THE MICROBIOLOGY OF THE ATMOSPHERE 



which are themselves affected by the temperature and humidity of the air 

 mass. 



This survey is therefore perhaps best regarded as a valuable contri- 

 bution to knowledge of the local air-spora near the ground; judging from 

 knowledge about the upper-air, the amount contributed by the air mass 

 is likely to have been small. Cladosporium and yeasts were the chief 

 constituents of all the air masses (even of fresh polar air), and on our 

 interpretation the abundance of PenkilUum is not surprising for samples 

 taken in a large city. Alternaria and Fusarium were commoner in tropical 

 air. Smut spores occurred in all air masses and at all seasons. Basidiospores 

 of agarics were suspected but not positively identified. Fungi were most 

 numerous in July and August, when 625 cultivable (8,610 visible) spores 

 were recorded per cubic metre, and least numerous from December to 

 February, when 36 (28) were recorded per cubic metre. Bacteria were 

 present in greatest numbers in polar air during spring and autumn, 

 rising from fewer than 70 per cubic metre during March to 710 per cubic 

 metre in June, then decreasing to the end of August and rising to a second 

 maximum during November. In air of maritime origin the trend was 

 irregular. 



AIR MASSES OVER THE NORTH ATLANTIC OCEAN 



In two flights from Montreal to London, England, at altitudes ranging 

 between 2,700 and 3,000 metres, the McGill workers were able to study 

 the relation between microbial concentration and air mass (Pady & Kelly, 

 1954; Pady & Kapica, 1955). Over the ocean polar air had generally 

 fewer bacteria and fungi than tropical air (Table XXIII). 



Over Quebec Province, in one air mass which was classified by 

 meteorologists as of polar origin, and which gave few bacteria or fungi in 

 culture, very many fungus spores \^'ere caught on a silicone-coated slide 

 in the slit sampler. The authors interpreted this as evidence of a load of 

 non-viable organisms which could only have originated in the tropics, and 

 suggested that the air had been carried into the Arctic, thence eastwards, 

 and finally southwards, during which passage most of the suspended 

 micro-organisms lost their viability. However, another explanation seems 

 possible on careful examination of the data. The visible total on the 

 silicone-coated slides, amounting to 18,700 per cubic metre, was made up 

 largely of yeasts (9,900 per cubic metre) and yellow-brown spores (7,500 

 per cubic metre). As 'about 50 per cent of the latter had an apiculus and 

 were considered to be basidiospores', the other 50 per cent were probably 

 also basidiospores lying in the alternative position (in which the apiculus 

 would be invisible). The flight may well have been through one or more 

 thermals arising from coniferous forests of Labrador and Quebec Province, 

 by which a polar air mass was becoming charged with the air-spora of 

 the ground layer. 



144 



