O2‐induced inactivation of nitrogenase as a mechanism for the toxic action of Cd2+ on Nostoc UAM 208

summary Direct involvement of Cd2+ in the inhibition of N, 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 O2 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 Cd24 in terms of the O2 mediated inhibition of N2 fixation is discussed.