AEROBIOLOGY 



diffusion and, to a minor extent, by gravity, but it may also be fed from 

 below, from new sources of soil and vegetation. Even over the Arctic in 

 winter, the air is only relatively sterile; samples of a few cubic metres may 

 or may not contain viable spores. Wind transport of microbes is a process 

 that is evidently going on continuously on a w^orld-wide scale. 



Many unsolved problems remain. Is there a biotic zone at the height 

 of a few^ thousand metres ? Or is the lower air cleared by rain, with microbes 

 re-concentrating transiently at the base of a cloud ? Is there a true aerial 

 plankton in the sense of a population permanently living and reproducing 

 at great heights, as suggested by R. C. McLean (1935, 1943) ? This would 

 seem improbable — unless a microbial population is permanently balanced 

 over tropical regions by rising air-currents and descending cloud droplets. 

 Do air masses retain a characteristic polar or tropical spora, or do they 

 rapidly receive and give up the spora of the land over w hich they pass ? 



Sooner or later, if they have not been already deposited by other 

 means, airborne microbes are removed from the atmosphere by collection 

 in rain, snow\ or hail. If the carrying droplet later evaporates below the 

 cloud base, there could possibly be an actual local increase in concentra- 

 tion high above ground. Rain, hail, and snow bring down a large microbial 

 flora and are factors making for non-uniform deposition, in contrast with 

 the more uniform diffusion which results from turbulence. 



Spores deposited on the ground, or on vegetation near to a source, 

 show a pronounced gradient (following the concentration gradient of the 

 wind-blown spore-cloud). This can be estimated by making assumptions 

 based on observed data, and can be used in predicting the danger of 

 contamination by foreign pollens, plant pathogens, and so on. In practice 

 the ideal gradient is often modified by topography — sometimes a decreased 

 wind velocity decreases spore deposition. 



In the past it may have been too easily assumed that the dispersion of 

 an organism around its origin is like a normal frequency distribution. This 

 may be true when dispersion is due to motile surface animals, actively 

 flying insects, and possibly even rain-splash. But wind-dispersal is not 

 'normal' (in a statistical sense) around the point of origin; by contrast, 

 it is a hollow curve. This throws light on a paradoxical situation: in the 

 w ind-dispersal of microbes liberated near ground-level, the gradient near 

 the source is very steep, and relatively short isolation-distances give good 

 protection. Pollens and plant pathogens with large dry spores liberated at 

 ground-level may have 90 per cent of their spores deposited within about 

 100 metres — or perhaps more than 90 per cent if allowance is made for 

 the unexplained large values of p observed within a metre or two of a 

 ground source. Yet spores can travel immense distances, and spores 

 found over the ocean clearly represent the tails of the distributions of all 

 the sources on the up-wind continent. The equation for Q^^. has the 

 curious property that the farther a spore has travelled, and the longer it 

 has survived deposition, the farther it is likely to travel. 



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