HYDROGEN ADAPTATION AND DE-ADAPTATION 133 



teria and fluorescent dyestuffs prove that oxygen appears in the sur- 

 rounding medium within 0.01 second after the beginning of photo- 

 synthesis, and that therefore the internal oxygen tension cannot be 

 markedly different from its external pressure. 



If this is so, then photochemical de-adaptation, at least, must be 

 attributed to an intermediate of photosynthesis, and not to free oxygen. 

 We shall designate this ''oxygen precursor" as {O2}, with the braces 

 indicating an "acceptor" or "carrier" molecule. The oxidant {O2} 

 is not likely to be the direct product of the primary photochemical 

 process. This is indicated by inhibition experiments (Chapter 12), 

 and by the observation of Rieke and Gaffron (1943) that the de-adapta- 

 tion in flashing light occurs at the same average intensity of illumination 

 as does the de-adaptation in continuous light. We therefore assume, 

 with Gaffron, that (at least) two successive enzymatic reactions are 

 required for the conversion of the primary photochemical product Z 

 into O2 (c/. Eqs. 7.10b and c): 



(6.7a) 2Z + H2O >H02!+2HZ 



(6.7b) §102) ^^02 



We attribute the photochemical de-adaptation to the reaction : 



(6.8) {02!+ 2 Eh >2EhO 



The rate at which the intermediate oxidant {O2} is produced in 

 light, must decrease with decreasing concentration of carbon dioxide, at 

 least in a certain range of concentrations (c/. Chapter 27, Vol. II). The 

 tolerance of the adapted state for hght should rise under these conditions. 

 In fact, if the carbon dioxide formed by fermentation is removed by an 

 alkaline absorber, the adapted state can be preserved in light which 

 would otherwise cause a rapid de-adaptation. 



Another treatment which prevents photochemical de-adaptation, is 

 poisoning with comparatively large quantities of hydroxylamine (c/. 

 Chapter 12). Apparently, this agent prevents the conversion of the 

 primary photochemical oxidation product into the hydrogenase-destroy- 

 ing intermediate (02}, i. e., it inhibits reaction (6.7a). 



Finally, the tolerance of adapted algae for light can be increased also 

 by the provision of added oxidation substrates, e. g., glucose. They 

 either accelerate the removal of the primary oxidation products, HZ, 

 and thus prevent the formation of the oxidant {O2} , or reduce this oxidant 

 in competition with the hydrogenase. 



Having thus attributed the photochemical de-adaptation to accumu- 

 lation of an intermediate oxidant {O2}, we ask whether dark de-adapta- 

 tion should be ascribed to a similar agent, or to free oxygen. Gaffron 



