OXYGEN EVOLUTION AND PHOTOREDITCTION 237 



reduction under more completely anaerobic conditions. This was done 

 with Warburg manometers, as is explained in the next section. 



MANOMETRIG OBSERVATIONS 



Larsen, Yocum, and van Niel (1) have pointed out that, if the 

 photochemical consumption of hydrogen and carbon dioxide by 

 normal adapted Scenedesmus consists of no more than photosynthesis 

 and the oxyhydrogen reaction running concurrently, then this com- 

 plex process should have a quantum yield for carbon dioxide uptake 

 that is 50% higher than the quantum yield of photosynthesis alone. 

 Another requirement of this kind of metabolism, however, is that the 

 rate of carbon dioxide uptake not exceed the maximum rate of 

 oxygen uptake in the oxyhydrogen reaction by more than 50%. 

 Manometric evidence presented here indicates that this requirement 

 is violated and that, therefore, a true photoreduction must exist in 

 normal adapted cells. 



Under conditions where oxygen tension is not limiting, the rate of 

 oxygen uptake in the oxyhydrogen reaction at 28.3°C. by Scenedes- 

 mus Ds is 3.6 cell volumes per hour. The maximum rate of carbon 

 dioxide uptake in a process consisting of photosynthesis and oxy- 

 hydrogen reaction running concurrently is, therefore, 5.4 cell volumes 

 per hour. However, it is possible manometrically to achieve rates of 

 carbon dioxide uptake in the light that reach and even exceed 8 

 cell volumes per hour before deadaptation sets in. True photoreduc- 

 tion must, therefore, accoimt for at least 30% of the photochemical 

 carbon dioxide uptake under these conditions. 



CONCLUSIONS 



Our data require an interpretation more complex than is provided 

 by heretofore prevailing views on the nature of the photochemical ac- 

 tivity of adapted Scenedesmus under hydrogen and carbon dioxide. 

 The present experiments make it clear that adapted Scenedesmus in 

 the presence of hydrogen and carbon dioxide can, under certain 

 conditions, perform photosynthesis at a rate which may even ap- 

 proach or reach that necessary to account for all its photochemical 

 activity. Under these circumstances a correspondingly large part of 

 the photochemical uptake of hydrogen and carbon dioxide by adapted 

 Scenedesm,us consists of the combination of photosynthesis and the 



