Utilizing the allelopathic effect of plants to inhibit algae growth has gained significant attention from researchers in recent years. However, there are large amounts of microplastics in the aquatic environment, which may influence the inhibitory effects of allelochemicals on algae. Consequently, this research aimed to examine the individual and combined toxicity of gallic acid (GA) and polyethylene glycol terephthalate (PET) microplastics on Microcystis aeruginosa. At a concentration of 50 mg/L, PET significantly enhanced algal cell growth and chlorophyll a synthesis, while inducing oxidative stress without damaging the cell membrane. On the other hand, low concentrations of GA promote algal cell growth. However, high concentrations of GA not only inhibit growth and photosynthesis but also result in cellular abnormalities and severe oxidative damage, as well as membrane disruption. Moreover, it was observed that the exposure of PET diminished the toxicity of GA to M. aeruginosa. The combined exposure to GA and PET resulted in a slight increase in chlorophyll a content, as well as phycocyanin (PC), alloxyphycocyanin (APC), and phycoerythrin (PE) content, along with maximum photochemical efficiency (Fv/Fm), compared to GA exposure alone. In the combined system of GA and PET, the contents of superoxide dismutase (SOD) and malondialdehyde (MDA) were lower than those in the single GA system. Additionally, both systems showed a degree of improvement compared to the control group. Scanning electron microscopy (SEM) observed that the presence of PET resulted in reduced damage to algal cells compared to exposure to GA alone. This research establishes a foundation for understanding the synergistic impacts of allelochemicals and microplastics on M. aeruginosa, thereby providing a reference for managing cyanobacteria in water blooms.
