OBSERVATIONS ON THE FORMATION OF THE PHOTOSYNTHETIC APPARATUS 305 



second point it may not always be safe to generalize about the mechanism of 

 photosynthesis from one special case. The observation that Clironiatiurn can 

 reduce pyridine nucleotides in the dark with molecular hydrogen, does not appear 

 to preclude the possibility that Chroniatiwn could reduce pyridine nucleotides 

 directly or indirectly by a photochemically generated reductant. 



Arnon : The point is that hydrogen gas has been known for almost 30 years 

 as a physiological electron donor for Chroynatium. The utilization of hydrogen gas 

 by this organism is not to be regarded as an experimental artifact. We do not build 

 but merely support our theory with the facts of Chroma tiiini photosynthesis. We 

 have presented other lines of evidence elsewhere. 



Frenkel: We have tried to grow Rhodnspirillnni ruhrutu with hydrogen but it 

 does not grow very well. 



Arnon: Chromatium grows very well with hydrogen gas. 



Bergeron : Coming back to the question of acetate-grown CJirumatium I should 

 like to point out the truth in both points of view. Recently Dr. Benedict was trying 

 to study carotenoid biosynthesis in Cluomatiinn using labelled acetate, and he was 

 using a medium containing a very small percentage of CO^. They were growing 

 very nicely. Some objection was raised to the carbonate so they were transferred 

 to a medium which was identical except for the fact that the minimum amount of 

 CO2 was taken away. The new cultures grew slowly but finally got going again on 

 the pure acetate. 



Arnon : Let me make it clear again that when I say that Cluamatiuni grows 

 without CO.j, I do not imply that CO._, is not used catalytically ; I firmly believe it 

 is. What I am saying is that we have grown Chromatium without any added supply 

 of CO., and under conditions where any large concentrations of endogenous CO2 

 would be swept out by bubbling gas. 



Frenkel: What is the pH of the medium ? 



Arnon : They grow at pH between 7 and 7 • 8. 



Frenkel : Under these conditions it may not be too simple to remove the CO.^ 

 which is produced metabolically at a rate adequate to prevent its re-utilization. 



Arnon : As I said earlier I firmly believe that the CO., is used catalytically. 



Frenkel: In studies on the effect of COj on the growth of micro-organisms, 

 experimental conditions are not always adequately described. At low gassing rates, 

 with actively metabolizing cells a steady state concentration of CO^ may be built- 

 up permitting continued growth of the micro-organisms. 



Kamen: I should mention that in practically all the chloroplasts structures 

 which are known there is a very high concentration of chlorophyll held to the 

 protein. In the bacteria, as you may have noticed in Table i (p. 2S0), the ratio 

 of chlorophyll to protein is about 15, whilst in the case of chloroplasts, it is some- 

 thing of the order of 1500. In the case of the nucleotides, also, the concentration 

 of the nucleotides in chromatophores is about i to 15 chlorophylls, while in 

 spinach chloroplasts it is over i to 2500 as Krogman showed, so there is a quantita- 

 tive difference between structures producing oxygen and those which don't. I 

 believe that this may have something to do with the Emerson effect which indicates 

 that at least two quanta are funnelled to each active site to get the oxygen off. It is 

 very difficult for an organism with limited amounts of chlorophyll to funnel the 

 quanta to where it wants it. 



VOL. n. — X 



