Non-Homogeneous Toxicity of Polyvinyl Chloride Microplastics on Microalgal Cells Regulated by Nutrient Availability

Microplastics (MPs) have been shown to exert time-dependent toxic effects on microalgae, with toxicity either intensifying or alleviating over prolonged exposure. However, the underlying mechanisms driving this phenomenon remain unclear, particularly regarding the role of external nutrient depletion in the surrounding medium. Herein, we performed bioassays and computational modeling to evaluate the effects of polyvinyl chloride microplastics (mPVC, 10, 50, and 100 mg L−1) on the growth and physiology of Desmodesmus sp. under varying initial medium nutrient levels (100%, 25%, 8%, and 4%) for 96 h. The results demonstrated that nutrient levels in the culture medium modulated the toxic impact of mPVC. Under a 25% nutrient concentration, exposure to 100 mg L−1 mPVC significantly inhibited algal growth by 6.5%, whereas at a 4% nutrient level, the growth of Desmodesmus sp. was significantly enhanced by 7.0%. Consistently, mPVC exposure at 25% nutrient concentration led to a sharper decline in intracellular pigment content (13.1–15.5%), soluble sugars content (8.2–27.0%), and photosynthetic efficiency (4.3–20.5%). In contrast, exposure to mPVC at 4% nutrient level improved the light use efficiency (11.9–15.4%) and electron transport rate (9.2–13.9%) of the algal cells. Independent action modeling revealed that as medium nutrient levels decreased, the interaction effects of mPVC exposure shifted from a synergistic to an antagonistic relationship. Overall, our findings identify medium nutrient availability as a critical regulatory factor influencing the toxic effects of mPVC on microalgae, potentially providing different insights into understanding the time-dependent toxicity of MPs.