The Evolution of Sediment Microorganisms During the Transition from Freshwater to Seawater and Their Dependence on Water Quality

Estuarine ecosystems, characterized by dynamic salinity gradients and complex physicochemical interactions, serve as critical transition zones between freshwater and marine environments. This study investigates the spatial evolution of sediment microbial communities across a freshwater–seawater continuum and their correlations with water quality parameters. Five sampling zones (upstream, midstream, downstream, transition zone, and ocean) were established in a typical estuary (Kuiyu Park, China). High-throughput 16S rRNA sequencing revealed significant shifts in microbial composition, with dominant phyla including Firmicutes, Bacteroidetes, Proteobacteria, and Actinobacteria. Alpha diversity decreased from freshwater to the transition zone but rebounded in seawater, suggesting habitat filtering and niche differentiation. Redundancy analysis identified salinity, dissolved oxygen, nutrients, and heavy metals as key drivers of microbial community structure. Functional predictions highlighted metabolic adaptations such as methanogenesis, sulfur oxidation, and aerobic chemoheterotrophy across zones. This study explores how sediment microorganisms adapt to water quality variations during the freshwater–seawater transition, offering insights into estuarine resilience under global change. These findings elucidate microbial assembly rules in estuarine ecosystems and provide insights for ecological management under global environmental change.