Wastewater treatment plants (WWTPs) are the hotspots for the spread of antibiotic resistance genes (ARGs) into the environment. Understanding the behavior and assembly processes of ARG communities in each treatment unit in WWTPs can promote ARG surveillance and control. Despite this, current ARG surveillance in WWTPs lacks comprehensive assessments of the city-level and short-term daily variations. Thus, in this study, 285 ARGs and ten mobile gene elements (MGEs) were monitored in seven WWTPs in Xiamen via high-throughput qPCR for seven days. The results showed that the average daily load of ARG to WWTPs was about 1.21E20 copies/d, and a total of 1.44E18 copies/d was discharged to the environment across the entire city. Genes conferring resistance to beta-lactam, multidrug, aminoglycoside, and macrolide-lincosamide-streptogramin B (MLSB) were dominant. Interestingly, there was no daily difference in the richness and abundance of ARGs as well as ARG community composition. Stochastic processes were the main force determining the assembly of ARG communities in WWTPs. The relative importance of stochastic processes in each treatment unit ranked in the order of influent (INF) > effluent (EFF) > active sludge (AS). The results of SourceTracker, variation partitioning analysis (VPA) and hierarchical partitioning (HP) analysis indicated that bacterial and ARG communities from upstream treatment units played an increasing dominant role in shaping ARG communities in AS and EFF, respectively, suggesting the importance of mass-immigration of bacteria and ARGs from the source on ARG transport in wastewater treatment units. This emphasizes the need to revise the way we mitigate ARG contamination but focus on the source of ARGs in urban wastewater.

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