The Allelopathic Inhibition of Submerged Macrophytes (Ceratophyllum demersum and Myriophyllum spicatum) in Response to Toxic and Non-Toxic Microcystis aeruginosa

The present study systematically explored the purification effects and response of submerged plants, <i>Ceratophyllum demersum</i> and <i>Myriophyllum spicatum</i>, on toxic and non-toxic strains of <i>Microcystis aeruginosa</i> via indoor co-culture experiments. The results showed that: (1) Both plants significantly inhibited the growth of <i>Microcystis</i> and reduced the concentration of chlorophyll-a (Chla) in the water by rapidly absorbing nutrients such as nitrogen and phosphorus, with no significant differences in the inhibition between toxic and non-toxic strains, indicating that nutrient competition might be the dominant mechanism for algal suppression. (2) <i>C. demersum</i> had higher nitrogen and phosphorus removal efficiency than <i>M. spicatum</i>, but the microcystins (MCs) released by toxic <i>M. aeruginosa</i> inhibited the nutrient removal capacity of both plants. (3) The plants promoted cell lysis of toxic <i>M. aeruginosa</i> and reduced extracellular MCs in the water while accumulating MCs internally, with <i>C. demersum</i> showing stronger MC accumulation and removal ability. (4) <i>Microcystis</i> stress activated the plants' antioxidant defense systems, increased activities of SOD (Superoxide Dismutase) and CAT (Catalase), and caused membrane lipid peroxidation, increased content of MDA (Malondialdehyde), with toxic <i>M. aeruginosa</i> inducing stronger oxidative stress, and <i>M. spicatum</i> being more severely affected. (5) Plant species and algal toxicity jointly drove changes in the attached microbial community structure. The rhizosphere of <i>M. spicatum</i> specifically enriched Bdellovibrionota, suggesting a potential microbial predation pathway for algal suppression, while <i>C. demersum</i> was more associated with <i>Bacillus</i> and other microbes with allelopathic potential. In summary, <i>C. demersum</i> performed better in nutrient removal, toxin accumulation, and physiological tolerance. This study provides further theoretical support for using submerged plants to regulate cyanobacterial blooms and remediate eutrophic water bodies.