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Mercury(Hg)is one of the top toxic metals in environment and it poses a great risk to organisms.This study aimed to elucidate the toxic effects of Hg2+ on energy conversion of photosystem I(PSI)and photosystem Ⅱ(PSⅡ),membrane potential and proton gradient of Microsorium pteropus(an aquatic plant species).Contents of chlorophyll a,chlorophyll b and carotenoids,quantum yield and electron transfer of PSI and PSⅡ of M.pteropus exposed to various concentrations of Hg2+ were measured.With increasing Hg2+ concentration,quantum yield and electron transport of PSI [Y(Ⅰ)and ETR(Ⅰ)] and PSⅡ [Y(Ⅱ)and ETR(Ⅱ)] decreased whereas limitation of donor side of PSI [Y(ND)] increased.At ≥165 μg L-1 Hg2+,quantum yield of non-light-induced non-photochemical fluorescence quenching in PSⅡ [Y(NO)] significantly increased but quantum yield of light-induced non-photochemical fluorescence quenching [Y(NPQ)] decreased.Membrane potential(Δ?)and proton gradient(ΔpH)of M.pteropus were reduced significantly at 330 ?g L-1 Hg2+ compared to control.Mercury exposure affected multiple sites in PSⅡ and PSⅠ of M.pteropus.