Cu2+ Inhibits Photosystem II Activities but Enhances Photosystem I Quantum Yield of Microcystis aeruginosa

Copper at low concentration is essential for many metabolic processes in plants and microorganisms. Cu²⁺ is an essential element but high levels of Cu²⁺ have toxic effects on organisms. Exposure to elevated concentrations of Cu²⁺ can increase content of reactive oxygen species and the non-selective conductivity of cell membrane and permeability of the plasmalemma. Extensive studies showed that photosystem II (PSII) is very sensitive to Cu²⁺ toxicity. However, Photosystem I (PSI) was found to be less sensitive than PSII under various environmental stresses, including Cu²⁺ stress.

In the this study, effects of copper on the activities and electron transport of PSI and PSII in Microcystis aeruginosa, one of the most common freshwater cyanobacteria species, were examined. The Dual-PAM-100 chlorophyll fluorometer was used to study the toxic effects of copper on PSI and PSII activities and the regulation mechanism between PSII and PSI in M. aeruginosa.

The results showed that cell growth and contents of chlorophyll α were significantly inhibited by Cu²⁺. Photosystem II activity [Y(II)] and electron transport [rETR_max(II)] were significantly altered by Cu²⁺. The quantum yield of photosystem II [Y(II)] decreased by 29% at 100 μg L⁻¹ Cu²⁺ compared to control. On the contrary, photosystem I was stable under Cu²⁺ stress and showed an obvious increase of quantum yield [Y(I)] and electron transport [rETR_max(I)] due to activation of cyclic electron flow (CEF). Yield of cyclic electron flow [Y(CEF)] was enhanced by 17% at 100 μg L⁻¹ Cu²⁺ compared to control. The contribution of linear electron flow to photosystem I [Y(II)/Y(I)] decreased with increasing Cu²⁺ concentration. Yield of cyclic electron flow [Y(CEF)] was negatively correlated with the maximal photosystem II photochemical efficiency (F_v/F_m). In summary, photosystem II was the major target sites of toxicity of Cu²⁺, while photosystem I activity was enhanced under Cu²⁺ stress.

The study was published in Biological Trace Element Research in August 2014.