SUBLETHAL CONCENTRATIONS OF COPPER STIMULATE PHOTOSYSTEM-II PHOTOINHIBITION IN CHLORELLA-PYRENOIDOSA

The effect of copper (CuSO4, 0.16-1.56 mu M) on photosystem II (PS II) was investigated by measuring the chlorophyll fluorescence in Chlorel la pyrenoidosa cells exposed to different irradiance levels and differ ent temperatures. The algal culture remained viable only if less than 0.7-0.8 mu M Cu2+ was added. In darkness, PS II inactivation was indic ated by a decrease in F-v/F-m ratio and was detected only at Cu2+ conc entrations higher than 0.7 mu M. In the light, Cu2+ stimulated PS II p hotoinhibition throughout the whole range of copper concentrations. Lo w temperature significantly enhanced the PS II photoinhibition in the presence of Cu2+. Kinetics of PS II inactivation monitored at algal gr owth light intensity in the presence oi chloramphenicol were only slig htly affected by the addition of Cu2+. However, Cu2+ markedly slowed d own (0.16 mu M) or completely suppressed (0.7 mu M) the recovery of PS II activity after the photoinhibitory treatment of the algae with a s trong light. The results suggest that sublethal concentrations of copp er can nonspecifically retard synthesis of the PS II D1 protein in Chl orella cells, thus causing PS II inactivation under light.