INFLUENCE OF OXYGEN ON INDUCTION 1365 



— on how long it takes for the oxygen produced by residual photcjsynthesis 

 to "burn up" the accumulated (and continuously produced) inhibitors 

 arising from the anaerol)ic metabolism. Whether this is at all possible is 

 determined by the extent of competition of other oxygen-consuming side 

 reactions, as well as by the exact rate of the residual oxygen production. 

 In hydrogen-adapted Scenedesmvs, competition comes, for example, from 

 the "oxyhydrogen" reaction (2 H2 -j- O2 —^ 2 H2O). If all the oxygen lib- 

 erated by photosynthesis is swept away by a stream of oxygen-free gas 

 (as was done in the experiments of Franck, Pringsheim and Lad), "anaero- 

 bic inhibition" can be prolonged practically indefinitely, even in strong 

 light, and reversible light curves of residual, anaerobic photosynthesis can 

 be obtained. 



Some algae {e. g., Scenedesmus) acquire, simultaneouslj^ with anaerobic 

 inhibition of oxygen production, the capacity for photochemical reactions 

 that involve absorption or liberation of hydrogen (cf. chapter 6). It was 

 the study of these phenomena that led Gaffron (1935, 1937 1-^, 1939'-, 

 1940) to the conclusion that the cause of induction generally lies in the de- 

 activation of a catal3^st or catalysts concerned primarily with the liberation 

 of molecular oxygen (Franck's "catalyst C"). In Scenedesmus, this de- 

 activation is coupled with the activation (probably, by reduction) of a 

 "hydrogenase," capable of transferring molecular hydrogen; in most other 

 green plants, no hydrogenase is produced, and the only effect of anaerobic 

 incubation is therefore an enhanced initial inhibition of ordinary photosyn- 

 thesis. In purple bacteria, which have no oxygen-liberating enzymatic 

 system to begin with, but which do contain an active hydrogenase, anaero- 

 biosis does not affect the capacity for photochemical metabolism at all; 

 often it even represents the only condition under which this metabolism is 

 possible (cf. chapter 5). 



The question arises as to whether the persistent inhibition of photosyn- 

 thesis observed under strictly anaerobic conditions is caused simply by a 

 more complete deactivation (or even destruction) of the oxygen-liberating 

 enzjrme (the autocatalytic regeneration of which by photosynthesis is thus 

 delayed), or by another, independent phenomenon. It seems that both fac- 

 tors play a role, so that "anaerobic inhibition" is the result of two processes 

 — one affecting the activity of the oxygen-liberating enzymatic system, and 

 the other, obstructing in a less specific way all catalytic agents, including 

 chlorophyll and the "finishing catalyst" Eb (Franck's "catalyst B"). 

 Perhaps the two inhibiting agents are identical, but widely different 

 amounts are needed for the specific and the nonspecific "narcotic" action. 

 (Perhaps the same hypothesis can be applied to "short" and "long" induc- 

 tion under aerobic conditions.) 



Among experiments that demonstrate the production by anaerobic 



