The Effect of Single and Combined Stress of Microplastics and Heavy Metals on Growth, Biochemical Components and Antioxidant Activity of Lemna gibba (Duckweed)

The increasing presence of microplastics (MPs) and heavy metals (HMs) in aquatic ecosystems poses a growing concern due to their potential ecotoxicological effects. While the individual toxicity of MPs and HMs has been widely investigated, limited attention has been given to their combined effects on aquatic macrophytes. In this study, we aimed to evaluate the single and combined impacts of two types of MPs [polypropylene (PP) and acrylonitrile butadiene styrene (ABS)] at concentrations of 25, 50, and 100 mg L⁻¹, along with three typical HMs (Zn²⁺, Cu²⁺, and Ni²⁺), on the growth, biochemical components, and antioxidant activity of the model macrophyte Lemna gibba under laboratory conditions over a 7-day exposure period. The results revealed that both contaminants alone negatively impacted growth and biochemical performance, but the combined application caused a more pronounced decrease, suggesting a synergistic inhibitory effect on plant metabolism. The simultaneous application of ABS-MP with nickel, copper, and zinc resulted in more pronounced adverse effects on L. gibba growth parameters, photosynthetic pigments, and carbohydrate content compared to single-pollutant exposures. Co-application of copper and nickel induced pronounced oxidative stress in plant tissues, as evidenced by increased malondialdehyde levels. Furthermore, significant reductions were observed in total protein, total phenolic, and total flavonoid content across all treatments. Conversely, total antioxidant activity showed variable results dependent on the specific contaminant and concentration applied. These findings provide preliminary evidence that co-occurring MPs and HMs may exert additive or synergistic stress effects on aquatic macrophytes. In particular, the comparative evaluation of ABS and PP microplastics, along with Cu, Zn, and Ni treatments, highlights polymer- and metal-specific toxicity patterns and integrated antioxidant response profiles that have not been previously reported for L. gibba.