Polystyrene microplastics enhance environmentally relevant prometryn-induced developmental toxicity, tissue damage, and gut microbiota dysbiosis in marine medaka (Oryzias melastigma).

Microplastics (MPs) and triazine herbicides are ubiquitous pollutants in marine ecosystems, but their combined toxicity under environmental concentrations to marine fish is still unclear. In this study, we exposed fertilized eggs of marine medaka (Oryzias melastigma) to 10 μg/L prometryn (Pro) alone and combined with 10-μm MPs at concentrations of 1, 10, and 100 μg/L for 60 days to assess their impacts on the development, tissue damage, and gut microbiota dysbiosis. The results revealed that the presence of MPs significantly exacerbated the toxicity of Pro. Specifically, the combination of Pro with 1 and 10 μg/L MPs for 21 days led to more severe developmental issues, including delayed hatching, reduced larval size/weight, and increased malformations. Prolonged exposure (60 days) further intensified these adverse effects, with higher MPs accumulation in the intestines, reduced survival, and stunted juvenile growth. Histopathological examination showed that MPs exacerbated the damage of Pro in the liver and intestinal tissues. Moreover, co-exposure disrupted genes linked to oxidative stress, immunity, and growth. Additionally, both Pro alone and combined with 100 μg/L MPs reduced microbiota diversity and induced dysbiosis. This study offers novel perspectives on the evaluation of environmental health risks of ubiquitous MPs and herbicide pollution.