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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 < 0.05) was observed after exposure to the mixture of Cd, B[a]P and MPs compared to joint exposure with individual contaminants and MPs. Significant changes in bioavailability for mt2 biomarker (p < 0.001) resulted after exposure to a Cd and B[a]P mixture with MPs compared to the same exposure without MPs. Biofilms significantly reduced bioavailability of B[a]P (cyp1a gene expression (p < 0.01)) but not Cd (mt2 gene expression) in the mixture with Cd and B[a]P (HDPE + BF + B[a]P + Cd) compared to the same treatment without biofilm (HDPE + B[a]P + Cd). Thus, compared to Cd, the biofilm could provide additional interactions with B[a]P, and new specific active sites on the MPs surface, that reduced B[a]P bioavailability. Additionally, the biofilm microbial community included hydrocarbon-degrading bacteria able to metabolize hydrophobic chemicals. These data indicated that in a mixture of co-contaminants, the biofilm selectively influenced their bioavailability and that the microbial composition of MPs biofilm may have played a key role in reducing B[a]P bioavailability. The results of this study highlight how in a complex exposure scenario characterized by a mixture of different co-contaminants, the polymer and chemical properties and micro-surroundings of the organisms may affect contaminants' bioavailability and/or exposure.