An increasing number of studies report on microplastics (MP) in nature and their interactions with microbial communities. Rivers, as major transport routes for MP into marine systems and as reservoirs for bacteria, are important focal points in understanding these interactions. This study aims to investigate MP as substrates for bacterial colonization and their potential to transport these communities from riverine to marine environments. This transport was simulated via a sequential incubation experiment, where 3 sample types, synthetic (HDPE, tyre wear) and natural (wood) particles, were enclosed in a cage and successively incubated at four sites, from freshwater (Weser estuary) to coastal seawater offshore (Helgoland, North Sea). After each incubation period, a subset of each particle type and surface waters were sampled and the cage moved to the subsequent site. A second cage was simultaneously incubated at the offshore site for the entire period. Bacterial communities differed significantly between sample types at each site and across the salinity gradient. Communities detected on HDPE were the most diverse, while those on tyre wear were the least. Percentage similarities between sample types were low overall while similarities within each type decreased across the salinity gradient. Of the total detected ASVs, only 0.7 % were successively found on HDPE across all sites, none of which were detected on HDPE from the second cage offshore. Our results show an almost complete turnover of bacterial communities on MP transported from riverine to marine systems