We investigated the colonization dynamics of different microplastic (MP) pellets, namely, polyethylene (PE), polyethylene terephthalate (PET), polystyrene (PS), polypropylene (PP) and polyvinyl chloride (PVC), either pristine or contaminated with polychlorinated biphenyls (PCBs), by an organohalide respiring marine microbial community and its biotransformation activity towards PCBs sorbed on MPs, in anaerobic laboratory microcosms of a marine sediment. All MPs were rapidly colonized by the microbial community within 2 weeks of incubation, when approximately 1010 16S rRNA gene copies cm-2 were detected on PVC, 109 copies cm-2 on PE, and 108 copies cm-2 on PET, PP and PS. A greater biofilm growth on PVC pellets than other MPs was confirmed by quantification of the reducing sugars of the EPS and biofilm staining with crystal violet. Illumina sequencing of the 16S rRNA genes and Principal Coordinate Analysis (PCoA) revealed that the biofilm community on MPs significantly differed from the sediment community, being enriched of chemoorganotrophic fermenting species, and was significantly affected by the type of polymer. The presence of sorbed PCBs did not significantly affect the overall community composition, and mainly resulted in the enrichment of Dehalococcoidia, i.e., of the organohalide respiring members of the community. Reductive dechlorination of PCBs sorbed to MPs was observed after 2 weeks of incubation, when the average number of chlorines per biphenyl molecule was reduced from 5.2 to 4.8-4.3, and was faster (35.2 ± 1.9 to 61.2 ± 5.8 μmol of Cl removed kgMP-1 week-1) than that of sediment-sorbed ones (33.9 ± 9.1 μmol of Cl removed kgsediment-1 week-1), which started only after 10 weeks of incubation. These data suggest that microbial colonization of contaminated MPs might change the composition of sorbed PCB mixtures and therefore the toxicity associated to PCB-polluted MPs.