THE AIR-SPORA NEAR THE EARTH S SURFACE 



ventilation and contamination by vital activity on board — at a level of 

 perhaps i per cent of that in dwellings in Paris. Nevertheless, Miquel 

 concluded, a ship travels in an atmosphere of self-contamination with 

 bacteria, moulds, and starch grains. 



On a voyage to the Caribbean, B. Fischer (1886) found very few 

 terrestrial microbes in ocean air — except near major land-masses, where 

 large numbers of bacteria appeared, apparently derived from the soil. 

 Flemming (1908) sampled air on a voyage from Hamburg to Rio de 

 Janeiro and Santos. Of his numerous 20-litre samples taken more than 

 200 km. from land, two-thirds were sterile — but even at this distance he 

 averaged thirty-four viable spores per cubic metre. These were mostly 

 of moulds and yeasts, though bacteria increased in proportion nearer to 

 land. 



Although over the sea the air is extremely pure in comparison with air 

 over land, most investigators on board ship have found bacteria, yeasts 

 and mould spores wherever tests have been made. Bisby (1935) exposed 

 Petri dishes on a voyage from Montreal to England and isolated bacteria, 

 Botrytis cinerea^ and Phoma hibernka, all near the coast of Ireland. 



The microbes of marine air have been studied at the Scripps Insti- 

 tution of Oceanography, California, by ZoBell & Mathews (1936) and 

 ZoBell (1942). They claimed that less than 5 per cent of bacteria in sea 

 water will grow in freshwater nutrient media, and a still smaller percen- 

 tage of freshwater bacteria will grow on sea-water media. Petri dishes 

 of nutrient media made up with distilled water (FW) or sea water (SW) 

 were exposed horizontally at distances of up to 1,600 metres inland from a 

 sea wall during a sea breeze of 5-8 m.p.h. (2-6 metres per sec). The 

 ^SW count decreased and the 'FW count increased with the distance 

 inland, the 'SW'/'FW ratio decreasing steadily from 10-20 at the sea 

 wall, to i-o at 400 metres, and 0-5 at 1,600 metres inland. The number of 

 mould spores usuallv increased with increasing distance from the sea. 



In a land breeze, littoral spray puts into the air salt-water bacteria 

 which can be detected at up to 8 km. out to sea, after which the ratio 

 'SW'/'FW goes down to i-o owing to the predominance of terrestrial 

 bacteria in the air for distances of 160 km. out to sea in fine weather. 

 Exceptionally at 880 metres height on Mt. Woodson, 32 km. inland, plates 

 exposed in a sea breeze following rain gave a ratio of 'SW/'FW = 2-06, 

 which was interpreted as indicating a predominance of marine bacteria 

 in the air in a region where soil bacteria normally predominate. It has 

 been calculated that an average of 127 cubic miles of sea water is put 

 into the Earth's atmosphere each year in the form of splash droplets, 

 and this would provide an average of only about one marine bacterium 

 per square centimetre of the Earth's surface per year — a small quantity 

 compared with the deposition rate from the land air-spora (ZoBell, 1942). 



Although all workers agree that marine air contains extremely few 

 bacteria, ZoBell points out that the use of sea-water media might be 



125 



