Silver nanoparticles (AgNPs) are recognized for their high phytotoxicity at elevated concentrations, yet the impacts of microplastics—particularly on plant photosynthesis—remain insufficiently explored. Furthermore, despite growing evidence of co-occurrence of AgNPs and microplastics in aquatic ecosystems, their combined effects on aquatic plants are largely unknown. This study investigates the individual and combined effects of AgNPs and polystyrene microplastics (PS-MPs) on the submerged leaves of Ottelia cordata, aiming to determine whether microplastics alter the phytotoxic response to AgNPs exposure. Exposure to 1 mg/L AgNPs resulted in marked cellular ultrastructural damage, substantial declines in chlorophyll content, bicarbonate utilization, and photosynthetic enzyme activities, collectively impairing photosynthetic performance. In contrast, 10 mg/L PS-MPs exhibited relatively mild toxicity, primarily reducing oxygen exchange rates and the activities of phosphoenolpyruvate carboxylase (PEPC) and pyruvate phosphate dikinase (PPDK), with negligible effects on chloroplast structure or other physiological parameters. Notably, co-exposure to AgNPs and PS-MPs significantly alleviated AgNPs-induced phytotoxicity. This mitigation is likely due to the adsorption of AgNPs onto the surface of PS-MPs, which reduced Ag accumulation in plant tissues and consequently lessened physiological disruptions. Indicators such as chlorophyll content, fluorescence parameters, bicarbonate uptake, and enzymatic activity partially recovered under co-exposure conditions. These findings reveal complex interactions between nanomaterials and microplastics, highlighting that PS-MPs may attenuate the toxicity of AgNPs in aquatic plants. This study contributes to a more nuanced understanding of contaminant interactions and underscores the importance of evaluating combined pollutant effects in ecological risk assessments.
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