Polysaccharides as a protective response against microcystin-induced oxidative stress in Chlorella vulgaris and Scenedesmus quadricauda and their possible significance in the aquatic ecosystem.

Toxic cyanobacteria occur worldwide in aquatic ecosystem, and their toxins have adverse effects on most aquatic organisms. However, some species of green algae can grow and flourish at environmentally relevant concentrations of microcystins (MCYSTs). Therefore, the present study aimed to investigate the possible adaptive response of two representatives of green algae, Chlorella vulgaris and Scenedesmus quadricauda to these toxins. Growth and antioxidative biomarkers of these algae were studied over a 14-day exposure to different concentrations of pure microcystin-LR (MCYST-LR) and crude MCYSTs. Both pure and crude MCYSTs significantly decreased the growth of the two algae compared to control cultures during the first 3 days of incubation. Meanwhile, increases in glutathione-S-transferase (GST), glutathione peroxidase (GPX) and lipid peroxidation, and decreases in glutathione (GSH) were also observed in toxin-treated cultures. All growth and biochemical variables were restored to control levels after 3 days of incubation and remained at levels near to those of control cultures during the remaining period of experiment. The changes in these variables correlated with polysaccharide contents of toxin-treated cultures, indicating the involvement of these polysaccharides in protecting the algal cells against MCYST-induced oxidative stress. The results of in vitro assay of antioxidant activity revealed that these polysaccharides had different activities, depending on their sulfate contents. This study provides an evidence for the first time that polysaccharides play a protective role in some microalgae against MCYST-induced oxidative stress.