DANIEL I. ARNON 495 



hypothesis thai liglii fuiutioned in photosynthesis by decomposing 

 COo into "C;" and Oo could no longer be maintained-. 



4. Reducing Power Versus Assimilatory Power 



II the iiuKtion of light was not the photodecomposition of COo, 

 where was the connnon role for light to be found in bacterial and 

 plant photosynthesis? This question was answered by van Niel's 

 hypothesis (159) that envisaged a basic similarity between bacterial 

 and plant photosynthesis in a common photochemical origin of the 

 reductant for CO^ assimilation. Van Niel proposed that the light 

 reaction in both groups of organisms is a photolysis of water which 

 results in the formation of a reductant (H) and an oxidant (OH) , 

 the latter being the precursor of molecular oxygen. 



To account for the lack of oxygen evolution in bacterial photo- 

 synthesis, van Niel pointed to the requirement of this process for an 

 external hydrogen donor. The external hydrogen donor was, in his 

 postulation, required for reducing the (OH) back to water, and in 

 this manner keeping the photosynthetic apparatus in bacteria active. 

 Only in green plants did (OH) give rise to molecular oxygen, and 

 in this capacity for oxygen evolution lay the difference between 

 bacterial and plant photosynthesis. 



The postulated photolysis of water by radiation at an energy 

 level of visible light was a reaction unknown in chemistry. There 

 were therefore no experimental restrictions on the surmises as to 

 the nature of the CO2 reductant, (H) . One postulation which has 

 figured in numerous hypotheses [see for example (143, 56)], con- 

 sidered (H) to be a "reducing power" of a potential sufficiently strong 

 to reduce CO2 to the level of carbohydrates by some mechanism that 

 is peculiar to photosynthesis and different from the known biochemi- 

 cal reactions for the dark assimilation of CO2. This hypothesis has 

 thus far remained speculative. 



The alternative view regarded COo assimilation in photosynthesis 

 as being fundamentally a reversal of the degradative reactions of 

 carbohydrates, i.e., a dark chemosynthetic process which depends on 

 only two products formed by light reactions, reduced pyridine nu- 

 cleotide and ATP. This hypothesis was formulated on theoretical 

 grounds in 1943 by Ruben (130) and incorporated earlier suggestions 



-This generalization would apply only to the overall process and would not 

 necessarily exclude the catalytic participation of COo in the mechanism of oxygen 

 evolution, as has recently been proposed by Warburg et al. (165). 



