PRIMARY PROCESS IN BACTERIA AND ADAPTED ALGAE 169 



van Niel, was that the primary product obtained by the oxidation of 

 water in bacteria, {OH}^, is somewhat different from that formed in 

 green plants, {OH}-^, and therefore incapable of conversion into oxygen. 

 For example, the energy content of {OH}^ could be insufficient for this 

 conversion, perhaps because this product is formed with the help of 

 infrared quanta, supplied by bacteriochlorophyll, which are about 30% 

 smaller than the red quanta made available by ordinary chlorophyll. 

 The difference between {OH}^ and {OHp may be in the nature of the 

 acceptor (symbolized by brackets), the simplest hypothesis being that 

 this acceptor is the sensitizing pigment itself, that is, chlorophyll in 

 green plants and bacteriochlorophyll in purple bacteria. 



If we accept van Niel's hypothesis, we must conclude that the 

 mechanism of photoreduction is somewhat different in hydrogen-adapted 

 algae and in purple bacteria. The former contain ordinary chlorophyll, 

 apparently unaffected by the adaptation process; the primary oxidation 

 product of water, {OH}^, is thus probably the same in the ordinary and 

 in the adapted state, and the difference in the final stages of oxidation 

 must be attributed to the activation of the hydrogenase system and the 

 simultaneous inactivation of the oxygen-liberating enzyme, Eo, as 

 suggested by Gaffron (c/. Chapter 6, page 134). The identity of the 

 primary processes in adapted and ordinary green algae is supported by 

 the observations of Rieke and Gaffron (1943) that the maximum quantum 

 yield and the saturation rate in flashing light are the same in the photo- 

 reduction by adapted algae as in the photosynthesis in the nonadapted 

 state. In the case of purple bacteria, on the other hand, the primary 

 oxidation product, {OH}^, is naturally incapable of conversion into 

 free oxygen; therefore, aerobic conditions may cause only a complete 

 cessation of synthesis (if they lead to an oxidative deactivation of the 

 hydrogenase) but cannot cause a transition to ordinary photosynthesis 

 (with water as reductant), as this occurs in the "de-adaptation" of 

 green algae. 



However, an even simpler description of the same facts becomes pos- 

 sible if one assumes, as we have done above, that the primary photo- 

 chemical process is the oxidation of an intermediate reductant, HZ, and 

 that, in the course of normal photosynthesis, the oxidation product, 

 Z, recovers hydrogen from water by a nonphotochemical reaction. In 

 adapted algae, this recovery is blocked, and a reaction with a substitute 

 reductant (e. g., H2) is made possible by a characteristic transformation 

 of the enzymatic system (activation of the hydrogenase, deactivation of 

 the deoxidase). This was the mechanism assumed in schemes 6.1 and 

 6. III. In purple bacteria, on the other hand, the primary reductant, 

 HZ^, is different from the corresponding compound in green plants, HZ^, 

 and its oxidation product, Z^, is incapable of oxidizing water, but capable 



