Polymer-specific microplastic risks and microbial community shifts in a freshwater ecosystem: Field evidence from the Hunter River, Australia

Microplastics (MPs) are an emerging concern in freshwater ecosystems due to their role in transporting pollutants and altering microbial dynamics. This study provides a snapshot of MP contamination, ecological risks, and associated microbial communities in the Hunter River, Australia, based on single-time-point sampling across five sites. MP concentrations ranged from 47 to 229 ± 42.63 particles L^-1, with fibres, white particles, and 100-200 μm fragments most abundant. Polyethylene (PE), polyethylene terephthalate (PET), and polyamide (PA) were dominant polymers, while Omadale, Aberdeen, and Wollombi were rated hazard level V on the Polymer Risk Index due to high levels of polyvinyl chloride (PVC) and polycarbonate (PC). Microbial profiling revealed significant site-specific differences in diversity and composition, with higher Chao1 and Shannon indices at Aberdeen and Denman. Actinobacteria, Proteobacteria, Cyanobacteria, and Chloroflexi dominated across sites, though their relative abundances varied. Nineteen plastic-degrading genera were identified from PlasticDB, including Sphingomonas, Pseudoxanthomonas, Rhodococcus, and Microbacterium, alongside predicted functional enzymes involved in the degradation of xenobiotics and plastics. Canonical correspondence analysis (CCA) linked specific polymers (PVC, PET, PC, PS) to microbial taxa, including Kapabacteria, Parcubacteria, and Actinobacteria. Co-occurrence networks of plastic degraders showed distinct structural differences, with Aberdeen hosting the most complex network and Singleton the sparsest, suggesting that local conditions influence microbial stability and degradation potential. Broader network analyses revealed that higher MP loads were associated with reduced modularity and increased fragmentation, indicating disrupted community cohesion. Indicator species analysis further identified site-specific taxa, emphasizing environmental drivers of community structure. Overall, these findings demonstrate that MPs pose ecological risks not only through their abundance but also via polymer-specific effects on microbial assembly. This study provides essential baseline data for Australian rivers, supporting targeted monitoring and management strategies to mitigate polymer-driven ecological impacts.