Microplastic selects for convergent microbiomes from distinct riverine sources

Microplastic is a contaminant of concern in freshwater ecosystems worldwide. Microplastic particles within aquatic habitats are colonized by dense microbial biofilms, and previous studies have shown that microplastic microbiomes are distinct in taxonomic composition from bacterial assemblages in the surrounding environment. However, questions remain about the degree to which microplastic selects for specific bacterial taxa across diverse aquatic habitats. We used laboratory microcosms inoculated with water from 3 rivers in northern Illinois watersheds with distinct land-use types to test the hypothesis that microbiomes present on microplastic would be similar even when the microplastic was incubated with microbial assemblages from different source waters. When microplastic from a commercial soap and ceramic tiles were incubated with water from each of the 3 rivers for ~1 mo, the bacterial assemblages that colonized the microplastic were remarkably consistent in taxonomic composition, whereas the planktonic and tile bacterial assemblages originating from the 3 rivers were distinct. The number of bacterial operational taxonomic units found within the microplastic microbiomes was consistent across source water treatments and ~3× lower than in the surrounding water and tiles. Some of the bacterial taxa that were over-represented in the microplastic microbiomes in our experiment can metabolize plastic or plastic-associated compounds. If microplastic microbiomes are metabolizing plastic polymers, this process could have significant implications for the long-term fate of microplastic in freshwater habitats.