298 KADIATION BIOLOGY 



sufficient to invalidate the more direct evidence derived from the 0^^ 

 experiments. 



The view that the photosynthetically produced oxygen originates from 

 water was held long before isotope studies were introduced into biological 

 research. This opinion arose from comparing the photosynthetic process 

 of green plants with that of purple sulfur bacteria (Van Niel, 1931). 

 These organisms, for example, realize the following over-all conversion: 



COo + 2H2S i (CH2O) + 2S + H2O. (5-3) 



A comparison of Eqs. (5-3) and (5-2) strongly suggests that in green 

 plants the relation between water and oxygen is the same as that between 

 •hydrogen sulfide and sulfur in purple sulfur bacteria. This means that 

 the oxygen in green-plant photosynthesis arises from a photochemical' 

 decomposition of water. 



Study of the photosynthetic process of purple sulfur bacteria (Thio- 

 rhodaceae), of the related green sulfur bacteria (Thiochloraceae), and of 

 the nonsulfur purple bacteria (Athiorhodaceae) reveals many interesting 

 facts, which in some respects have made these bacteria advantageous 

 over green plants for the study of the process. The most obvious feature 

 is that carbon dioxide is photochemically reduced by these bacteria at 

 the expense of substances other than water.* In green-plant photosyn- 

 thesis, only water, so far, has been known to act as a hydrogen donor 

 for the photochemical process, whereas with most of the bacteria already 

 mentioned various reduced substances can act as such. Purple sulfur 

 bacteria, for example, can reduce carbon dioxide not only with hydrogen 

 sulfide, sulfur, sodium thiosulfate, and other reduced sulfur compounds 

 but also with various organic substances, especially aliphatic acids, e.g., 

 butyric acid (Muller, 1933), and with molecular hydrogen. Instead of 

 carbon dioxide, organic carbon compounds, especially acids of a more 

 oxidized type, e.g., malic acid, can be used as substrates for photosyn- 

 thetic conversions (ibid.). Whereas the Thiorhodaceae in their develop- 

 ment are obligately anaerobic and obligately photosynthetic (Roelofsen, 

 1935), some Athiorhodaceae can develop either anaerobically photosyn- 

 thetically or aerobically heterotrophically, in each case using much the 

 same substrates (Van Niel and Muller, 1931). 



Although certain Thiorhodaceae (e.g., Chromatium, strain D) in fairly dense 

 suspensions may remain alive and motile in the open air for a very long time, 

 development from a small inoculum often fails to start when the compounds in 

 the medium are not sufficiently reduced. The present writer, for example, occa- 

 sionally observed that cultures in stoppered bottles completely filled with boiled 

 culture medium did not grow unless a drop of a sodium sulfide solution was added 

 (unpublished work). 



1 In this connection the term "photochemical" does not imply that in this process 

 no dark-reaction steps are included. 



