Disseminated polymers (PM) and their associated pollutants are distributed both horizontally, from the coasts to remote places, and vertically, from surface to deep sea food webs, which is a major environmental concern. PM fate primarily depends on their buoyancy which can be modified by biofouling. Microbial biofilms, including pollutions-sensitive or tolerant diatoms (Bacillariophyta) communities (DC), quickly colonize PM surfaces and produce biopolymers matrices. Trace metals (TM) are likely to bind to these matrices, but the implication for both marine diatoms and PM-consuming organisms remains unclear. As part of the MicroplastiX project (2020-2023) funded by JPI Oceans, this study sought inter alia to assess TM accumulation dynamics and DC diversity and composition in biofilms. 8 buoyant or sinking polymers were immersed during 3 months at the 4 seasons between 10/21 and 9/22 at 4 different locations. 2 in NW Mediterranean Sea (Toulon,FR ; Naples,IT) and 2 in Atlantic coast (Galway,IR ; La Coruña,SP). Biofilm growth by crystal violet staining, DC assessment by metabarcoding and TM quantification by ICP-MS were performed after 7, 14, 28, 56 and 84 days of continuous immersion. In Toulon, PS, PLA and PET displayed the highest biofilms development whatever the season. Spring was significantly the most favourable one their growth (p<0.001). Overall, seasonal variations were highlighted regarding bioaccumulated trace metals with specific fluctuations among sites. Processing of the diatom diversity data is ongoing to evaluate the influence of TM concentrations on DC.