This study investigates young biofilms formed on polyethylen, polystyrene and wood particles along a 2000 km transect within the Baltic Sea. It aims at illuminating the principal drivers of community assembly on microplastics, in terms of surface-specific assembly versus the influence of environmental factors. Assemblages on the particles were also compared to the assemblages on natural particles and in the free-living water fraction. This study provides thus further inside into bacterial assemblages on microplastics and their potential impacts on marine ecosystems.