514 LIGHT AND LIFE 



conclusion would have conferred on chloride the then unwarranted 

 status of an essential element for green plants. However, they en- 

 visaged the possibility, which has since been documented by Broyer 

 and associates (.84) and Martin and Lavollay (102) , that chloride 

 may prove to be an essential micronutrient for green plants. 



A reinvestigation by Bove et al. (30) 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 bac- 

 terial particles and chloroplasts. Thus, in the absence of chloride, 

 chloroplasts behaved like bacterial chromatophores. They were able 

 to carry out the anaerobic cyclic photophosphorylation but were un- 

 able to evolve oxygen. 



Effect of Ferricyanide 



The key premise in the proposed mechanisms for cyclic photophos- 

 phorylation is that the electron expelled from the chlorophyll mole- 

 cule 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 photophosphory- 

 lation should be abolished if the electrons are prevented from com- 

 pleting 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 (74) , and confirmed in this laboratory, ferri- 

 cyanide has a great affinity for trapping electrons during photophos- 

 phorylation. Thus by adding ferricyanide, in the absence of chloride, 

 cydic j)hotophosphorylation in both chloroj)lasts 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 4 shows that this theoretical prediction has been experi- 

 mentally verified. The addition of ferricyanide abolished cyclic photo- 

 phosphorylation both in chlorojilasts and in chromatophores. Adding 

 this ion in its reduced form, as ferrocyanide, was without effect. The 

 reduction of ferricyanide with ascorbatc either prior to, or during, 

 illumination of the photosynthetic particles restored in full their 



