134 ANAEROBICALLY ADAPTED ALGAE CHAP. 6 



pointed out, in support of the second viewpoint, that the maximum 

 rate of hydrogen consumption by the oxyhydrogen reaction, attained 

 when dark de-adaptation sets in, is approximately equal to the maximum 

 rate of hydrogen consumption by photoreduction, reached just prior to 

 photochemical de-adaptation. This equality finds a plausible explanation 

 in the assumption that de-adaptation is caused by oxidation inter- 

 mediates, which in both cases must be removed by the hydrogenase 

 system. As long as the removal keeps pace with the photochemical or 

 enzymatic supply of the oxidants, the adapted state is stable; whenever 

 the supply becomes too rapid, an accumulation of oxidants occurs and 

 brings about the de-activation of the hydrogenase. In other words: 

 the maximum attainable rates of photoreduction (in light) and of the 

 oxyhydrogen reaction (in the dark) are the same, because they are both 

 limited by the quantity of available hydrogenase. 



One may further ask whether the intermediate oxidant which causes 

 de-activation in the dark is identical with the intermediate {O2} of 

 photosynthesis and photoreduction, or merely similar to it in its capacity 

 to oxidize the hydrogenase. This question is important because if the 

 first alternative were correct, the oxygen evolution in photosynthesis 

 (reaction 6.7b) would have to be considered as a reversible process, its 

 direction depending on the concentration of {O2} and the partial pressure 

 of oxygen. 



The following considerations speak against this concept. In the first 

 place, almost the only known reversible oxygen acceptor in nature is 

 hemoglobin, and it is doubtful whether a similar catalyst exists in plants 

 (cf. Chapter 11). In the second place, Gaffron concluded from poisoning 

 experiments that the enzyme Eo ("deoxidase," cf. Chapter 11), which 

 catalyzes the oxygen-liberating reaction (6.7b), is de-activated, in the 

 course of anaerobic adaptation, simultaneously with the activation of the 

 hydrogenase. Eh. If this conclusion is correct, a reversal of reaction 

 (6.7b) in adapted algae is impossible, even if this reaction were thermo- 

 dynamically reversible in the first place. 



Gaffron's argumentation in favor of a de-activation of the enzyme, 

 Eo, in the adapted state was as follows: Cyanide prevents adaptation; 

 if applied after completed adaptation, it causes a slow de-adaptation in 

 light. This is best explained by assuming that an oxidation of the 

 hydrogenase occurs continuously during photoreduction, but does not 

 lead to de-adaptation as long as the autocatalytic re-adaptation (reaction 

 6.5) holds step with the de-adapting reaction (6.8). Cyanide blocks 

 re-adaptation (by "freezing" the hydrogenase in its oxidized state, EhO), 

 and thus causes a cumulative de-adaptation in light. 



This hypothesis implies that a small quantity of the intermediate 

 oxidants {O2} is formed even in the adapted state, where the great 



