144 ANAEROBICALLY ADAPTED ALGAE CHAP. 6 



final result is identical with that of the dark reaction (6.6b). In the 

 second alternative, reaction (6.15) is replaced by (7.10d,e). 



Comparing scheme 6. Ill with scheme 6.1, we find two new features. 

 In the first place, it provides for an enzymatic link between the catalytic 

 systems on both sides of the primary photochemical hydrogen transfer, 

 by means of the reaction: 



(6.13) 2HX + Ah >H2Ah + X 



This means that the primary reduction product in photosynthesis is 

 capable of supplying hydrogen back to the same acceptor which serves 

 as a reductant for the primary photochemical oxidation product, Z, 



CO, . Hx .^ "g*'^ 



HoO 



{C^^aO} TflSd: 



- - -■ X 



Scheme 6.III. — Photochemical and dark reactions in adapted algae. Simplified 

 representation. Arrows indicate hydrogen transfers between two oxidation-reduction 

 systems. FuU equations given in text and referred to by figures in parentheses. 



< Normal photosynthesis (6.7a, b), (7.10a), (7.10d, e). 



-< Photochemical and dark hydrogen liberation. The first step of the dark 



reaction (6.6b') is by-passed in light via (6.14), (7.10a) and (6.13). 



•< Photochemical and dark hydrogen consumption. The last step of the dark 



reaction (6.6b) is by-passed in light, via (6.6c), (7.10a) and (6.15); alterna- 

 tively hydrogen may go in Ught to CO2 (by reactions 7.10d, e) instead of R 

 (by reaction 6.15). 



by means of reaction (6.6c). The necessity for a nonphotochemical 

 linkage between the two catalytic systems already was emphasized in 

 connection with the mechanism of the chemosynthetic reduction of 

 carbon dioxide, where an enzymatic link had to be provided from the 

 hydrogenase system to carbon dioxide. 



The second feature of scheme 6. Ill are the direct links (6.14) and 

 (6.15): 



(6.14) H2R' + 2Z ).R' + 2HZ 



(6.15) R -I- 2 HX > H2R + 2 X 



