Early toxic effects of zinc, cobalt, and cadmium on photosynthetic activity of the green alga Chlorella pyrenoidosa Chick S-39

Early toxic effects of heavy metals (HMs) Zn, Co, and Cd at concentrations from 0.01 to 100 mM on photosynthetic activity of the green alga Chlorella pyrenoidosa Chick S-39 were studied. The early effect of HMs was manifested as a rapid (within 0.5-2 h) reduction of photoinduced oxygen release by the algal cells. The suppressed relative yield of variable chlorophyll fluorescence (F-v/F-m) by HMs as well as its dynamics in C. pyrenoidosa provided evidence for rapid inactivation of photosystem II (PS II). Analysis of the induction curve of delayed chlorophyll fluorescence in Chlorella cells suggested that the early toxic effects of Zn, Co, and Cd at the above concentrations manifested itself not only in inhibited electron transport in PS II, but also in reduced energization of photosynthetic membranes. Hence, the early toxic effect of Zn, Co, and Cd was primarily related to the decreased efficiency of the light reactions of photosynthesis, which subsequently resulted in reduced productivity of the alga.