Species-specific bioaccumulation and biotransformation of two novel brominated flame retardants in fish: Insights from isotope fractionation.

1,2-dibromo-4-(1,2-dibromoethyl)-cyclohexane (TBECH) and 1,2-bis(2,4,6-tribromophenoxyethane) (BTBPE), two novel brominated flame retardants, have been widely detected in diverse environmental media. However, information about their fate in aquatic environments is limited. In this study, two aquatic food chains comprising a prey species (tiger barb, TB) and two predator species (oscar fish, OF, and redtail catfish, RF) were established to examine the species-specific biotransformation and bioaccumulation of TBECH and BTBPE in fish. Higher biomagnification factors of TBECH and BTBPE were observed in RF ((3.44 ± 0.31)-(3.62 ± 0.14)) compared to OF ((2.77 ± 0.06)-(3.18 ± 0.05)). Debrominated and hydroxylated biotransformation products (BPs) of BTBPE and α-/β-TBECH were identified in OF livers, whereas no debrominated BPs were detected in RF. The changes in enantiomer fractions (EFs) of α-TBECH and β-TBECH in both RF and OF indicate selective biotransformation of specific enantiomers within each fish species. Additionally, the stable carbon isotope (δ13C) of α-/β-TBECH and BTBPE, as well as α-/β-TBECH enantiomers further substantiates a higher metabolic conversion potential was observed in OF compared to RF, supports the hypothesis that debromination exclusively occurs in OF. Meanwhile, the carbon isotope enrichment factor (εC) of α-/β-TBECH, BTBPE, and α-TBECH enantiomers in OF ((-2.43 ± 0.21)-(-3.41 ± 0.16)) were significantly higher than those in RF ((-1.74 ± 0.18)-(-2.16 ± 0.22); p < 0.05), indicating potential disparities in biotransformation mechanisms between these two fish species. In vitro incubation experiments using OF liver microsomes further demonstrated that εC values of β-TBECH and BTBPE during the debromination process (-2.93 ± 0.14 and -3.55 ± 0.21) were notably higher compared to oxidative processes (-1.90 ± 0.23 and -2.74 ± 0.18). These results provided valuable insights into species-specific bioaccumulation and biotransformation of chemicals in fish, while stable carbon isotope fractionation can potentially reveal distinct transformation pathways of chemicals within fish.