| Summary: | Mercury (Hg) poses high toxicity to organisms including algae. Studies showed that the growth and photosynthesis of green algae such as <i>Chlorella</i> are vulnerable to Hg stress. However, the differences between the activities and tolerance of photosystem I and II (PSI and PSII) of green microalgae under Hg exposure are still little known. Responses of quantum yields and electron transport rates (ETRs) of PSI and PSII of <i>Chlorella</i> <i>pyrenoidosa</i> to 0.05–1 mg/L Hg<sup>2+</sup> were simultaneously measured for the first time by using the Dual-PAM-100 system. The photosystems were isolated to analyze the characteristics of toxicity of Hg during the binding process. The inhibition of Hg<sup>2+</sup> on growth and photosystems was found. PSII was more seriously affected by Hg<sup>2+</sup> than PSI. After Hg<sup>2+</sup> exposure, the photochemical quantum yield of PSII [Y(II)] decreased with the increase in non-photochemical fluorescence quenching [Y(NO) and Y(NPQ)]. The toxic effects of Hg on the photochemical quantum yield and ETR in PSI were lower than those of PSII. The stimulation of cyclic electron yield (CEF) was essential for the stability and protection of PSI under Hg stress and played an important role in the induction of non-photochemical quenching (NPQ). The results showed a strong combination ability of Hg ions and photosystem particles. The number of the binding sites (n) of Hg on PSII was more than that of PSI, which may explain the different toxicity of Hg on PSII and PSI.
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