Abstract: Nanoparticles (NPs) have attracted considerable attention because of their wide range of applications. Interactions between NPs and heavy metals (eg., lead) in the aquatic environment may modify toxicant bioavailability and consequently the toxicant’s environmental fate and toxicity. Therefore, we investigated the influence of silicon dioxide nanoparticles (nano-SiO₂) on the bioavailability and toxicity of Pb, and also its effects on the thyroid endocrine system of zebrafish (Danio rerio) larvae. Zebrafish embryos after postfertilization of 2 hours were exposed to Pb alone (0, 5, 10 and 20 μg/L) or in combination with nano-SiO₂ (25 μg/L) until 144 hours. The results showed that the bioconcentration of Pb significantly enhanced when combined with nano-SiO₂ than without (P<0.05). The exposure of zebrafish embryos/larvae to either Pb or nano-SiO₂ alone did not cause any significant difference in the thyroid hormone contents (T4 and T3) (P>0.05), whereas the contents of both T4 and T3 significantly decreased under the co-exposure of Pb and nano-SiO₂(P<0.01 and P<0.05). The considerable upregulation of tshβ gene were observed in Pb groups no matter with or without nano-SiO₂ treatment, while the transcription of ttr and tg genes were remarkably downregulated by Pb treatment alone, and even further down-regulated under the co-exposure (P<0.05). These results indicate that nano-SiO₂ enhances the bioconcentration of Pb, thus leading to thyroid endocrine disruption in zebrafish larvae. 

Key words: lead, nanoSiO2, coexposure, thyroid disruption, zebrafish (Danio rerio)