THE MICROBIOLOGY OF THE ATMOSPHERE 



property shared by all airborne microbes — visibility — and no other method 

 can reveal the whole range of the air-spora and disclose what numbers 

 and kinds are in the air awaiting identification. More precise methods can 

 then be applied to the study of smaller groups. 



The air-spora is very imperfectly explored, but sampling has already 

 shown that bacteria, algae, yeasts, spores of fungi, mosses and ferns, 

 pollens, and protozoa, commonly occur in the air. The air-spora near the 

 ground is extremely variable, both from time to time and from place to 

 place. It changes with season, and with weather — the concentration of 

 constituent types often changing several thousand-fold in the course of an 

 hour or two. It changes regularly in composition and concentration 

 throughout the day and night. Visual methods indicate that fungus spores 

 are normally in the majority over other components of the air-spora — 

 probably outnumbering bacteria. However, we lack methods for the 

 complete enumeration of the bacteria in outdoor air, and improved tech- 

 niques may reveal the presence of far more bacteria than we can recognize 

 at present. 



We now know that basidiospores (ballistospores) form an important 

 part of the air-spora, often carrying electrical charges, and probably out- 

 numbering even Cladosporiiwi (which is the dominant mould almost 

 everywhere). The tardy recognition of the abundance of basidiospores is 

 partly explained by their inefficient collection by standard samphng 

 methods, and partly by the unfamiliarity of many microbiologists with the 

 spores of the higher fungi (cf. Plate 6). Damp night air has its character- 

 istic spora, but at dawn the night-spora disappears suddenly; where it 

 goes we do not know. Rain removes the dry-air spora and substitutes a 

 different one. 



Vegetation is the main source of the air-spora, but most bacteria come 

 from blown soil or splashed water. The source of the yeasts, which are 

 sometimes recorded in large numbers, is still obscure — unless they prove 

 to be Sporobolomycetes. Air near the ground at times contains from tens 

 of thousands to hundreds of thousands of micro-organisms per cubic 

 metre. The air-spora near the ground is commonly dominated by con- 

 tributions from local and intermediate sources; but local effects are 

 smoothed out at high altitudes, over the oceans, and in polar regions. 



Spore concentration over the land usually decreases with altitude, 

 though not always regularly. Often a large proportion of the air-spora 

 must occur at altitudes higher than lo metres. Far out at sea, a few metres 

 above sea-level, the microbial content of the air is usually small ; but, at an 

 altitude of a few thousand metres over the ocean, the air often contains a 

 few bacteria and from tens to hundreds of fungus spores per cubic metre. 

 Current information suggests that, over the oceans, concentrations are 

 greater in the upper air than near sea-level. Clearing the lowest zone of 

 the atmosphere from microbes is most evident over the ocean. For above 

 land the spore-cloud near the ground is not only fed from above by 



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