Membrane lipid peroxidation, cell viability and Photosystem II activity in the green alga Chlorella pyrenoidosa subjected to various stress conditions

The unicellular green alga Chlorella pyrenoidosa was subjected to a variety of stress conditions (strong illumination, incubation with Cu 1+ or Zn 2+, exposure to high temperatures). The amplitude of thermoluminescence (TL) peak at 125°C, accumulation of thiobarbituric acid reactive substances (TBARS), which indicate an accumulation of lipid peroxidation products, efficiency of Photosystem II reactions ( F v/ F M ratio) and the percentage of viable cells were measured in stressed culture. Exposure of algae to strong (5000 μmol photons m 2 s 1) or to low (60 μmol photons m −2s −1) light combined with the addition of 1.6 μM Cu 2+ or 30 μM Zn 2+ inactivated Photosystem II, decreased the viability of Chlorella cells, and, finally, significantly enhanced TL and the accumulation of TBARS, which was accompanied by chlorophyll bleaching. TL emission started to rise after a lag-period of about 30 min in algae subjected to strong illumination, 2–3 h in copper-treated algae, and 10 h in zinc-treated algae. A vast majority of cells were nonviable to the end of the lag-period. The addition of Cu 2+ or ZN 2+ in darkness caused a slight decrase in the F v/ F M ratio without significant changes in TL emission. Incubation of algae at 50°C for 10 min did not affect the F v/ F M ratio nd cell viability, whereas no viable cells and Photosystem II activity were detected in the culture incubated at 55°C. Heat stress at temperatures above 55°C significantly enhanced the amplitude of the 125°C TL peak and the accumulation of TBARS when the algae were further incubated at low light at room temperature. We conclude that, under the stress conditions used in this study, (i) lipid peroxides and products of their degradation are not responsible for the cytolethal effect in Chlorella and (ii) lipid peroxidation arises mainly upon illumination of dead cells.