Exposure route affects the distribution and toxicity of polystyrene nanoplastics in zebrafish

The widespread use of polystyrene (PS) products in a myriad of consumer products has resulted in widespread contamination of PS nanoplastics (PSNPs) in aquatic ecosystems. Fish early life stages are exposed to nanoplastics dermally and via gills. Additional routes of exposure include oral via the ingestion of contaminated prey and maternal transfer. However, there is limited amount of work studying the impact of exposure route in the toxicokinetics and toxicodynamics of PSNPs. The objective of this study was to compare the effects of exposure routes (aqueous and microinjection) on the organ distribution and toxicity of PSNPs. We "mimicked" the maternal exposure of PSNPs to zebrafish by injecting a known concentration of fluorescent particles directly into 2-cell stage embryos. Endpoints were collected starting at 96 h post-fertilization until several weeks post-hatch to evaluate depuration. Although both exposure routes led to the accumulation of PSNPs in the yolk sac followed by brain, eyes, gut and swim bladder, the aqueous exposure caused higher PSNP concentrations in the brain and eyes and the injection exposure caused PSNP accumulation mainly in the trunk area. A waterborne exposure also reduced antioxidant gene expression; increased frequency of developmental abnormalities such as bent tails, jaw deformities and pericardial edema; and resulted in lower growth rates and hypoactivity. Overall, a waterborne exposure to PSNPs resulted in higher transfer to the brain and caused greater toxic effects to zebrafish compared to an injection exposure and highlights the key role of exposure routes in the uptake, localization and subsequent distribution of nanoparticles.