445 



Norman I. Bishop and H. Gaffron 



THE EFFECT OF INHIBITORS WHICH PREVENT OXYGEN EVOLUTION 

 ON PHOTOHYDROGEN FORMATION 



Poisons that specifically inhibit oxygen evolution while leav- 

 ing photoreducti on undisturbed have been known for some time^-''. 

 The most effective ones in this respect are certain substituted 

 phenylureas ^ ' . 



When such an "oxygen poison" is added to normal, adapted algae, 

 little or no hydrogen is evolved in the light as compared to the 

 control, regardless of the light intensity (Fig. 2), At a OCMU 

 concentration of 6 x 10"5m, where under aerobic conditions photo- 

 synthesis would be completely blocked, no hydrogen production 

 occurs. At lower concentrations of DCMU (6 x lO'^M and 6 x 10"' 

 M) , photosynthesis and photohydrogen production are inhibited to 

 the same degree. 



Simazine, another specific inhibitor of oxygen production^'', 

 also prevents hydrogen evolution. The results obtained with this 

 herbicide were indistinguishable from those shown in Fig. 2 for 

 DCMU. 



MANGANESE DEFICIENCY AND HYDROGEN EVOLUTION 



Manganese deficient cells of several algae are known to have a 

 diminished rate of photosynthesis and of the quinone-Hill reaction 

 while in algae that possess an adaptable hydrogenase the capacity 

 for photoreducti on of carbon dioxide with hydrogen remains unim- 

 paired^"'^' ^^' . The ability of manganese deficient cells of Sce - 

 nedesmus to produce hydrogen photochemical 1y was tested and com- 

 pared to normal cells (Fig. 3). Again the correlation is appar- 

 ent that the capacity to evolve hydrogen in the light parallels 

 that of the oxygen evolving system. As anticipated, the addition 

 of 10~5m MnCl2 to deficient cells restores equally the capacity 

 for hydrogen and oxygen evolution. 



THE SIMULTANEOUS EVOLUTION OF HYDROGEN AND OXYGEN 



Horwitz and Allen observed that the activation of a hydrogen- 

 ase in an alga does not necessarily require a corresponding inac- 

 tivation of its oxygen-evolving systemv^O. Photosynthesis and 

 photoreduction can proceed simultaneously as long as the emerging 

 oxygen does not inactivate the hydrogenase. Thus the possibility 

 had to be considered that what had been called an evolution of hy- 

 drogen was perhaps the evolution of both hydrogen and oxygen. If 

 correct, the volume of gas produced during a light period should 



