Shaping microbial communities in coastal systems through DOM transformation in river plumes.

River plumes substantially influence dissolved organic matter (DOM) composition and microbial community structure, but the mechanisms linking DOM molecular traits to microbial interactions in these dynamic systems remain poorly understood. We investigated DOM-microbe interactions in the Pearl River plume using high-resolution biogeochemical surveys in the northern South China Sea. By integrating optical, mass spectrometric, and 16S rRNA gene sequencing analyses, we traced DOM molecular composition and bacterial community responses along a salinity gradient. Riverine and local biological inputs enriched bio-labile DOM (BLDOM) in plume waters, which in turn enhanced DOM biochemical transformations and intensified the molecular specificity of bacteria-DOM associations. The enrichment of BLDOM was associated with intensified microbial competition and a shift toward r-strategists with higher ribosomal RNA gene operon (rrn) copy numbers and narrower habitat niche breadths. This pattern supports and extends the "Hunger Games" hypothesis by demonstrating that DOM bioavailability, rather than just inorganic nutrients, acts as a key driver of microbial competition and cooperation in dynamic coastal environments. The dominance of r-strategists in plume waters, characterized by rapid growth and efficient utilization of BLDOM, accelerated DOM turnover and promoted the transformation of bioavailable compounds into more refractory forms, thereby positioning river plumes as hotspots for coastal carbon sequestration. These findings provide new insights into the mechanistic links between DOM dynamics, microbial adaptation strategies, and carbon cycling in coastal ecosystems.