over the 

 tropics acts as a barrier to its spread between the northern and southern 

 hemispheres. 



A thermonuclear bomb of the megaton range, exploded near the 

 ground, puts a large proportion of its sub-microscopic dust into the 

 stratosphere, where it behaves quite differently from the radioactive dust 

 in the troposphere. The cloud spreads uniformly over all latitudes and is 

 removed much more slowly, depletion to 50 per cent of the original 

 quantity being variously estimated as taking from 5 to 10 years. Dust from 

 successive tests thus accumulates in the stratosphere. As no natural 

 mechanism has been suggested which would convey micro-organisms into 

 the stratosphere on a similar scale, the results of thermonuclear tests are of 

 less immediate interest to aerobiology than are tests of lower power or 

 so-called 'normal bombs'. 



Possibly a hint at the ultimate horizontal dispersal-limit of micro- 

 organisms in air is to be found in a study of the geographical distribution 

 of species. In a world survey of the fungus, Schizophyllum commune^ the 

 numerous incompatibilit}'' factors appeared to be randomly distributed 

 (Raper et al., 1958), though the same may not be true oiCoprinus. On the 

 whole, fungi are believed to show a wider natural geographical distribution 

 than flowering plants. Europe and North America share more species of 

 H^menomycetes (mushrooms, toadstools, and their allies) than of flowering 

 plants. The species of fungi in the tropics and the south-temperate regions 

 differ considerably from those of the north-temperate regions (Bisby, 

 1943). Many saprophytic species of fungi, especially soil moulds, tend to 

 be cosmopolitan. How far wind has operated in their transport, and how 

 far other means, especially man, are responsible for the world-distribution 

 of these micro-organisms, is unknown; but the limits of the rust fungi give 

 us some evidence. 



Viability 



Although, under ideal conditions, dispersal of airborne microbes is a 

 limitless process, phenomena discussed in this chapter indicate that 

 practical limits exist. One factor or another may reduce to negligible 

 proportions the amount of inoculum transmitted between distant places. 



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