Inhibition of Photosystem II in the Green Alga Scenedesmus obliquus by Nickel

The toxic effect of Ni²⁺ ions on photosynthetic electron transport was investigated by monitoring Hill activity, fluorescence, oxygen evolution and thermoluminescence properties in the green algae Scenedesmus obliquus 276-1. Nickel strongly inhibited 2,6-dichlorophenolindophenol (DCPIP) photoreduction in the broken cells of Scenedesmus, and the activity lost could not be restored by adding 1,5-diphenyl carbazide (DPC). Oxygen evolution both measured polarographically and under flash light conditions decreased by increasing the nickel concentration. Fluorescence intensity measured at room temperature decreased upon addition of nickel chloride, both in the presence and absence of DCMU. The maximum fluorescence could not be restored by addition of artificial electron donors. Thermoluminescence studies revealed that the S₂Q_A^- charge recombination, however, was inhibited with increasing concentrations of nickel chloride. The results suggest that Ni²⁺ does not block the electron flow between the primary and secondary quinone electron acceptor, but modify the Q_B site or interact with the non-heme iron between the Q_A and Q_B, leading to the impairment of photosystem II.