138 METABOLISM AND PHYSIOLOGY 



apparently has a genetic block in the oxygen-producing mechanism. 

 This organism, consequently, isincapableof producing O2 when illumi- 

 nated, but it retains the capacity for photoreducing CO2 with molecular 

 hydrogen. It seems reasonable that further investigation of algae of the 

 kind under discussion may provide valuable insights into the biochem- 

 ical evolution of photosynthesis. On the basis of the data at hand, 

 Gaffron (44) has concluded that it is: 



. . . extremely plausible that whatever else had to change to make the 

 release of free oxygen possible— an indispensable part of this last evolu- 

 tionary step must have been the addition of certain catalysts to an already 

 existing complete mechanism for the fixation of carbon dioxide, for photo- 

 phosphorylation, and for photo reduction with hydrogen. In place of the 

 reaction leading to molecular oxygen there has been originally a coupling 

 to hydrogenases or dehydrogenases. 



According to this point of view, O2 formation represented the acquisi- 

 tion of a new and singular property. Perhaps there is really no need 

 to ask why the bacteria do not produce O2; the significant question 

 may be why green plants do, 



Gaffron (45) subscribes to the view that the function of the accessory 

 hydrogen donor in autotrophic photosynthesis of the bacterial type is 

 to reduce the potential 02-precursor (photooxidant) and has marshaled 

 arguments against an alternative hypothesis, viz., that the accessory 

 donor is, in fact, used as a source of hydrogen for CO2 reduction. In 

 his terms, the alternative stipulates that water is no longer oxidized, 

 but instead NADP is reduced"by a more direct photocatalytic hydrogen 

 (or electron) transport," If we use the formalism of light- dependent 

 "water cleavage," this seems to imply that the only alternative is one 

 in which the "water cleavage" system is entirely eliminated. We can 

 formulate schemes, however, which retain "water cleavage" for the 

 purpose of producing the reductants and oxidants required for cyclic 

 photophosphorylation, or generation of energy- rich precursors of ATP, 

 and postulate that the hydrogen necessary for net reduction is derived 

 from the accessory donor. This type of conception would relegate O2 

 production to the category of a unique feature of green plant photosyn- 

 thesis which is not readily rationalized as a minor comparative bio- 

 chemical variation. 



The notion that the accessory donor serves as the source of hydro- 

 gen for CO2 reduction is an old idea which can be found in some of the 

 earliest discussions (e,g,, see ref, 1) on the mechanism of bacterial 

 photosynthesis. Similarly, the concept of a hydrogen transfer, between 

 the added hydrogen donor and CO2, which is "driven" in some way by 

 light- energy also was proposed in relatively modern terms many 

 years ago (46). The knowledge accumulated during the past fifteen 

 years on electron transfer processes and phosphorylation in a variety 



