Cyanobacteria have an evolved antioxidant system that catalyzes the harmful oxy radicals produced during photosynthesis. The differential responses of superoxide dismutase (SOD), nitrogenase, growth and physiological processes in the presence of five different chelators at varying concentrations with different chelating properties were investigated in the heterocystous cyanobacterium, Anabaena variabilis. Growth and enzyme activities were found to be influenced by the chelator properties. Ethylene diamine tetra acetic acid (EDTA) was the most potent chelator while 2, 2, bipyridyl (2,2-BP) stimulated growth. Nitrogenase and SOD activities decreased in the presence of EDTA, diamino tetraethyl penta acetic acid (DTPA) and 2,2-BP. The absence of SOD activity at higher concentrations of EDTA and DTPA was also detected in the native gel electrophoresis assays. Of the five chelators, EDTA, DTPA and 1, 10-phenanthroline (Phen) degraded cells at high concentrations. Increased nitrogenase activity with a decreased SOD activity at low chelator concentration and decreased nitrogenase activity with an increased antioxidant activity was observed at high concentrations of 8-hydroxyquinoline (HQ). These results suggest that for cyanobacteria, even tightly bound iron is biologically available but iron bound to some extremely strong chelators may become biologically unavailable.