348 Annals New York Academy of Sciences 



growth in environments in which the chemical potential of water is low. Noth- 

 ing more than speculative attention has been given to the possibility of micro- 

 bial growth in nonaciueous media. It should not be forgotten, however, that 

 the water dependent metabolism of all living organisms that are known must 

 be at least to some extent the end result of selection on a water rich earth. It 

 is not known whether life could form on a planet on which the predominant 

 lifjuid was some other compound than water. One should also remember that 

 under aerobic conditions of metabolism water is one of the main excretory com- 

 pounds formed by living organisms. Mechanisms for the selective retention of 

 metabolically formed water might enable some organisms to persist and grow 

 in liquid media with low water contents. 



Other Factors 



In relation to natural radiations, direct sunlight is known to be lethal for 

 many microorganisms, but the effects probably result from dehydration and 

 high temperatures in most cases. ZoBell and McEwen (1935) were unable to 

 detect any lethal effect when marine bacteria were exposed in layers of water 

 greater than 5 mm. in thickness to full noon sunlight on a roof top in La JoUa, 

 Cahfornia. Two halophilic bacteria isolated by Browne (1922) withstood in- 

 definite exposure to "the brightest sunlight." 



The effect of ultraviolet light on microorganisms has been studied by many 

 workers; however, most of the data refer to high dosages for short times. It 

 would be of much interest to know the maximal levels of continuous ultraviolet 

 radiation that can be tolerated by actively growing cultures. Although ultra- 

 violet light in high doses is harmful to all organisms, it must be remembered 

 that deleterious effects are much less pronounced above M)() m/x- than below for 

 equal energies of incident light (Meier, 1936). There is also a great variation 

 in the sensitivity of different microorganisms to ultraviolet light. Siliceous 

 tests of diatoms apparently afford no protection (Ursprung and Bloom, 1917). 

 Because the possibility of shielding and the well known photoreactivation phe- 

 nomenon, whereby the lethality of ultraviolet light is partly reversed by later 

 application of visible light, it is probably incorrect to assume, as many have 

 done, that an ozone free earth would necessarily be sterile. 



The biological effects of gamma- and other types of ionizing radiations have 

 also been studied by many investigators. Single large doses have usually been 

 used. Populations of many microorganisms will survive single doses in the 

 range of 10'' r. (Shields et al., 1961). Saccharomyces cerevisiae has been cultured 

 under continuous exposure to 50 mr. per day of radium emanations (Maisin et 

 al., 1960), however, this is doubtlessly far below the maximal level that can be 

 tolerated. According to Prince (1960) a good place to look for radiation re- 

 sistant microorganisms would be in nuclear reactors. He states that it is 

 "common knowledge that some bacteria can adapt even to the water in a swim- 

 ming-pool-type nuclear reactor." 



A few other case histories will serve to round out the picture that has been 

 presented. Some of these refer to survival rather than to growth and repro- 

 duction. The cases are as follows. 



(1) The growth of several bacteria and fungi in concentrated CuS04 solu- 

 tions. The subject has been reviewed by Starkey and Waksman (1943). 



