DISCRIMINATION BETWEEN O ISOTOPES 1917 



originating in CO2, and a carbohydrate with Vs of its oxygen atoms coming 

 from the same source; (37D.4C) suggests that a secondary process dikites 

 O, the "carbon dioxide oxygen," accumulated in the carbohydrate, by an 

 exchange reaction with water, reducing the 3 : 1 ratio resulting from reaction 

 (37D.4B), to the experimentally found ratio of 1:1. 



It must be noted that the 0-contribution to photosj^nthetic oxygen, claimed by the 

 Japanese authors, is sufficient to account for only one-half of the "excess" 0(18) in the 

 atmosphere (compared to water), and thus does not obviate the necessity of a special 

 hypothesis to explain this excess. 



The results of Yosida et al. were not confirmed by the measurements of 

 Dole and Jenks (1944). In agreement with Vinogradov and Teis, they 

 found that oxygen produced by various plants (aquatic higher plants, 

 Chlorella, sunflower, Coleus) gave water which was similar to, or — more 

 exactly — only 0.6-1. 87/I. heavier than the water from which it was liber- 

 ated by the plants. The excess density corresponds to that predicted for 

 the isotopic equilibrium (at 25° C.) between liquid water and oxygen gas. 

 It could be therefore suggested that "photosynthetic" oxygen, produced 

 indiscriminately from H20(16) and H20(18), is, after its hberation, equili- 

 brated isotopically with water. However, this equilibration is known to be 

 a very slow process in vitro: if it were catalytically accelerated in living 

 plant cells the same final isotopic composition could result also from pri- 

 mary oxygen liberation from carbon dioxide, thus invalidating the whole 

 method. 



A satisfactory explanation of the isotopic difference between the oxygen 

 in H2O and in photosynthetically liberated O2 remains to be given; but as to 

 the amount of this difference, the smaller figure of Dole, Vinogradov, 

 Kamen and their co-workers seem, by their approximate agreement, to 

 carry more weight than the much larger figures of Yosida et al. 



The discrepancy between the composition of the photosynthetic oxygen 

 and of oxygen in the atmosphere, in any case, remains to be interpreted. 

 The hypothesis of the photostationary state, referred to above, was tested 

 by experiments of Dole, Hawkings and Barker (1947), who measured the 

 isotopic composition of oxygen taken up from the air by soil bacteria. 

 These organisms — which account for a large proportion of the total con- 

 version of oxygen to water on the face of the earth — were found to dis- 

 criminate in favor of 0(16), but only to an extent far too small to account 

 for the excess 0(18) found in the air. Dole et al. therefore considered the 

 earliest hypothesis, proposed by Dole in 1936, as the most satisfactory one. 

 (This hypothesis suggested that an isotopic equilibrium between oxygen 

 gas and carbon dioxide is established in the stratosphere, under the in- 

 fluence of ultraviolet radiation, at —55° C, and is "frozen" when the gases 

 descend into the lower atmosphere, where they are protected from ultra- 



