168 PRIMARY PHOTOCHEMICAL PROCESS CHAP. 7 



HX is reoxidized by reaction with the complex, {CO2}, an exhaustion of 

 {CO2} may lead to an accumulation of reduced intermediates, HX, and 

 exhaustion of the quenching species, X. 



On the basis of all these considerations, without pretending to be 

 able to give a final answer to the problem of the primary photochemical 

 process in photosynthesis, it seems that eight primary processes of the 

 type assumed in scheme 7. VI, perhaps, with chlorophyll identified with 

 the reductant, HZ (c/. Chapter 19, page 552), provides the best working 

 hypothesis. 



Scheme 7.V, which contains two sets of different primary processes 

 but leaves open the possibility of the same intermediate reductants 

 occurring in photosynthesis and chemosynthesis, is our second choice, 

 and would become the first one if the existence of two interconvertible 

 green modifications of chlorophyll — one a photo-oxidant and one a 

 photoreductant — would be definitely confirmed by experiments in vitro. 



8. The Primary Process in Bacteria and Adapted Algae 



In hydrogen-adapted algae and in bacteria, molecular hydrogen, hy- 

 drogen sulfide, or other inorganic or organic hydrogen donors replace 

 water in the role of the ultimate reductant in photosynthesis. Does this 

 substitution mean a change in the primary photochemical process, or 

 merely a different course of secondary catalytic reactions? 



Nakamura (1938), van Niel (1941), Franck and Gaffron (1941), and 

 Gaffron (1942) all suggested, for different reasons, that the substitute 

 reductants do not participate in the primary photochemical process. 

 One of van Niel's arguments was the observation (c/. page 110) that 

 organic reductants are used up by Spirillum riibrum at the same rate in 

 the dark and in light. This indicates a preliminary enzymatic transfor- 

 mation of these reductants, e. g., hydrogen transfer to the hydrogenase 

 system (cf. Chapter 6, Eq. 6.6b), which they have to undergo both in 

 respiration and in photoreduction. 



Since van Niel and Gaffron considered the oxidation of water as one 

 (or even the only) primary photochemical reaction in ordinary photosyn- 

 thesis (as in Scheme 7. II), the assumption that substitute reductants do 

 not participate in the photochemical process led them to the logical con- 

 clusion that, in bacterial photoreduction too, the primary photochemical 

 process is the oxidation of water. The fact that purple bacteria do not 

 evolve oxygen in light could then be explained in two ways. One hy- 

 pothesis, suggested by Gaffron, was that the intermediate product of 

 water oxidation, {OH}, can only be reduced in bacteria by substitute 

 reductants — hydrogen, hydrogen sulfide, etc.— (and not by water), 

 because these organisms contain an active hydrogenase system, but not 

 the oxygen-liberating enzyme, Eq. The other hypothesis, proposed by 



