Reduced pigmentation and thyroid hormone disruption in zebrafish embryos caused by industrial sludge near Bohai Bay, China

In recent years, pollution caused by the discharge of industrial wastewater into Bohai Bay has posed a potential threat to the health of surrounding residents. Sludge was collected from the outlet of a factory that discharges effluent into Bohai Bay, and alcohol extracts of sludge (SE) were prepared. We confirmed by UPLC-MS/MS analysis that the SE contained PAHs, including fluorene, pyrene, and phenanthrene. Zebrafish embryos as animal models were exposed to 0.1, 0.3, 0.5, 1 and 5 ​mg/mL SE from 2 to 4 ​h post-fertilization (hpf) until 120 hpf. The results showed that SE caused a concentration-dependent increase in mortality and a decrease in hatchability. We found that SE significantly reduced eye pigmentation and decreased the movement of embryos and larvae. In addition, SE decreased triiodothyronine (T3) content and down-regulated the mRNA expression of some thyroid hormone-related genes including TPO and Thrβ, and caused the up-regulation of Dio2 and Dio3 at 120 hpf. Exposure to three individual PAHs found in SE, namely fluorene, pyrene, and phenanthrene, caused morphological and transcriptional changes that were similar to those caused by SE exposure. These findings indicate that PAHs in SE can reduce the pigmentation of zebrafish, which may be related to the genetic changes associated with thyroid hormones, and that zebrafish eye pigmentation can be used as an indicator of PAHs exposure. •SE significantly reduced eye pigmentation and decreased zebrafish movement.•SE decreased T3 content and affected some thyroid hormone-related genes.•UPLC-MS/MS analysis showed that the SE contained fluorene, pyrene, and phenanthrene.•Individually, fluorene, pyrene, and phenanthrene caused changes similar to those caused by SE exposure.