THE MICROBIOLOGY OF THE ATMOSPHERE 

 VERTICAL GRADIENT OVER THE OCEAN 



The McGill workers showed conclusively that the upper air contains 

 an appreciable spore-load in all the places which they examined. Even in 

 the high-arctic winter, the evidence proves that samples of a few cubic 

 metres of air may or may not be sterile. Cladosporium appears to dominate 

 the upper-air spora, often together with many yeasts and bacteria. Pos- 

 sibly the purest air is to be found near sea-level in mid-ocean. The few 

 observations suggest that samples taken on board ship are collected in a 

 purified layer of the atmosphere, and that higher up over the ocean surface 

 the spore concentration is greater than at sea-level. The troposphere is 

 always more or less contaminated with micro-organisms. From Erdtman's 

 (1937) results a ship in the North Atlantic in spring would be in a region 

 of about one pollen grain per 100 cubic metres, whereas Pady & Kapica 

 (1955), at 3,000 metres over the same ocean, recorded up to 25 pollen 

 grains (with moss spores) per single cubic metre. 



On a flight from New Zealand to Australia, at about 1,000 metres 

 above the Tasman Sea, Newman (1948) exposed sticky slides behind a 

 leading wire in the hope of improving the trapping efficiency by breaking 

 the stagnant layer. At a position 1,100 km. off Australia, he estimated 

 pollen grains at 0-73 per cubic metre, and fungus spores at 0-70 per cubic 

 metre; at 340 km. from Australia he found pollen to be 8-75 and fungus 

 spores 1 6-8 per cubic metre — which is about 100 times as numerous as the 

 concentration of pollen grains and fern spores at ship's mast-level on 

 the North Atlantic crossing recorded by Erdtman (cf. p. 127). Newman's 

 values for fungus spores are somewhat similar to those of the McGill 

 University workers for the upper air over the Atlantic. It seems likely 

 that Erdtman's samples, taken on board ship, were from a zone of surface 

 air which had been largely cleaned by rain-wash, sedimentation, and 

 contact with the ocean, and only partially replenished from the stock in 

 the air mass overhead, where aircraft samples were taken. 



Summary 



Knowledge of upper-air microbiology is based on occasional samples 

 and is affected by place, season, weather, air mass, and so on. There are 

 no continuous records; but there are some hints that a 'biological zone' 

 occurs at middle height, which can probably be explained in terms of 

 temperature inversions, air masses, and precipitation. 



Molisch (1920) introduced the concept o^ aero plank ton to denote the 

 microbial complex referred to in this book as the air-spora. It has been 

 argued that the word 'plankton' suggests organisms based on the air 

 during at least a vigorous phase of growth, whereas the air-spora is only 

 airborne temporarily, even though adapted to wind transport as a means of 

 dissemination. Clearly this argument is valid for pollens and plant spores; 



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