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 evolut ion and thermoluminescence properties in the green algae Scenedesmus o bliquus 276-1. Nickel strongly inhibited 2,6-dichlorophenolindophenol (DCPIP) photoreduction in the broken cells of Scenedesmus, and the act ivity lost could not be restored by adding 1,5-diphenyl carbazide (DPC ). Oxygen evolution both measured polarographically and under flash li ght conditions decreased by increasing the nickel concentration. Fluor escence intensity measured at room temperature decreased upon addition of nickel chloride, both in the presence and absence of DCMU. The max imum fluorescence could not be restored by addition of artificial elec tron donors. Thermoluminescence studies revealed that the S2Q(A)- char ge recombination, however, was inhibited with increasing concentration s of nickel chloride. The results suggest that Ni2+ does not block the electron flow between the primary and secondary quinone electron acce ptor, but modify the Q(B) site or interact with the non-heme iron betw een the Q(A) and Q(B), leading to the impairment of photosystem II.