Intensive mariculture promotes molecular reorganization of dissolved organic matter in coastal waters.

Mariculture effluents represent a growing anthropogenic perturbation to coastal environments, yet their impacts on dissolved organic matter (DOM) composition and microbially mediated transformation processes remain insufficiently understood. Here, we applied molecular-level DOM analysis combined with microbial community profiling to elucidate DOM biogeochemical behavior in a typical high-density mariculture bay in China. Mariculture effluents substantially increased DOM molecular diversity, yielding up to 1406 unique molecular formulas in tailwater, and introduced abundant nitrogen- and sulfur-containing compounds. Sulfur-containing DOM was markedly enriched in Xiangshan Bay (12.11%) relative to riverine water (7.35%), while inner and mid-bay DOM shared far more molecular formulas with mariculture tailwater than with riverine water. In contrast, outer-bay DOM was dominated by lignin-rich, aromatic compounds associated with the Yangtze River diluted water plume, producing a sharp spatial segregation of DOM sources within the bay. Microbial communities closely mirrored these patterns, showing strong overlap between tailwater and bay waters and enrichment of heterotrophic and sulfur-related pathways. Co-occurrence networks revealed highly connected and modular DOM-microbe interactions in Xiangshan Bay, suggesting potential microbial involvement in sulfur-containing DOM transformation. These results demonstrate that mariculture effluents are closely associated with distinct DOM composition and DOM-microbe coupling patterns, highlighting intensive aquaculture as an important contributor to organic matter transformation in semi-enclosed coastal systems.