MANGANESE IN OXYGEN EVOLUTION 245 



photoreduction at very high light intensities than is manganese de- 

 ficiency. This is quite understandable since the concentration of 

 poison required inhibits photosynthesis completely, whereas man- 

 ganese deficiency does not. Other differences between poisoned and 

 deficient algae are the following: (7) The above-mentioned poisons, 

 if added prior to adaptation, will inhibit the process of adaptation to 

 hydrogen (4,7). With manganese-deficient algae, no such inhibition 

 of adaptation is observed. (^) High concentrations of poisons reduce 

 the quantum yield of photoreduction (i.e., the rate at low light in- 

 tensities) up to 50% (4,7). With increasing strength of manganese 

 deficiency, however, up to an age of five weeks for the deficient cul- 

 tures, the rate of photoreduction in the light-limited range remains 

 absolutely constant. At the same time, the protection of photoreduc- 

 tion against deadaptation at higher light intensities increases steadily. 

 (By contrast, the yield of photosynthesis drops to about one-fourth 

 during the first 7 to 10 days of deficiency.) Later the manganese- 

 deficient cells become chlorotic, the yield of photoreduction de- 

 creases, and the algae deteriorate rapidly. 



An addition of manganese to deficient cells results under aerobic 

 conditions in the well-known increase in photosynthesis (1-3); with 

 adapted algae, however, a decrease of photoreduction occurs be- 

 cause deadaptation is accelerated and starts at lower light intensi- 

 ties. 



In the light-limited range, the amounts of CO2 reduced per unit of 

 time are approximately the same for normal algae in photosynthesis 

 and photoreduction and for manganese-deficient algae in photoreduc- 

 tion. These deficient algae, however, can reduce only one-fourth as 

 much CO2 in aerobic photosynthesis. 



Studies performed with algae deficient in phosphate and in iron 

 (c/. 2) show that the effect of manganese is quite specific for this 

 ion. Phosphate deficiency inhibits photosynthesis and photoreduc- 

 tion to about the same degree; the percentage inhibition of photo- 

 synthesis increases with increasing light intensity. The effect of 

 phosphorus deficiency resembles the action of dinitrophenol on 

 photoreduction as observed by Gaffron with Scenedesmus D3 (4). 

 Iron deficiency inhibits photoreduction much more strongly than 

 photosynthesis. In this connection it should be mentioned that 

 photoreduction has been found to l^e very sensitive toward cyanide 

 and carbon monoxide (4). 



