ZoBell — 72 — Marine Microbiology 



lengths shorter than 2800 A is very small. Virtually all of the radiations 

 which are most bactericidal are absorbed by the first meter of sea water. 

 The intensity of the most lethal radiations is reduced nearly 50 per cent 

 by passage through only 10 cm. of sea water. The lethal action of ultra- 

 violet radiations decreases exponentially with decreasing intensity. If the 

 intensity of sunlight is sufficiently intense at the surface of the water to 

 kill a given bacterium in ten seconds, it would require more than 100 sec- 

 onds to kill such a bacterium protected by a layer of sea water 40 cm. 

 thick, and 1000 seconds to kill it in 70 cm. of water. 



The absorption curves in Figure 8 are based upon observations by 

 HuLBURT (1928) in which the angle of incidence of the radiations is 90° 

 and in perfectly quiet, clear sea water free of suspended matter. The 

 transmission of radiations is reduced proportionately as the angle of inci- 

 dence decreases. The transmission is also reduced when the surface of the 

 water is ruffled and by the presence of particulate matter. So few as 100 

 bacteria per ml. of water greatly decrease the transmission of ultraviolet 

 radiations. 



HuLBURT (1928) found that sea water is much less transparent to ultra- 

 violet radiations than is pure distilled water, the absorption coefficient for 

 the wave length of 3030 A being 0.017 in sea water as compared with 

 0.005 i^ distilled water. The transmission of ultraviolet radiations by 

 fresh water in lakes or rivers is primarily a function of its content of elec- 

 trolytes and suspended materials, its turbulence, the angle of incidence, 

 and the intensity of the incident radiations. 



In natural fresh water, Buchner (1893) noted only a feeble lethal 

 action of sunlight on bacteria, the bactericidal power penetrating less 

 than 3 meters. Jordan (1900) found that in river water, sunlight is vir- 

 tually without bactericidal action. Even in clear calm water it is doubtful 

 whether abiotic radiations are active more than 5 feet from the surface, 

 and due to the turbidity and constant movement of most bodies of water 

 in nature, it is improbable that bacteria are subjected to radiations for 

 sufficient time or in sufficient intensity to be killed. 



From his data on the seasonal and vertical distribution of bacteria in 

 Lake Windermere, England, Taylor (1940) detected no bacteriostatic 

 effect of the sun's rays. During an eleven-week period he found almost 

 as many bacteria in samples collected from the immediate surface as in 

 samples from a depth of i meter, and there were generally more bacteria 

 in surface water than in samples from a depth of 10 meters. 



It has been pointed out by Prescott and Winslow (1931) that the 

 tendency of bacteria to settle in standing water has been misinterpreted 

 as a lethal action of sunlight. This criticism is applied to the work of 

 Clemesha who attributed very great importance to the action of light 

 in the self-purification of lakes and rivers in India after observing more 

 bacteria in bottom water than in the superficial layers of water. 



The increasing abundance of bacteria from the surface downward as 

 shown by the curve in Figure 7 on page 67 may appear at first sight to 

 indicate a direct inhibitory effect of sunlight in surface waters, but there 

 are other more cogent factors which account for the vertical distribution 

 of bacteria. Unquestionably light does have an indirect effect upon the 

 diurnal, seasonal, horizontal, and vertical distribution of bacteria in sea 

 water or lake water through the intermediary of photosynthetic organ- 

 isms and to a lesser extent of phototropic organisms. In the latter cate- 

 gory are certain zooplankton which migrate towards the surface during 



