This paper presents the inhibition of photosynthetic electron transport chain of
Nostoc muscorum by divalent Ni
2+ and monovalent Ag
+. PS I (DCPIP/ASC→MV) and PS II (H
2O→PBQ) activities were markedly inhibited by Ni
2+ and Ag
+ in a concentration-dependent fashion but PS II was more susceptible than PS I to Ni
2+. Ag
+ was more toxic to PS I than PS II. The greater sensitivity of PS II to Ni
2+ was further confirmed by the inhibition of DCPIP photoreduction and Chl
a fluorescence. Restoration of Ag
+ -induced inhibition of DCPIP photoreduction and Chl
a fluorescence by artificial electron donors (DPC, NH
2OH, MnCl
2) and their failure to restore Ni
2+-induced inhibition suggests that Ni
2+ inactivates PS II by causing alteration and destruction of photosynthetic membranes, but Ag
+ inhibits the electron flow at the oxidizing side of PS II. Nevertheless, the suppression of the fluorescence intensity at low concentrations of both metal cations points to the involvement of phycobilisomes in the inhibition of PS II activity.
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