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.