Shaping of microbial community structure and function by effluent-derived dissolved organic matter in a receiving river across seasons.

Wastewater treatment plant (WWTP) effluent is a significant source of environmental impacts, introducing substantial dissolved organic matter (DOM) and nutrients into receiving waters. However, the specific role of effluent-derived DOM (EDOM) in shaping microbial community structure and key metabolic functions remains insufficiently understood. We systematically explored the effects of EDOM on the seasonal dynamics of bacterial communities in effluent-receiving waters in a typical water-scarce region of North China. Our results revealed that WWTP effluent input significantly altered DOM characteristics, shifting them toward greater humification, increased recalcitrance, lower molecular mass, and simpler structure, against a background of elevated nutrient levels. The interplay between transformed EDOM and resident microorganisms collectively restructured the bacterial community and its potential metabolic functions. These changes included (i) increased taxonomic richness and a more specialized community composition; (ii) enhanced microbial network complexity with a polycentric architecture dominated by DOM components; (iii) seasonal convergence of microbial network structure, as strong EDOM-driven interactions in the dry season were attenuated by wet season hydrology; and (iv) strengthened coupling between DOM molecular traits and biogeochemical cycling potential. Crucially, EDOM was identified as the key driver controlling microbial richness and core metabolic processes, with its influence exhibiting significant seasonal dynamics across the carbon, nitrogen, and sulfur cycles. While current Chinese wastewater discharge standards regulate nutrient inputs, they lack consideration of ecological risks posed by EDOM fractions and their dynamic interactions with microorganisms. Our findings highlight the need to integrate EDOM characteristics into effluent risk assessments to improve WWTP discharge standards.