Estuaries locate in the land-ocean interface, serve as an actively “dissolved organic matter (DOM) processing factory”. Bacteria are considered to be important shapers of DOM composition. However, the interactions between bacterial communities and DOM molecules in global estuaries are unclear, which limits the assessment of the carbon storage potential of estuarine DOM pools. Here, we collected samples from four estuaries (Jiaozhou Bay and Xiamen Bay, pEstuaries; Chesapeake Bay and Mission-Aransas Estuary, aEstuaries). DOM composition and bacterial community structure were analyzed by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), three-dimensional fluorescence spectroscopy, and 16S rRNA genes amplicon sequencing. Results showed that the DOM content and molecular formula number were significantly higher in aEstuaries than in pEstuaries, but the bacterial diversity was lower in aEstuaries. This might be attributed to the different DOM molecular composition in the two regions. Specifically, the aEstuaries had a higher proportion of lignin, tannin components, while the pEstuaries DOM was enriched in labile DOM components (CHOS and saturated compounds), which meant that aEstuaries’ DOM pool was relatively refractory and was not conducive to bacterial utilization. In contrast, the labile DOM pool in pEstuaries contributed to the diversity and complexity of bacterial communities (as confirmed by the network topology parameters), thereby accelerating the consumption of DOM. Notably, water physiochemical parameters could indirectly affect estuarine carbon cycling by influencing the bacterial-DOM interaction system. Structural equation modelling (SEM) analyses indicated that higher concentrations of dissolved inorganic nitrogen and dissolved oxygen could improve DOM bioavailability by affecting bacterial community structure and carbon metabolism gene abundance, which was unfavorable to the long-term accumulation of DOM in the estuary. In addition, DOM molecular stoichiometry showed high water temperature would weaken the DOM reactivity and promote DOM accumulation, which was also confirmed by the SEM pathways. This study reveals the interactions between DOM molecular composition and bacterial community in typical estuaries and provides a deeper understanding of microbial roles in marine biogeochemical cycling.

land-sea interface, bacterial community, ecological networks, SPE-DOM, PLS-SEM