Halogenated dissolved organic matter regulates bacterial community composition and organic carbon transformation in aquatic ecosystems.

Bacteria-dissolved organic matter (DOM) interactions profoundly influence organic carbon fate along the land-ocean aquatic continuum. Halogenated DOM (H-DOM) is recognized as a widespread component of the DOM pool in natural waters, but its interactions with bacterial communities and ecological functions remain poorly understood. Using non-targeted ultra-high-resolution mass spectrometry and amplicon sequencing, we characterized H-DOM, non-halogenated DOM (N-DOM), and bacterial communities across three hydrological seasons in the Minjiang River, the largest mountainous river in southeastern China, and in the nearby coastal plume system of the Pearl River. Results showed that N-DOM exhibited higher relative abundance in the high-flow season and was spatially homogeneous. In contrast, the relative abundance of riverine H-DOM was highest during the low-flow season and exhibited significant spatial heterogeneity, with higher H-DOM relative abundances observed in aquaculture zones, farmlands, and urban areas strongly affected by anthropogenic activities. Despite spatiotemporal variations in DOM composition, network analyses revealed that riverine bacteria consistently showed highly positive correlations (99.35% ± 0.45%) with H-DOM across all seasons, and exhibited tighter associations with H-DOM than with N-DOM. Similar network structure was observed in the coastal plume system. Structural equation modeling further demonstrated that, compared with plume H-DOM, riverine H-DOM with greater anthropogenic impact exerted a stronger regulatory effect on bacterial community composition, thereby altering bacteria-mediated DOM transformation. Overall, these findings reveal the previously overlooked ecological role of H-DOM in bacteria-DOM interactions. Integrating this component into the carbon cycling framework will facilitate a more comprehensive understanding of carbon transformation across land-ocean aquatic ecosystems.