ZoBell — 70 — Marine Microbiology 



intensity of light noted by various workers has led some of them to con- 

 clude that bactericidal radiations restrict the bacterial population in 

 surface waters (Pfennigee, 1902; Minder, 1920; DtJGGELi, 1924). 



Half a century ago Frankland and Frankland (1894) recorded the 

 following observations: (7) There is no question that bacteria are injured 

 by sunlight, particularly ultraviolet radiations. (2) A complicating factor 

 in the study of the effect of light on bacteria is the influence of the sur- 

 rounding medium, (j) Most divergent results have been obtained by 

 different workers as regards the precise duration of exposure to sunlight. 

 {4) Although the exact time that bacteria will endure insolation in water 

 varies greatly, the balance of evidence tends to show that they are less 

 rapidly destroyed in water than when exposed in culture media, probably 

 due to peroxide formation in the latter. (5) Evidence on the depth of 

 water to which the sun's rays take effect is very contradictory, but it 

 seems to be the concensus of most workers that solar rays are largely, if 

 not entirely, deprived of their bactericidal effect by passing through from 

 5 to 60 cm. of water. "In its special connection with the bacteriology of 

 water we must, therefore, recognize in sunshine a powerful bactericidal 

 agency, but one the importance of which there has been a considerable 

 tendency to magnify and exaggerate." 



Fischer (1894a) found more bacteria in surface water from the North 

 Atlantic at sunrise than in the afternoon. Schmidt-Nielson (1901) at- 

 tributed to the lethal action of sunlight the difference between 26 bac- 

 teria per ml. of surface sea water and 420 per ml. found at a depth of 25 

 meters. Similarly Gaarder and Sparck (1931) ascribed to the bacteri- 

 cidal effect of sunlight the paucity of bacteria in Norwegian oyster pools 

 in the summer as compared with their greater abundance in winter. 

 Minder (1920) sought to explain the summer minima and low bacterial 

 counts in surface water in Lake Zurich on a basis of the lethal action of 

 sunlight. ZiH (1932) offered the same explanation to account for the 

 greater abundance of bacteria in winter than in summer in Lake Lunz, 

 Switzerland. 



While it has been amply confirmed in nearly all parts of the world that 

 there is often an apparent negative correlation between the abundance of 

 bacteria in water and the intensity of light, there are no data which prove 

 that the bactericidal action of sunlight is directly responsible for the 

 diurnal, seasonal, or vertical distribution of bacteria in either the ocean or 

 large lakes. Fred et al. (1924) found no evidence that light influences the 

 bacteria in surface water of Lake Mendota, Wisconsin. Although the 

 number of bacteria observed by Lloyd (1930) in the surface layers of 

 water in the Clyde Sea, Scotland, increased slightly during the hours of 

 darkness, even on sunny summer days there were more bacteria at the 

 surface than in the underlying strata, leading her to deduce that the bac- 

 tericidal effect of sunlight is negligible. Reuszer (1933) found no cor- 

 relation between the number of bacteria in surface sea water and exposure 

 to sunlight during the summer months near Cape Cod, Massachusetts. 

 From their studies on the Cal-'fornia coast, ZoBell and Feltham (1934) 

 concluded that if there is any direct harmful effect of sunlight on bacteria 

 in sea water, it is obscured by other factors. 



Failing to lind any evidence for the lethal effect of sunlight upon the 

 diurnal, seasonal, or vertical distribution of bacteria in the sea, ZoBell 

 and McEwen (1935) exposed shallow layers of sea water in glass vessels 

 to direct mid-summer sunlight at La JoUa, California. The results, which 



