Strong reprocessing of dissolved organic matter along South-to-North water diversion

<p>Freshwater ecosystems contain a diverse mix of dissolved organic matter (DOM) molecules, which play a significant role in the global carbon cycle. Here, we discovered an absorbing pattern of DOM-microbe interactions along the Middle Route of the South-to-North Water Diversion Canal in China, the largest water transfer project in the world. Using ultrahigh-resolution mass spectrometry, we tracked the molecular composition of DOM along the canal and analyzed its relationship with planktonic bacterial communities and physicochemical conditions. More oxidized and recalcitrant DOM formulae were accumulated in the canal waters, inducing the enrichment of oligotrophic taxa. Homogenization of DOM composition and bacterial communities was observed in the canal waters. Additionally, negative interactions between DOM molecules and bacterial taxa were more abundant in the canal waters (69.5%) than those in natural waters (32.2%). These results together suggest that the semi-enclosed nature of the canal and the sustained microbial and photochemical transformations drive DOM toward more homogeneity and recalcitrance, necessitating systematic quantification of carbon emissions specific to the South-to-North Water Diversion project. Incorporating carbon reduction strategies and technologies into the design and management of the national water network is scientifically imperative.</p>