Quercetin alters microcystin-LR partitioning and proteomic stress responses in Microcystis aeruginosa

Plant-derived allelochemicals are increasingly explored for cyanobacterial bloom control, but their effects on cyanotoxin accumulation and partitioning remain insufficiently understood. This study investigated the effects of quercetin on growth, photosynthetic performance, microcystin-LR (MC-LR) dynamics, and proteomic responses in Microcystis aeruginosa (M. aeruginosa). At low concentrations (≤ 1 mg/L), quercetin caused limited effects on cell density, while increasing extracellular MC-LR at selected sampling times. At growth-inhibitory concentrations (10-50 mg/L), quercetin suppressed algal growth and total MC-LR production, whereas the extracellular MC-LR fraction varied over time. At algicidal concentrations (≥ 70 mg/L), quercetin caused severe growth inhibition, impaired photosynthetic performance, and substantial leakage of intracellular MC-LR, resulting in persistent extracellular MC-LR risk. Short-term exposure further showed that quercetin rapidly altered intracellular and extracellular MC-LR partitioning. Proteomic analysis after 5 d of exposure to 50 mg/L quercetin revealed increased abundance of proteins involved in membrane transport, nutrient acquisition, and stress defense, together with decreased abundance of proteins associated with photosynthetic electron transport, carbon fixation, ribosomal function, and amino acid biosynthesis. Glutathione-related proteins were also upregulated, suggesting enhanced redox defense. Overall, these findings indicate that quercetin alters MC-LR partitioning of M. aeruginosa. This highlights the need to evaluate flavonoid-based algicides in terms of both bloom suppression and toxin-release risk.