Multi-omics reveals microbial nitrogen metabolism driving biphasic dynamics of N-nitrosodimethylamine precursors along effluent-receiving river
The effluent of wastewater treatment plant is discharged with significant amounts of organic and inorganic nitrogen into receiving rivers, increasing the risk of carcinogenic N-nitrosodimethylamine (NDMA) formation from precursors. The fate of NDMA precursors was closely associated with the growth and metabolic processes of river microorganisms. However, microbial mechanisms governing NDMA precursors dynamics along receiving river sediments are not yet fully understood. This study conducted time-series incubation experiments for sediments collected along receiving rivers (before and after discharge of effluent), in combination with multi-omics analyses (16S rRNA gene sequencing, metagenomics, and metabolomics) , tracking microbial functional metabolism dynamics to clarify the precursors transformation mechanism. A biphasic pattern of total NDMA precursors (defined as formation potential, FP) revealed different phases including initial formation followed by degradation, driven by shifts in microbial community structure and function. NDMA FP reached a peak concentration of 1258 ng/L during incubation of downstream sediment. The early phase was dominated by r-strategist taxa such as Bacillota, which promoted organic nitrogen mineralization and nitrification, leading to the accumulation of precursors and nitrate. With anoxic conditions developing, a functional shift toward the K-strategist taxa (e.g., Pseudomonadota and Chloroflexota) occurred, enhancing co-metabolism of precursors by methane monooxygenase and consumption of precursors by ammonia monooxygenase. Then precursors were transformed into organic nitrogen compounds (e.g., short-chain peptides, amino acids, and peptide analogues) and inorganic nitrogen (e.g., nitrite). Notably, long-term discharge of effluent formed a stable and functionally specific microbial legacy, which increased the peak NDMA FP in sediments collected from downstream of effluent by 158% and 80.7% compared to upstream and outfall area, respectively. These findings illustrated that effluent-induced microbial succession established a self-regulating metabolic network, enhancing the peak risk of NDMA precursors through ecologically retained functional traits. This study provided a theoretical foundation for assessing NDMA precursor transformation and risk in sediments along effluent-receiving river.