Phytotoxic Mechanisms of Polystyrene Microplastics in Myriophyllum spicatum Under Saline Conditions: Insights from Physiology, Transcriptomics, and Phyllosphere Microbiota

Microplastics are emerging contaminants widely present in aquatic environments, yet their toxic effects on submerged plants and associated microbial communities under saline conditions remain unclear. In this study, Myriophyllum spicatum was exposed to polystyrene (PS) microplastics (0, 10, 30, 60, and 100 mg·L−1) under 0.5% salinity. We investigated plant growth, physiological responses, nitrogen and phosphorus removal, transcriptomic changes, and phyllosphere microbial communities. Results showed a concentration-dependent response, with low-dose stimulation and high-dose inhibition. At 30 mg·L−1, PS promoted growth, maintained membrane integrity and photosynthetic pigment levels, and enhanced nutrient removal. In contrast, 100 mg·L−1 PS caused membrane damage, photosynthetic inhibition, oxidative stress, and reduced nutrient uptake, indicating clear toxic effects. Transcriptomic analysis revealed that high PS significantly affected genes related to photosynthesis, antioxidant defense, energy metabolism, and nutrient transport. Microplastics promoted biofilm formation on leaf surfaces but did not significantly alter overall microbial community structure or diversity, instead shifting functionally related taxa associated with plant oxidative responses and nutrient removal. These findings demonstrate that PS microplastics exert phytotoxic effects under saline conditions by disrupting physiological processes and are associated with shifts in functional microbial groups, with potential implications for aquatic ecosystem health.