Low-pollution types of heavy metals reshape bacterial community structure in an urban river receiving treated wastewater.

This study selected a river in an industrial city of Northeast China as a representative case and systematically investigated the impacts of receiving treated wastewater on river water quality, heavy metal pollution, and bacterial community dynamics. The results demonstrated that receiving treated wastewater led to a decline in water quality grade, increased heavy metal pollution and ecological risks of varying degrees, and enhanced bacterial richness and diversity. Co-occurrence network analysis revealed distinct core bacterial communities among river segments with different water sources. Robustness analysis further indicated that the bacterial community structure in the treated wastewater-receiving segment (TW) was more stable compared to other segments. Deterministic processes (specifically Homogeneous Selection) dominated the assembly of bacterial communities in the TW segment, whereas stochastic processes prevailed in the rainwater-dominated segment (RW). Structural Equation Modeling (SEM) elucidated that water quality parameters and nutrients were the primary drivers of bacterial composition in RW, while water quality parameters and low-pollution types of heavy metals (LHMs) exerted more pronounced influences on bacterial composition in TW. The inhibition of nutrient cycling functions of bacteria by LHMs has been demonstrated. Additionally, absolute abundance analysis based on 16S rRNA gene high-throughput absolute quantitative sequencing compensated for the limitations of relative abundance analysis in reflecting quantitative differences. These findings are of significant importance for water quality management and aquatic ecological conservation of urban wastewater-receiving rivers.