142 



ANAEROBICALLY ADAPTED ALGAE 



CHAP. 6 



link between the hydrogenase-oxidase system on the "oxidation side" 

 of the primary photochemical process (c/. Scheme 6.1) and the catalytic 

 system on the "reduction side" of this primary process (which takes 

 care of the reduction of carbon dioxide in photosynthesis). A similar 



e;|(6.9) 



{Oa} 



H, 



Ah 





J [(6.lla) 



"[(6.6o) 



HzEh 

 J 



Eh 



I 



2/H2O2} 4H2A„ Cp2 



2HjO 



1'^"'=) 



1 



R 



4H20 



(6'2) 



4A„ 



{CH2O} 



Scheme 6. 1 1 

 A. Mechanism of the oxyhydrogen 

 reaction in adapted algae. 



B. Carbon dioxide reduction 

 coupled with the second stage 

 of the oxyhydrogen reaction. 



(Figures in parentheses refer to equations in text.) 



link is required also for the explanation of the photochemical absorption 

 and evolution of hydrogen which will be discussed in the next section. 



4. The Photochemical Reactions of Adapted Algae 



(V) and (VI). Photochemical Absorption and Liberation of Hydro- 

 gen. — As mentioned on page 129, illumination accelerates both evolution 

 and absorption of hydrogen by adapted Scenedesmus cells (cf. Fig. 13). 

 The dependence of both effects on light intensity is so different, that 

 sometimes a change in light intensity can convert hydrogen evolution 

 into hydrogen consumption. For example, on the left side of figure 13, 

 (which corresponds to 0.2% H2 in the air), hydrogen absorption prevails 

 at 800 lux and hydrogen evolution at 3500 lux. On the right side of the 

 same figure (in an atmosphere of pure hydrogen), even an illumination 

 of only 400 lux causes a rapid saturation of the cells with hydrogen. 



The photochemical liberation of hydrogen can be observed only in 

 absence of carbon dioxide. If the latter gas is admitted, it acts as an 

 acceptor for hydrogen, and photochemical hydrogen liberation is trans- 

 formed into photoreduction. Since acid fermentation liberates carbon 

 dioxide continuously as long as the cells are deprived of oxygen, experi- 

 ments on hydrogen liberation in light have to be carried out with alkali 

 in a side arm of the manometer, and allowance must be made for the 

 amount of carbon dioxide consumed by photoreduction before it had 



