Dietary accumulation of hexabromocyclododecane diastereoisomers in juvenile rainbow trout ( Oncorhynchus mykiss ) I: Bioaccumulation parameters and evidence of bioisomerization

Juvenile rainbow trout ( Oncorhynchus mykiss ) were exposed to three diastereoisomers (α, β, γ) of hexabromocyclododecane (C 12 H 18 Br 6 ) via their diet for 56 d followed by 112 d of untreated food to examine bioaccumulation parameters and test the hypothesis of in vivo bioisomerization. Four groups of 70 fish were used in the study. Three groups were exposed to food fortified with known concentrations of an individual diastereoisomer, while a fourth group were fed unfortified food. Bioaccumulation of the γ‐diastereoisomer was linear during the uptake phase, while the α‐ and β‐diastereoisomers were found to increase exponentially with respective doubling times of 8.2 and 17.1 d. Both the β‐ and the γ‐diastereoisomers followed a first‐order depuration kinetics with calculated half‐lives of 157 ± 71 and 144 ± 60 d (±1 × standard error), respectively. The biomagnification factor (BMF) for the α‐diastereoisomer (BMF = 9.2) was two times greater than the β‐diastereoisomer (BMF = 4.3); the large BMF for the α‐diastereoisomer is consistent with this diastereoisomer dominating higher‐trophic‐level organisms. Although the BMF of the β‐diastereoisomer suggests that it will biomagnify, it is rarely detected in environmental samples because it is present in small quantities in commercial mixtures. Results from these studies also provide evidence of bioisomerization of the β‐ and γ‐diastereoisomers. Most importantly, the α‐diastereoisomer that was recalcitrant to bioisomerization by juvenile rainbow trout in this study and known to be the dominant diastereosiomer in fish was bioformed from both the β‐ and the γ‐diastereoisomers. To our knowledge, this is the first report of bioisomerization of a halogenated organic pollutant in biota.