536 



PHOTOCHEMISTRY OF PIGMENTS IN VIVO 



CHAP. 19 



(with glucose as a source of organic matter). All curves, regardless of 

 the treatment of the algae, approached the same maximum rate of 

 oxygen uptake in very strong light, a rate equivalent to between two and 

 four times the dark respiration. This final rate was not affected by 

 variations in carbon dioxide supply. The addition of cyanide (0.01 mole 

 per liter) which inhibited photosynthesis practically completely and 

 caused the oxygen consumption to begin its increase at light intensities 

 as low as 1,000 f.-c, also left the final rate of autoxidation in strong Hght 

 (20,000 f.-c.) unaffected. 



It was mentioned in chapter 13 (page 329) that Franck and French 

 (1941) attributed the inactivation of photosynthesis by excess oxygen to 



18 24 



Light intensity 



X 1000 f.-c. 



Fig. 61. — Oxygen liberation by Chlorella as a function 

 of light intensity in very strong light (after Myers and 

 Burr 1940). A, Cells grown in 5% CO2 and 450 f.-c; 

 B, cells grown in air (0.03% CO2) and 450 f.-c. 



a photoxidation of the "carboxylase," Ea, which supplies the photo- 

 synthetic mechanism with the carbon dioxide-acceptor complex, {C02}. 

 The same explanation can be suggested also for the inactivation by 

 intense light. Whenever enzyme Ea is inactivated, the concentration of 

 the normal oxidant in the primary photochemical process becomes de- 

 pleted, and oxygen is given a chance to act as a "substitute oxidant." 

 We have all reason to assume that the inhibition by excess light is 

 essentially the same phenomenon as inhibition by excess oxygen, narcoti- 

 zation, or carbon dioxide starvation. However, it would be important 

 to prove that this type of inhibition, too, is chlorophyll-sensitized and 

 therefore equally strong in red and blue light. Montfort (1941) asserted 

 that the "sunstroke" which marine algae suffer in intense light is caused 

 only by the short-wave part of the spectrum. 



