Wastewater discharges and polymer type modulate the riverine plastisphere and set the role of microplastics as vectors of pathogens and antibiotic resistance

A limited understanding of the factors shaping microbial communities in microplastics (MPs) hinders effective risk assessment and mitigation. This study investigated how wastewater treatment plant (WWTP) discharges and polymer type shape the riverine plastisphere in situ by analyzing microbial community composition, pathogens, antibiotic resistance genes (ARGs), and mobile genetic elements (MGEs). Polyethylene (PE), polypropylene (PP), polyethylene terephthalate (PET), and a polymer mixture (Mix) were incubated upstream and downstream of a WWTP discharge. High-throughput sequencing revealed that MPs harbored significantly higher microbial diversity and richness compared to the surrounding water and sand. The plastisphere communities were distinct before and after the WWTP discharge, sharing only <8 % of ASVs. Quantitative PCR data showed that WWTP discharges led to a 2.3-fold increase in ARGs abundance on MPs (vs. pre-WWTP MPs), with clinically relevant genes such as bla CTX-M , bla VIM , and bla IMP enriched in the plastisphere compared to water. Polymer type also influenced the microbial community's composition, resistome, and mobilome. The Mix, PET, and PP communities harbored 14 %, 10 %, and 14 % unique ASVs, respectively. Mix MPs were enriched in potential pathogens, PP biofilms exhibited the highest abundance of sulfonamide resistance genes, MGEs, and integrases, while PET biofilms had the highest abundance of multidrug resistance genes. This study enhances our understanding of the freshwater plastisphere under realistic conditions, emphasizing the crucial role of WWTP discharges and polymer type in shaping these communities and their associated risks. • WWTPs and polymer type influence on riverine plastisphere were investigated in situ. • WWTP effluent shaped plastisphere communities, with <8 % ASVs shared between sites. • WWTP effluent increased pathogens and ARGs in MPs, with ARGs rising ≈1.2-fold. • Exclusive ASVs were found in Mix (14 %), PET (10 %), and PP (14 %). • Mix, PET, and PP communities showed distinct resistomes and mobilomes.