Excess Ca2+ does not alleviate but increases the toxicity of Hg2+ to photosystem II in Synechocystis sp. (Cyanophyta)

•Effects of excess Ca on Hg toxicity to cyanobacterium was investigated.•Hg slows down electron transport on both donor side and acceptor side of PSII.•Excess Ca increases Hg toxicity to PSII of Synechocystis sp.•Hg may pose greater risk to ecosystem due to high level of Ca in karstic waters.

This study demonstrated that excess Ca2+ increased the toxicity of Hg2+ to PSII of cyanobacterium Synechocystis sp. using fast rise chlorophyll fluorescence test. Excess Ca2+ increased the inhibitory effect of Hg2+ on O2 evolution. Exposure to Hg2+ caused increase in functional antenna size (ABS/RC), trapping rate of reaction center (TR0/RC), dissipated energy flux per reaction center (DI0/RC) and maximum quantum yield of non-photochemical deexcitation (φDoφDo), indicating that some reaction centers were transformed to dissipation sinks under Hg2+ stress. Hg2+ stress slowed down electron transport on both donor side and acceptor side and caused accumulation of P680+. Excess Ca2+ intensified all the Hg2+ toxic effects on PSII function and led to dysfunction of PSII. The number of reaction centers that were transformed into dissipation sinks increased with increasing Ca2+ concentration.