356 DANIEL I. ARNON 



plants. However, they envisaged the possibiHty, which has since been 

 documented by Broyer and associates [loi] and Martin and Lavollay [102]. 

 that chloride may prove to be an essential micronutrient for green plants, 

 A reinvestigation by Bove et a/. [103] of the role of chloride in the 

 photochemical reactions of chloroplasts confirmed Warburg's conclusion 

 that chloride is essential for those photosynthetic reactions in which oxygen 

 is liberated. Chloride was not required, however, for the anaerobic cyclic 

 photophosphorylation that is shared by bacterial particles and chloroplasts. 

 Thus, in the absence of chloride, chloroplasts behaved like bacterial 

 chromatophores. They were able to carry out the anaerobic cyclic photo- 

 phosphorylation but were unable to evolve oxygen. Oxygen evolution, 

 not included in the mechanism of cyclic photophosphorylation, appeared 

 therefore as an additional secondary feature of photosynthesis, not essen- 

 tial to the primary conversion of light energy into the pyrophosphate bonds 

 of ATP, and peculiar to green plants, 



EFFECT OF FERRICYANIDE 



The key premise in the proposed mechanisms for cyclic photophos- 

 phorylation is that the electron expelled from the chlorophyll molecule in 

 the primary photochemical act is not removed from the "closed circuit" 

 within which it travels before it returns to the chlorophyll. If this basic 

 postulation is correct it follows that cyclic photophosphorylation should be 

 abolished if the electrons are prevented from completing the cycle because 

 of capture by an external electron acceptor. To be convincing, such an 

 experiment should be carried out with an electron acceptor which would 

 be free from the suspicion that it prevented phosphorylation by acting as 

 an uncoupler, or in some toxic manner. 



An electron acceptor that fulfills these requirements is ferricyanide. 

 As shown by Jagendorf [104], and confirmed in this laboratory, ferri- 

 cyanide has a great affinity for trapping electrons during photophosphoryla- 

 tion. Thus, by adding ferricyanide, in the absence of chloride, cyclic 

 photophosphorylation in both chloroplasts and chromatophores should be 

 inhibited, if the proposed hypothesis is correct. The cyclic flow of electrons 

 in the closed circuit would be interrupted when the electrons are trapped 

 by, and used in, the reduction of ferricyanide. 



Table III shows that this theoretical prediction has been experi- 

 mentally verified. The addition of ferricyanide abolished cyclic photo- 

 phosphorylation both in chloroplasts and in chromatophores. Adding this 

 ion in its reduced form as ferrocyanide, was without efi^ect. The reduction 

 of ferricyanide with ascorbate either prior to, or during illumination of the 

 photosynthetic particles, restored in full their capacity for cyclic photo- 

 phosphorylation. The conclusion seemed justified therefore that the 



