A simple procedure to determine Ca2+ bound to low- and high-affinity sites of Synechococcus oxygen-evolving particles was developed. The method consists of determination of Ca2+ in the particle suspensions with and without treatment with a chelating resin, Chelex 100, to remove the metal cations contaminating the suspension medium as well as those weakly bound to the particles. It was found that the particles contain one tightly bound Ca2+ per PS II reaction center which cannot be extracted with Chelex 100 and a larger amount of weakly associated and resin-extractable Ca2+.
