Cadmium exerts its toxic effects on photosynthesis via a cascade mechanism in the cyanobacterium, Synechocystis PCC 6803.

Despite intense research, the mechanism of Cd(2+) toxicity on photosynthesis is still elusive because of the multiplicity of the inhibitory effects and different barriers in plants. The quick Cd(2+) uptake in Synechocystis PCC 6803 permits the direct interaction of cadmium with the photosynthetic machinery and allows the distinction between primary and secondary effects. We show that the CO(2) -dependent electron transport is rapidly inhibited upon exposing the cells to 40 µm Cd(2+) (50% inhibition in ∼15 min). However, during this time we observe only symptoms of photosystem I acceptor side limitation and a build of an excitation pressure on the reaction centres, as indicated by light-induced P700 redox transients, O(2) polarography and changes in chlorophyll a fluorescence parameters. Inhibitory effects on photosystem II electron transport and the degradation of the reaction centre protein D1 can only be observed after several hours, and only in the light, as revealed by chlorophyll a fluorescence transients, thermoluminescence and immunoblotting. Despite the marked differences in the manifestations of these short- and long-term effects, they exhibit virtually the same Cd(2+) concentration dependence. These data strongly suggest a cascade mechanism of the toxic effect, with a primary effect in the dark reactions.