Is there a direct chloride cofactor requirement in the oxygen-evolving reactions of photosystem II?

The dark incubation at room temperature of photosystem II (PS II) membrane fragments in a chloride-free medium at pH 6.3 slowly leads to large chloride-restorable and non-restorable O2 evolution activity losses with time as compared with control samples incubated in the presence of 10 mM NaCl. The chloride requirement in O2 evolution generated under these conditions reveals a complex interplay among various experimental parameters, including the source of the plant material, the times of incubation, the sample concentration, the chloride concentration, as well as those treatments which are believed to specifically displace chloride from PS II such as alkaline pH pretreatment and Na2SO4 addition. The results indicate that secondary, structural changes within the PS II complex are an important factor in determining the influence of chloride on the O2 evolution activity and raise the question whether or not chloride ions actually play a direct cofactor role in the water-oxidizing reactions leading to O2 evolution.