Single and combined toxicity of polystyrene nanoplastics and PCB-52 to the aquatic duckweed Spirodela polyrhiza

As nanoplastics and persistent organic pollutants are broadly distributed in aquatic ecosystems and pose a potential threat to ecosystem, most pertinent studies have focused on aquatic animals, while studies on freshwater plants have been rarely reported. Therefore, we analyzed the single and combined toxicological impacts of various concentrations of 80 nm polystyrene nanoplastics (PS-NPs) including 0.5, 5, 10, and 20 mg/L and polychlorinated biphenyl-52 (PCB-52, 2,2′,5,5′- tetrachlorobiphenyl) at 0.1 mg/L on the aquatic plant Spirodela polyrhiza (S. polyrhiza) after a 10-day hydroponic experiment. Laser confocal scanning microscopy (LCSM) showed the accumulation of PS-NPs mainly in the root surface and the lower epidermis of leaves, and the enrichment of PS-NPs was aggravated by the presence of PCB-52. PS-NPs at 10 mg/L and 20 mg/L alone or in combination with PCB-52 notably inhibited the growth of S. polyrhiza, reduced the synthesis of chlorophylls a and b, and increased the activities of superoxide dismutase (SOD) and peroxidase (POD) as well as malondialdehyde (MDA) levels, and induced osmotic imbalance (soluble protein and soluble sugar contents) (p < 0.05). However, a single treatment with low levels of PS-NPs had positive effects on the growth (0.5 mg/L) and photosynthetic systems (0.5, 5 mg/L) of S. polyrhiza, while co-exposure exacerbated the damaging impacts of PS-NPs on the antioxidant defense system of S. polyrhiza, which was more pronounced in the roots. Furthermore, correlation analysis revealed that plant growth parameters were positively correlated with chlorophyll a and b content and negatively correlated with soluble sugars, antioxidant enzymes, lipid peroxidation, and carotenoid content (p < 0.05). These results provide data to improve the understanding of the single and combined ecotoxicological effects of nanoplastics and polychlorinated biphenyls (PCBs) in aquatic plants and their application in phytoremediation measures.