THE AIR-SPORA NEAR THE EARTH'S SURFACE 



In practice the problem of recording an air-spora requires the recogni- 

 tion of a number of categories for the organisms most commonly present 

 in the sample, including a miscellaneous group, which may ultimately 

 contain from lo to 15 per cent of the total. Pollens of flowering plants can 

 often be identified to the species level, and so can a few fungal spores — 

 especially of the Urcdineae and some other plant pathogens (Appendix I, 

 p. 207). In samples of the outdoor air, bacteria can seldom even be 

 recognized visually as such, let alone identified, and the only sampling 

 devices suitable for their study involve making cultures. In practice the 

 categories adopted are of varying degrees of arbitrariness, and the names 

 are applied to them for convenience of reference ; but the different cate- 

 gories behave so differently that it would be intolerable to have no way of 

 referring to them. 



Taxonomic Groups Needing Study in the Air-Spora 



We now have some knowledge of the occurrence of bacteria, fungi, and 

 pollens as components of the outdoor air-spora, but there are some groups 

 whose presence is obvious enough yet about which we have scarcely any 

 quantitative information. Thus I know of no systematic records having 

 been kept to assess the concentration of actinomycetes, and of moss and 

 liverwort spores, in the atmosphere. 



(i) Protozoa. For these we have the estimate by Miquel (1883) of an 

 average of o-i to 0-2 airborne protozoan 'eggs' per cubic metre at the 

 Observatoire Montsouris, Paris. Using a Pasteur-type filter, Puschkarew 

 (191 3) sampled air near ground-level on the right bank of the Neckar 

 downstream from Heidelberg. In forty-nine tests, on different occasions 

 and at different times of the day, his catch works out at an average of 2-5 

 protozoan cysts per cubic metre of air. His cultures included species of 

 Amoeba, Bodo, Monas, Calpoda, etc. Curiously enough, he concluded 

 that this concentration was too small to account for the observed almost 

 world-wide uniformity of species of freshwater protozoa, and that other 

 dispersal routes must be important (as no doubt they are). This study 

 awaits convenient techniques. 



(ii) Algae. Microscopic terrestrial and freshwater algae occur in the 

 air, but have been little studied. A few samples were taken on the roof 

 of buildings at Leiden by Overeem (1937), using the 'standard aeroscope' 

 and Rettger bubbler. At least forty algae were obtained from a total of 

 20 cubic metres of air, including: Chlorococctwi, Chlorella, Pleurococcus, 

 Stkhococcus, and Navicula. 



The occurrence of blue-green algae resembling species of Gloeocapsa 

 or other members of the Chroococcaceae was recorded by Gregory et 

 al (1955) from continuous sampling with a Hirst trap at Thorney Island 

 in Chichester Harbour, England, from 30 June to 13 July, 1954. Concen- 

 trations averaged 1 10 colonies per cubic metre of air (averaging 8 cells per 



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