Mercury-induced alterations of chlorophyll a fluorescence kinetics in cyanobacteria: Multiple effects of mercury on electron transport

Changes in the fluorescence yield of chlorophyll were monitored to investigate the effect of HgCl 2 on the cyanobacteria Spirulina platensis and Anacystis nidulans; weak modulated light, high intensity actinic light and additional strong sources of illumination were used. Depending on the concentration of HgCl 2, three distinct types of change in the fluorescence yield of chlorophyll a were observed. At low concentrations (1.5 μM), HgCl 2 behaves in a similar manner to diuron in that it increases the fluorescence intensity F 0 in weak modulated light. This increase may be due to blockage of electron flow on the reducing side of photosystem II. At slightly increased levels of mercury (3 μM), the quenching of the variable fluorescence of chlorophyll suggests a decrease in electron flow on the donor side of photosystem II. This effect becomes much more evident in the presence of diuron. At sufficiently high concentrations (18 μM), a pronounced quenching of the chlorophyll fluorescence is observed, which may be due to both the blocking of photosystem II on the donor side and structural changes in the antenna pigments. These multiple effects of mercury are also observed in intact cells of Anacystis and in spheroplasts prepared from Anacystis. In the latter, the effects of mercury saturate at lower concentrations than those observed in intact cells. The effect of mercury cannot be reversed by ethylenediaminetetraacetic acid, suggesting that mercury binds with the pigment—protein complexes of the cyanobacteria.