Leaf aqueous (LA) extracts of medicinal plants Achillea ageratum L. and Origanum compactum Benth. were tested to explore their potential algicidal effects on Microcystis aeruginosa. The growth of M. aeruginosa and cell morphology changes in response to LA extracts was investigated. The concentrations of total phenols, total flavonoids, and total tannins in LA extracts were analyzed to reveal the potential allelochemical compounds. The results demonstrate that both A. ageratum and O. compactum

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... extracts inhibit M. aeruginosa growth in a concentration-dependent way. For both LA extracts, the highest inhibition rate exceeded 80% since the fifth day of exposure. The induction of inhibition effects was translated by a decrease in photosynthetic pigments (chlorophyll-a and carotenoids) and with other morphological modifications. Our results illustrate that both A. ageratum and O. Compactum LA extracts can offer an effective solution for control of Microcystis blooms, and these are recommended in the restoration of aquatic environments contaminated by these types of algal blooms.