O-2-INDUCED INACTIVATION OF NITROGENASE AS A MECHANISM FOR THE TOXIC ACTION OF CD2+ ON NOSTOC UAM-208

Direct involvement of Cd2+ in the inhibition of N-2 fixation in Nostoc UAM 208 has been investigated. Significant inhibition of nitrogenase activity was detected 2 h after addition of Cd2+ to the culture medium. Under anaerobic conditions the inhibitory effect of Cd2+ was largely prevented, suggesting that this ion induced inactivation of nitrogenase by O-2. Addition of Ca2+ to Cd2+-treated cultures markedly decreased the inhibition of nitrogenase activity, and this amelioration of Cd2+ toxicity was dose-dependent, indicating an antagonistic interaction between the two ions. Ultrastructural examination of cultures treated with Cd2+ showed heterocyst deterioration and a decrease in the cohesiveness of the polysaccharide layer of the heterocyst envelope. A possible antagonistic relationship between Ca2+ and Cd2+ in terms of the O-2-mediated inhibition of N-2 fixation is discussed.