10 PHOTOSYNTHESIS IN NATURE CHAP. 1 



of infrared radiation from the earth). Photosynthesis was therefore more 

 intense than now, and the cover of vegetation denser. The removal of 

 carbon dioxide by the minerahzation of coal and oil, the formation of 

 carbonate rocks and tlie increase in the concentration of alkaline earth 

 ions in the oceans, have caused a decrease in photosynthesis and drop in 

 temperature, and thus contributed to the advent of the glacial period. 



We cannot dwell here much longer on the role of photosynthesis in 

 the evolution of the earth and its influence on the geochemical distribution 

 of the elements. It must be mentioned, however, that in addition to the 

 carbon dioxide cycle, photosynthesis gives rise also to a natural cycle of 

 oxygen. The atmosphere contains approximately 2.8 X 10^^ tons of this 

 element. If the living organisms consume annually the equivalent of 

 15 X 10^" tons of carbon dioxide, i. e., 12 X 10^° tons of oxygen, they must 

 renew all the oxygen in the air in a little over two thousand years, and 

 decompose all the water in the oceans in about two million years. This 

 is still a short period compared with the age of life on earth ; we can thus 

 conclude that all oxygen now present on the surface of the earth, as H2O 

 or O2, has repeatedly passed, in previous geological ages, from the atmos- 

 phere through the biosphere into the hydrosphere and back. 



Since this cycle is composed of a photochemical forward reaction and a nonphoto- 

 chemical back reaction, it does not lead to a thermodynamic equilibrium, but rather to 

 a steady " photostationary state." This fact may be of importance for the distribution 

 of oxygen isotopes between air and water. It has been revealed by the measurements of 

 Dole (1936), Greene and Voskuyl (1936) and others, that the oxygen in the air is about 

 7.5 X 10~^ atomic weight units heavier than oxygen in the water of the oceans. If air 

 and water were in a thermodynamic isotopic equilibrium at the average temperature 

 prevailing at sea level, the difference should be three times smaller. Among several 

 attempts to explain this discrepancy, Greene and Voskuyl suggested that photosynthesis 

 may play a part in it. They pointed out that, if plants convert oxygen from carbon 

 dioxide (two O atoms) and water (one O atom) into free oxygen without discrimination 

 between O^^ and O^^, the oxygen produced in this way must be 1 X 10~^ atomic weight 

 units heavier than oxygen in water (because the heavy isotope is more abundant in 

 carbon dioxide than in water). However, this explanation presumes that oxygen in the 

 air is the product of a single photosynthesis, whereas we have seen above that it 

 must have undergone repeated back and forth transfers. Furthermore, it has been 

 long suspected, and recently confirmed by exi)eriments with radioactive tracers, (c/. 

 page ')'■>), that all oxygen produced in photosynthesis comes from water. Thus, the 

 hypothesis of Greene and Voskuyl is untenable. However, photosynthesis may have 

 influenced the oxygen isotope distribution between air and water in a different way. 

 It was suggested above that this distribution corresponds to a photostationary state, and 

 not to a thermodynamic equilibrium. If oxygen is produced indiscriminately from 

 HoO'8 and H2O"' in photosynthesis (as seems to be indicated by the results of Vinogradov 

 and Teis 1941) but the heavy isotope reacts slower in tlic thermal liack reaction, tiiis 

 must bring about an accunmlation of the lieavy isotope in the atmosphoro, and may 

 thus account for tlie liighcr density of atmospheric oxygen. 



