Abstract: he changes of Mn2+ contents in Anabaena variabilis were probed by EPR. Treatments with CaCl2 and Ca (NO3)2 at high concentrations induced the release of bound Mn and the decrease of oxygen-evolving activity of the cyanobacterium. When the release percentage of bound Mn reached up to 57%, the oxygen-evolving activity decreased to zero. MgCl2 treatment resulted in less effectiveness than CaCl2 MnCl2 at high concentration inhibited cyanobacterial oxygen evolution, as the indication of EGTA. In the comparision with control the low temperature fluorescence emission spectra of the cyanobacterium treated by CaCl2, MgCl2 and MnCl2 changed with the shoulder disappearance at 686 nm and the decline of ratio of F730/F695 The possible competitive substitutions among ions at their binding sites in oxygen-evolving complex were discussed.

Key words: Anabaena variabilis Oxygen evolution, EPR, Manganese, calcium, magnesium, Substitution