Microbial Control on Dissolved Organic Matter Composition along the South-to-North Water Diversion Middle Route Project

Drinking water safety is vital in large-scale water transfer systems such as the South-to-North Water Diversion Middle Route Project (SNWDP-M). Dissolved organic matter (DOM) significantly affects water quality, although factors driving its compositional changes along the SNWDP-M remain insufficiently explored. This study integrated DOM spectral analyses and ultrahigh resolution mass spectrometry with 16S rRNA gene sequencing to explore DOM–bacterial community (BC) interplay. Results demonstrated that the midstream section (MS) of the SNWDP-M exhibited higher protein-like content compared to the upstream section (US) and downstream section (DS), while the DS had relatively higher humic-like content. DOM molecular complexity and aromaticity increased from US to DS was observed, alongside a rise in bacterial diversity. Bipartite network analysis revealed evolving DOM-BC interactions, with generalized metabolic interactions DS, where BCs exerting a stronger influence on DOM composition downstream (RDA, R2 = 0.62) compared to the US (R2 = 0.29) and MS (R2 = 0.24). It highlights enhanced microbial control in the DS, emphasizing the need to integrate microbial dynamics into water quality management. Taken together, this study elucidates the dynamic DOM processes along the SNWDP-M and the BC influence, providing a basis for improving water quality management in large-scale water transfer projects.