246 E. KESSLER 



Previously it was suggested (3,9) that manganese might be pri- 

 marily involved in the basic photochemical reaction of photosyn- 

 thesis. However, from our results it follows that its main role in 

 photosynthesis must be concerned with oxygen evolution, the only 

 process of photosynthesis which is not needed in photoreduction. 

 Also the results of Gerretsen (10) with chloroplasts from higher 

 plants suggested the importance of manganese for the formation of 

 peroxides. Recently Kenten and Mann (11) have demonstrated a 

 light-dependent oxidation of manganese in chloroplast preparations 

 and have discussed its possible role in photosynthesis. It might well 

 be that there is an additional and perhaps smaller requirement 

 for manganese in CO2 reduction, as suggested by the work of Arnon 

 et al. (12) with chloroplasts. This, however, could be detected only 

 at saturating light intensities which cannot be reached in photoreduc- 

 tion because of deadaptation. 



The hypothetical scheme I might serve to explain the observation 

 that the inhibition of photosynthesis by manganese deficiency is 



"^ »H,0 



HYDROGENASE ^ 



Kv + X+Y+KO 



XH—»CCO^- REDUCTION) 



Scheme I. 



independent of light intensity, although an enzymatic reaction, 

 namely, oxygen evolution, is affected primarily. We assume that 

 manganese is specifically involved in the transformation of the first 

 oxidized product of photosynthesis, YOH, to a peroxide, Z(0H)2, 

 which eventually gives off oxygen {cf. 6). Normally, in the presence of 

 manganese in algae not adapted to hydrogen, all the YOH formed 

 will be disposed of to evolve oxygen. In adapted algae, however, 

 most of the YOH reacts with hydrogen activated by hydrogenase to 



