Sublethal Concentrations of Copper Stimulate Photosystem II Photoinhibition in Chlorella pyrenoidosa

The effect of copper (CuSO 4, 0.16-1.56 μM) on photosystem II (PS II) was investigated by measuring the chlorophyll fluorescence in Chlorella pyrenoidosa cells exposed to different irradiance levels and different temperatures. The algal culture remained viable only if less than 0.7-0.8 μM Cu 2+ was added. In darkness, PS II inactivation was indicated by a decrease in F v/F m ratio and was detected only at Cu 2+ concentrations higher than 0.7 μM. In the light, Cu 2+ stimulated PS II photoinhibition throughout the whole range of copper concentrations. Low temperature significantly enhanced the PS II photoinhibition in the presence of Cu 2+. Kinetics of PS II inactivation monitored at algal growth light intensity in the presence of chloramphenicol were only slightly affected by the addition of Cu 2+. However, Cu 2+ markedly slowed down (0.16 μM) or completely suppressed (0.7 μM) the recovery of PS II activity after the photoinhibitory treatment of the algae with a strong light. The results suggest that sublethal concentrations of copper can nonspecifically retard synthesis of the PS II D 1 protein in Chlorella cells, thus causing PS II inactivation under light.