The community dynamic alterations mechanisms of traveling plastics in the Pearl River estuary with the salinity influence.

Most ocean plastics originate from terrestrial emissions, and the plastisphere on the plastics would alter during the traveling due to the significant differences in biological communities between freshwater and marine ecosystems. Microorganisms are influenced by the increasing salinity during traveling. To understand the contribution of plastic on the alteration in biological communities of plastisphere during traveling, this study investigated the alterations in microbial communities on plastics during the migration from freshwater to brackish water and saltwater. The results revealed that the migrated plastics can form unique microhabitats with high bacterial and eukaryotic diversity. Compared with the natural carrier (stone), the communities in plastisphere had fewer variations with salinity, indicating that plastisphere can offer stronger protection for freshwater organisms. The hydrophobicity of plastics promoted algal colonization, providing a stable nutrient source for the community during salinity fluctuations. This reduced material exchange between the plastisphere and the surrounding high-salinity environment, facilitating greater community stability. Additionally, the abundant Ochrophyta and Bryozoa of eukaryotes on migrated plastics can facilitate further colonization and promote species diversity. Plastisphere microbial networks revealed that the reduction of salt-intolerant organisms during traveling had fewer effects on the abundance of associated organisms. A more stable community on migrated plastics led to the proliferation of pathogens and carbon cycle-degrading microorganisms. And the increasing relative abundance of carbon cycling functions indicated that the traveling plastics could pose higher environmental risks and exhibit enhanced carbon metabolic capabilities. The study highlighted the biofilms on migrated plastics as a unique ecological niche in estuarine environments, offering a crucial reference for evaluating the ecological risks linked to plastic travel from rivers to the ocean.