Coupled Salinity-Redox Gradients Shape Dissolved Organic Matter characteristics and Its Linkages with microbial Functional Potential in Intertidal Sediments.

Estuarine intertidal sediments occur in freshwater-seawater mixing zones where hydrological processes simultaneously alter salinity and redox conditions, potentially regulating dissolved organic matter (DOM) transformation and microbial processes. However, how DOM and its interactions with microbial communities respond to salinity and redox gradients in natural intertidal environments remains poorly understood. Here, surface sediments and porewater from the Yellow River Estuary were analyzed by integrating constructed composite salinity and redox indicators, spectroscopy-based two-dimensional correlation analysis, metagenomics, and multivariate statistics. Results showed that overall DOM variation was significantly associated with both gradients, with redox showing stronger links to chromophoric and protein-like components, molecular-weight properties, and redox-sensitive aromatic, phenolic, oxygen-containing structures. In contrast, neither gradient was significantly associated with whole-community taxonomic composition or functional potential, whereas carbon- and nitrogen-related taxa and functions showed environmental associations. Multiple analyses consistently indicated that DOM covaried with selected functional modules, with more reducing conditions linked to DOM accumulation and fermentation-related functions, and higher salinity linked to microbially derived DOM and substrate-utilization functions. Variation partitioning attributed 20.7% of DOM variation and 39.6% of the main joint DOM-microbe pattern to the shared salinity-redox fraction, while redox retained a significant unique association with DOM (20.3%). Exploratory path models further suggested that DOM characteristics may link salinity-redox gradients to selective rather than community-wide microbial functional variation. Together, these findings support considering salinity and redox as a coupled environmental background and highlight DOM chemistry as a potential interface between their variation and selective microbial functional responses in intertidal sediments.