Polystyrene microplastics inhibit the neurodevelopmental toxicity of mercury in zebrafish (Danio rerio) larvae with size-dependent effects

Insufficient evidence exists regarding the effects of microplastics (MPs) on the neuronal toxicity of heavy metals in the early stages of organisms. Herein, the effects of micro-polystyrene (μ-PS; 157 μm) and nano-polystyrene (n-PS; 100 nm) particles on the neurodevelopmental toxicity of mercury (Hg) in zebrafish embryos were compared. Zebrafish embryos exposed to Hg at the concentration of 0.1 mg L-1 revealed blood disorders, delayed hatching, and malformations such as pericardial oedema and tail deformity. The length of the larval head was significantly reduced (P < 0.01) and in vivo expression of atoh1a in the cerebellum of neuron-specific transgenic zebrafish Tg(atoh1a:dTomato) larvae was inhibited by 29.46% under the Hg treatment. Most of the toxic effects were inhibited by the combined exposure to μ-PS or n-PS with Hg, and n-PS decreased the neurodevelopmental toxicity of Hg more significantly than μ-PS. Metabolomic analysis revealed that in addition to inhibiting the amino acid metabolism pathway as in the μ-PS+Hg treatment, the n-PS+Hg treatment inhibited unsaturated fatty acid metabolism in zebrafish larvae, likely because of a greater reduction in Hg bioavailability, thus reducing the oxidative damage caused by Hg in the larvae. The combined effects of MPs and heavy metals differ greatly among different species and their targeted effects. We conclude that the combined toxicity mechanisms of MPs and heavy metals require further clarification.