Bacterial and fungal assemblages and functions associated with biofilms differ between diverse types of plastic debris in a freshwater system.

Once in aquatic ecosystems, plastics can be easily colonized by diverse microbes, and these microbial communities on plastics-the 'plastisphere'-often differ from the communities in the surrounding water and other substrates. However, our knowledge of plastic-associated bacterial and fungal communities on diverse plastics in freshwater is poor, especially for fungal communities. Furthermore, intraspecies interactions among bacterial and fungal communities colonized on diverse plastics are poorly known. Here, we characterized the taxonomic composition and diversity of bacteria and fungi on three types of plastics in a lab-scale incubator with freshwater from an urban river. High-throughput sequencing revealed that the alpha diversity of bacterial communities was higher on polyethylene microplastics (MPs) than on polyethylene (PE) and polypropylene (PP) sheets. The structure of bacterial communities on MPs differed from those on plastic sheets. In contrast, no striking differences in alpha diversity and taxonomic composition were observed for fungal communities on different types of plastics. Members of Ascomycota, Basidiomycota, Blastocladiomycota and Mucoromycota dominated fungal assemblages on plastics. Co-occurrence network analysis revealed that the biotic interactions between bacteria and fungi on MPs were less complex than those on PE and PP sheets. The three types of plastics shared no keystone taxa. The functional profiles (KEGG) predicted by Tax4Fun showed that the pathways of alanine, aspartate, glutamate and biotin metabolism were enriched in biofilms on MPs. Nonetheless, the higher complexity of plastic sheet-associated biofilms might make them more resistant to environmental perturbation and facilitate the maintenance of microbial activities.