Disruption of light–dark rhythms alters embryonic development toxicity and neurotoxicity of nanoplastics on zebrafish larvae by regulating thyroid pathways

Disruption of light–dark rhythms may increase the susceptibility of organisms to contaminants. Herein, the toxicities and related mechanisms of nanoplastics (NPs) under different light–dark rhythms were investigated. Zebrafish embryos exposed to polystyrene NPs (PS-NH₂ NPs; 50 nm) within environmentally relevant concentrations caused deformities (small eyes, pericardial edema, and spinal curvature) and increased swimming distances in zebrafish larvae. Both continuous light (LL) and continuous darkness (DD) increased the developmental toxicity of nanoplastics in zebrafish embryos, with DD having the stronger effect. The reason was that DD conditions down-regulated the pathways of thyroid, visual development, and cytochrome P450, as NP exposure did, whereas LL conditions down-regulated the first two pathways but up-regulated the cytochrome P450 pathway, which has detoxification effects in organisms. However, LL and DD conditions decreased the swimming distances of NP-treated larvae because they promoted the inhibition effect on eye development and elevated T4 levels in zebrafish larvae. Regulation of light–dark rhythms dominated the swimming behavior of zebrafish larvae under combined exposure. Our findings suggest that the influence of environmental factors on the potential health risks of NPs cannot be ignored.