The effect of cadmium on the photosynthetic activity of Synechocystis PCC 6803 was monitored in this study. The oxygen evolving capacity of Synechocystis treated with 40 muM CdCl(2) was depressed to 10% of the maximum in 15 min, indicating that Cd(2+) penetrated rapidly into the cells and blocked the photosynthetic activity. However, neither photosystem II (PSII) nor photosystem I (PSI) activity showed a significant short-term decrease which would explain this fast decrease in the whole-chain electron transport. Thermoluminescence measurements have shown that the charge separation and stabilization in PSII remains essentially unchanged during the first few hours following the Cd(2+) treatment. The electron flow through PSI was monitored by following the redox changes of the P700 reaction centers of PSI. Alterations in the oxidation kinetics of P700 in the Cd(2+)-treated cells indicated that Cd(2+) treatment might affect the available electron acceptor pool of P700, including the CO(2) reduction and accumulation in the cells. Perturbed angular correlation of gamma-rays (PAC) using the radioactive (111m)Cd isotope was used to follow the Cd(2+) uptake at a molecular level. The most plausible interpretation of the PAC data is that Cd(2+) is taken up by one or more Zn proteins replacing Zn(2+) in Synechocystis PCC 6803. Using the radioactive (109)Cd isotope, a protein of approximately 30 kDa that binds Cd(2+) could be observed in sodium dodecyl sulfate polyacrylamide gel electrophoresis. The results indicate that Cd(2+) might inactivate different metal-containing enzymes, including carbonic anhydrase, by replacing the zinc ion, which would explain the rapid and almost full inhibition of the photosynthetic activity in cyanobacteria.