Responses of Microcystis Colonies of Different Sizes to Hydrogen Peroxide Stress

Microcystis blooms have become a ubiquitous phenomenon in freshwater ecosystems, and the size of Microcystis colonies varies widely throughout the year. In the present study, hydrogen peroxide (H₂O₂) was applied to test the effect of this algaecide on Microcystis colonies of different sizes and to evaluate the colonies' antioxidant strategy. The results showed that Microcystis populations collapsed under treatment with 5 mg/L H₂O₂ at colony sizes smaller than 25 μm. A dosage of 20 mg/L H₂O₂ was necessary to efficiently control Microcystis colonies larger than 25 μm. The enzymatic and non-enzymatic antioxidant systems of different colonies exhibited various strategies to mitigate oxidative stress. In small colonies, superoxide dismutase (SOD) activity was readily stimulated and operated with catalase (CAT) activity to eliminate reactive oxygen species (ROS). In colonies larger than 25 μm, the antioxidant enzyme CAT and antioxidant substance glutathione (GSH) played major roles in mitigating oxidative stress at H₂O₂ concentrations below 20 mg/L. In addition, application of the algaecide led to the release of intracellular-microcystins (MCs), and oxidatively-driven MCs reached high concentrations when colony size was larger than 100 μm. Algaecide control measures should be implemented before the formation of large colonies to limit the algaecide dosage and MC release.