Similar Drivers of Bacteria-Mediated Dissolved Organic Matter Transformation across Geographically Distinct Estuaries

Estuaries are dynamic processing hubs for dissolved organic matter (DOM), and bacteria play key roles in DOM transformation. However, the processes and drivers of estuarine bacteria-mediated DOM transformation remain poorly understood. We investigated the DOM composition, bacterial communities, and environmental factors across four geographically distinct estuaries: Jiaozhou Bay (JZB), Xiamen Bay (XMB), Chesapeake Bay (CB), and Mission-Aransas Estuary (MAE). During the sampling period, JZB and XMB were enriched in autochthonous DOM, while CB and MAE were dominated by terrestrial DOM. Bacterial diversity was significantly greater in JZB and XMB than in CB and MAE. Co-occurrence networks showed that Proteobacteria (41.4% ± 9.8%) were extensively involved in estuarine DOM metabolism; bacterial communities exhibited a metabolic preference for carboxyl-rich alicyclic molecules (71.7% ± 7.1%) in all estuaries, and more complex communities had greater capacity to degrade recalcitrant DOM. Structural equation modeling further indicated that bacterial-mediated DOM transformation in all estuaries was regulated by similar environmental factors (temperature and dissolved inorganic nitrogen). Warming and nitrogen limitation could reduce bacterial diversity and promote the accumulation of recalcitrant DOM. Overall, this study explored estuarine bacteria-mediated DOM transformation at the molecular level and identified common environmental drivers, providing deeper insight into the fate of estuarine DOM.