Copper-induced inhibition of PSII photochemistry in cyanobacterium Spirulina platensis is stimulated by light

The role of light in the modifications of copper toxicity on PSII photochemistry in Spirulina platensis was investigated in vivo by use of chlorophyll fluorescence. The results demonstrated that copper (up to 300 mmol m^-3) substantially inhibited PSII photochemistry in the light but exhibited no significant effects on PSII photochemistry when the cells were incubated in the dark for 12 h. Moreover, the higher the light intensity, the more significant effect the copper treatment induced. It was found that copper inhibited PSII photochemistry, which was linked to the reduction of Q(A). Copper also decreased the proportion of the active PSII reaction centers, but showed no effects on the proportion of Q(B)-non-reducing PSII reaction centers. The results on PSII photochemistry under the steady state of photosynthesis demonstrated that copper induced an increase in the non-photochemical quenching while it resulted in a decrease in the photochemical quenching and the efficiency of energy excitation capture by open PSII reaction centers, of which both resulted in a decrease in the quantum yield of PSII electron transport.