In addition to aiding in digestion of food and uptake of nutrients, microbiota in guts of vertebrates are responsible for regulating several beneficial functions, including development of an organism and maintaining homeostasis. However, little is known about effects of exposures to chemicals on structure and function of gut microbiota of fishes. To assess effects of exposure to polycyclic aromatic hydrocarbons (PAHs) on gut microbiota, male and female fathead minnows ( Pimephales promelas ) were exposed to environmentally-relevant concentrations of the legacy PAH benzo[ a ]pyrene (BaP) in water. Measured concentrations of BaP ranged from 2.3 × 10 −3 to 1.3 μg L −1 . The community of microbiota in the gut were assessed by use of 16S rRNA metagenetics. Exposure to environmentally-relevant aqueous concentrations of BaP did not alter expression levels of mRNA for cyp1a1 , a “classic” biomarker of exposure to BaP, but resulted in shifts in relative compositions of gut microbiota in females rather than males. Results presented here illustrate that in addition to effects on more well-studied molecular endpoints, relative compositions of the microbiota in guts of fish can also quickly respond to exposure to chemicals, which can provide additional mechanisms for adverse effects on individuals. • Female and male fathead minnows exhibited significantly different gut microbiota. • Exposure to BaP altered structures in female gut microbiota, but not in males. • Exposure to BaP altered predicted functions in gut microbiota of fathead minnow. • Gut microbiome was more sensitive to a low dose BaP than host’s ahr1 and cyp1a1.

• A concentration dependent increase of B[ a ]P metabolites was observed • No induction of phase I or II activity was observed with increasing B[ a ]P exposure • Biotransformation of B[ a ]P was successfully implemented into in silico models • The models accurately predicted life stage-specific abundances of B[ a ]P metabolites Understanding internal dose metrics is integral to adequately assess effects environmental contaminants might have on aquatic wildlife, including fish. In silico toxicokinetic (TK) models are a leading approach for quantifying internal exposure metrics for fishes; however, they often do not adequately consider chemicals that are actively biotransformed and have not been validated against early-life stages (ELS) that are often considered the most sensitive to the exposure to contaminants. To address these uncertainties, TK models were parameterized for the rapidly biotransformed chemical benzo[ a ]pyrene (B[ a ]P) in embryo-larval and adult life stages of fathead minnows. Biotransformation of B[ a ]P was determined through measurements of in vitro clearance. Using in vitro-in vivo extrapolation, in vitro clearance was integrated into a multi-compartment TK model for adult fish and a one-compartment model for ELS. Model predictions were validated using measurements of B[ a ]P metabolites from in vivo flow-through exposures to graded concentrations of water-borne B[ a ]P. Significantly greater amounts of B[ a ]P metabolites were observed with exposure to greater concentrations of parent compound in both life stages. However, when assessing biotransformation capacity, no differences in phase I or phase II biotransformation were observed with greater exposures to B[ a ]P. Results of modelling suggested that biotransformation of B[ a ]P can be successfully implemented into in silico models to accurately predict life stage-specific abundances of B[ a ]P metabolites in either whole-body larvae or the bile of adult fish. Models developed increase the scope of applications in which TK models can be used to support environmental risk assessments.