Inhibitory Effects of Artemisia argyi Extracts on Microcystis aeruginosa: Anti-Algal Mechanisms and Main Allelochemicals

Harmful cyanobacterial blooms (CyanoHABs) threaten freshwater ecosystems and human health. Inhibiting cyanobacteria through plant allelopathy is an effective and environmentally friendly approach for CyanoHAB control. In this study, we evaluated the inhibitory activities of several organic solvent extracts from <i>Artemisia argyi</i> against the common bloom-forming cyanobacterium <i>Microcystis aeruginosa</i>, explored the anti-algal mechanism of the active fraction, analyzed its secondary metabolites using liquid chromatography-high-resolution mass spectrometry (LC-HRMS), and screened the potential allelochemicals. The results showed that the crude extract of <i>A. argyi</i> leaves (CE) exhibited significant inhibitory effects on <i>M. aeruginosa</i>. Among several solvent fractions of CE, the dichloromethane extract (DE) demonstrated the strongest inhibitory effect, with a 7-day IC₅₀ of 70.43 mg/L. After treatment with DE, the contents of chlorophyll a (Chl a), carotenoids, and phycobiliproteins (PBPs) in <i>M. aeruginosa</i> were significantly reduced. Meanwhile, an excessive accumulation of reactive oxygen species (ROS), reduction of catalase (CAT) activity, increase in malondialdehyde (MDA) content, and shrinkage of the membrane were found in <i>M. aeruginosa</i> cells under DE treatments. There were 81 secondary metabolites annotated in DE by LC-HRMS. Among them, hispidulin, jaceosidin, 5,7,3'-trihydroxy-6,4',5'-trimethoxyflavone, and eupatilin possessed strong inhibitory activities, with 7-day IC₅₀ values of 26.23, 27.62, 32.02, and 34.98 mg/L, respectively. These results indicated that the <i>A. argyi</i> extracts possess significant allelopathic activities on <i>M. aeruginosa</i>, and DE was identified as the primary active fraction. It inhibits algae growth by suppressing photosynthesis and inducing peroxidation, ultimately leading to cell death. Flavonoids in DE were the main allelochemicals responsible for the inhibition on algae of <i>A. argyi</i> extracts.