DISCUSSION 569 



with molecular hydros^cn. But thiosulfatc puts a demand of some 70 kcal 

 per mole CO. fixed, a figure pretty close to the 112 kcal required it water 

 is the reductant as in aerobic photosynthesis. Chromntium therefore must 

 have a photosynthetic machinery about as potent as the one of green plants. 

 In the peculiar case of feeding with hydrogen or other "easy" donors, this 

 big motor must be idling or short circuited, since only the ATP generated 

 by, let's say, the fuel pump is required for the job. 



Dr. Arnon: Well, I think we can only answer this question by discussing 

 the COo fixation, which we cannot do here now. Let me summarize in a nut 

 shell the reason why we thought that if in Chromatium all that light does is 

 make ATP, it should then be possible to replace light by exogenous ATP in the 

 dark. Unlike Rhodospirillum rubrum we have no alternatives here. Rhodo- 

 spirillum rubrum but not Chromatium can also have oxidative metabolism. 

 The experimental evidence is that in a cell-free Chromatium system light is 

 replaceable by ATP. The products of CO= or acetate assimilation are exactly 

 the same in both cases. 



Dr. Kok: You agree, though, that this does not prove that this is all the 

 light is doing. 



Dr. Arnon: I am now at the point where I feel I do not wish to go beyond 

 these data which we have. I will leave everything else to future evidence. 

 Dr. Frenkel: I believe this question can be handled experimentally in 

 the Rhodospirillum system where you can reduce DPN in the breakdown 

 of succinate. If Dr. Arnon's mechanism is correct, you should be able to 

 start with labeled succinate, and you might be able to get the incorporation 

 of hydrogen into DPN. If the data are positive, it would strongly suggest 

 the Krasnowsky reaction of sensitized reduction. If it is negative, you can't 

 tell anything. 



Dr. Strehler: I also have the impression that you are retreating a little 

 from the idea you expressed-that the only function of light in the photo- 

 synthetic bacteria is to produce ATP-since you later said that possibly the 

 hydrogen from succinate went down to chlorophyll and then was kicked 

 up to TPN. Wouldn't it to more parsimonious to assume (as has been sug- 

 gested earlier) that the ATP that you generate in the light is used to 

 drive the electrons from the potential of succinate up to that of TPN? 

 Dr. McElrov: Dr. Petrack, have you looked at the fluorescence? 

 Dr. Petrack: No. 



