Wastewater treatment plant effluent drives coupled changes of viral and bacterial community structure and function in impacted rivers

The discharge of wastewater treatment plant (WWTP) effluent containing bacteria and viruses has significant ecological and public health implications for aquatic ecosystems. While viruses infecting bacterial hosts are abundant and diverse in wastewater, their environmental fate, host association, and functional impact in affected river ecosystems remain poorly understood. Using a metagenomic approach, we characterized double-stranded DNA viral communities across nine WWTPs and impacted riverine habitats, including water, suspended particles, sediment, and epilithic biofilm. River water was the most affected habitat by WWTP effluent, with viral diversity increasing by 22 % (±15 %) downstream. In contrast, no significant differences were observed in either viral or bacterial community structures across locations in biofilm or sediment. Among 38,826 viral operational taxonomic units (vOTUs) recovered from 148 metagenomes, 18 % were shared exclusively between effluent and downstream habitats, primarily with river water (99 % of the vOTUs). These wastewater-associated vOTUs were predicted to infect key bacterial taxa involved in carbon and nitrogen cycling (e.g., Nitrosomonas and Methylomonadaceae) and potential human pathogens (e.g., Vibrio and Ralstonia). Additionally, WWTP effluent increased the diversity of virus-encoded auxiliary metabolic genes, especially those involved in carbon, nutrient, and drug metabolism, suggesting potential roles in shaping host fitness and environmental adaptation. Overall, our findings demonstrate that WWTP effluent drives coupled changes in viral and bacterial communities in river water, highlighting the potential ecological consequences of virus-host interactions in wastewater-impacted aquatic environments.