135 



and cancers ha\e out\vardl>' been entirely unaffected by con- 

 centrated or dilute DjO (23, 29, 30, 37). 



One experimenter failed to note an\' sij^nificant effect of 94 

 per cent DoC) on cell division of germinating wheat embryos 

 (19) and another reported that the growth of Staphylococcus 

 sp. and the typhus bacillus was unaffected over a period of 70 

 days by 92-94 per cent D2O (10). 



Stimulation of organisms exposed to dilute D2O — i.e., con- 

 centrations less than 1 per cent — has been reported. Improved 

 growth of various species of protozoa, bacteria, algae, fungi, 

 and green plants has been said to result from treatment with 

 0.05-0.13 per cent D2O (1, 9, 20, 26). Slight inhibition of the 

 germination of Lupinus seeds and growth of the seedlings has 

 also been reported (18). 



Other workers, however, have been unable to detect any in- 

 fluence of carefully purified dilute heavy water on living organ- 

 isms. In a number of experiments cultures of several species of 

 protozoa, luminous bacteria, soil bacteria, and different patho- 

 genic organisms (in vivo and in vitro) were not influenced by 

 dilute D2O (5, 11, 27). In other tests, fungi and the roots of 

 wheat plants grew equally well in 0.46 per cent D2O and in H2O 

 (7,22). 



Influence of D2O on photosynthesis. — A study of the influence 

 of D2O on photosynthesis is of particular interest, since water 

 enters directly into the chemical reactions at one stage of the 

 process. Many investigators have determined the effect of vary- 

 ing CO2 concentration, light intensity, temperature, and other 

 factors upon the photosynthetic process, and important hypoth- 

 eses and theories of the mechanism of photosynthesis have 

 been derived from their observations. For obvious reasons, how- 

 ever, it has been difficult to determine the role of H2O in the 

 process, but now the use of D2O suggests an approach to this 

 important problem. 



Preliminary reports from different laboratories show that 

 photosynthesis is considerably reduced in heavy water. For ex- 

 ample, it was found from direct measurements that with 99.9 

 per cent DoO photosynthesis in the unicellular green alga Chlo- 

 rella sp. was 0.41 of that with H2O (8). This figure closely ap- 

 proximates the earlier estimate (based on chemical analyses of 

 algae cultured in D2O) that D is assimilated 41 per cent as 

 rapidly as H (24). 



