132 



ANAEROBICALLY ADAPTED ALGAE 



CHAP. 6 



mechanism (6.5) provides for rapid re-adaptation whenever small 

 amounts of the hydrogenated enzyme still are present. 



De-adaptation can be enforced also in the dark, by means of oxygen. 

 While 0.5% oxygen (constantly renewed to prevent exhaustion by 

 respiration) is sufficient to prevent adaptation, the tolerance for oxygen 

 may rise, after adaptation, to 1 or 2%. This is due to the capacity of 

 the adapted cells for the oxyhydrogen reaction (III) ; only when the rate 

 of oxygen fixation by the cells becomes higher than the maximum 

 possible rate of this reaction, does de-activation become inevitable. 



c 



o 



c 

 u 



» 



i- 



a. 



E 

 E 





 o 



O 

 O 



20 40 60 80 



Time, minufcs 



100 



120 



Fig. 8. — The "de-adaptation" of anaerobically adapted 

 Scenedesmus by light increase from 500 to 5000 lux is not 

 reversed by return to 500 lux (after Gaff r on 1941). 



If the reduction of an enzyme is the basis of adaptation, its reoxidation 

 must be the basis of de-adaptation. This oxidation may be attributed 

 either to free oxygen, or to cellular oxidants, formed as intermediates 

 either in the photochemical reduction of carbon dioxide (in the case of 

 photochemical de-adaptation), or in the oxyhydrogen reaction (in the 

 case of dark de-adaptation). 



If one assumes that photochemical and dark de-adaptation are both 

 caused by free oxygen, one cannot help noticing the difference between 

 the partial pressure of oxygen at the moment of photochemical de-adapta- 

 tion (which is exceedingly low) and the comparatively high pressure 

 required for de-adaptation in the dark. To explain this difference, one 

 could suggest that when oxygen is produced photochemically within the 

 cell, a high internal pressure has to be built up before any gas can escape 

 into the atmosphere; so that chemical and photochemical de-adaptation 

 could correspond to the same internal oxygen tension in the chloroplasts. 

 Gaffron argued, however, that experiments with the oxygen electrode 

 {cf. Volume II, Chapter 33), as well as observations with luminous bac- 



