Metal-induced inhibition of photosynthetic electron transport chain of the cyanobacterium Nostoc muscorum

This study presents information on the mechanism of inhibition of the photosynthetic electron transport of Nostoc muscorum by chromium (Cr) and lead (Pb). Photosystem II (PS II) was found to be more sensitive both to low and high concentrations of test metals used. A considerable inhibition of photosystem I (PS I) was, however, observed at high concentrations only. Although Cr-induced inhibition of DCPIP photoreduction and lowering of chlorophyll a (Chl a) fluorescence intensity (F685) could not be reversed by artificial electron donors (diphenyl-carbazide (DPC), NH2OH, MnCl2 and benzidine) of PS II, these electron donors did substantially reverse the Pb-induced inhibition of DCPIP photoreduction as well as the lowering of Chl a fluorescence. Nevertheless, an increase in Chl a fluorescence at high concentrations of Pb suggested that this metal also arrests electron flow on the reducing side of the PS II reaction centre. Besides this, the suppression of fluorescence intensity of phycocyanin at low concentrations of both metals points to the involvement of phycobilisomes in the inhibition of PS II activity. The present study demonstrates that the modes of action of Cr and Pb on PS II are quite different