Toxic effects of mercury on PSI and PSII activities, membrane potential and transthylakoid proton gradient in Microsorium pteropus

•Responses of PSI and PSII of M. pteropus to Hg were simultaneously analyzed.•Y(I), ETR(I), PSII and ETR(II) decreased with increasing Hg concentration.•Damage to PSII by Hg was due to failure of dissipation of the excessive excitation energy.•Δψ and ΔpH of M. pteropus significantly decreased at high levels of Hg.•Formation of zeaxanthin and H+ efflux from the lumen to the stroma were inhibited.

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 (Δψ) and proton gradient (ΔpH) of M. pteropus were reduced significantly at 330 μg L−1 Hg2+ compared to control. Mercury exposure affected multiple sites in PSII and PSI of M. pteropus.