Influence of microplastic-associated biofilms on the bioavailability of a mixture of cadmium and benzo[a]pyrene by the analysis of biomarker gene expression in larval zebrafish

•BaP or Cd sorption to MPs reduces bioavailability of both contaminants.•MP-associated biofilm selectively influence biomarkers expression in Cd/BaP mix.•In presence of BaP, biofilm on MPs affects the transfer of Cd to aquatic environment.•Biofilms provide additional interactions sites for BaP compared to Cd.•Biofilm microbial communities include hydrocarbon-degrading bacteria. Microplastics (MPs, 1 µm – 5 mm) in aquatic environments undergo complex weathering transformations such as those induced by microbial colonization and biofilm formation, that affect their ability to interact with environmental contaminants (co-contaminants). In this study, the microbial composition of MP biofilms and its influence on the sorption and bioavailability of two co-contaminants with different physicochemistry, benzo[a]pyrene (B[a]P) and cadmium (Cd) in a mixture, were assessed. Aqueous-phase bioavailability was measured by assessment of biomarker gene expression for these toxicants (cytochrome P450 1A, cyp1a, and metallothionein 2, mt2, for B[a]P and Cd respectively) in larval zebrafish, Danio rerio. Significant induction of cyp1a and mt2 gene expression (p