Characteristics of microbial communities and nitrogen cycling in large and small estuaries

With the change of global climate and the intensification of human activities, the pollutant load in estuary is increasing. Large estuary and small estuary have different environmental characteristics and ecological functions. This study compared sediment physicochemical properties, microbial communities, and functional genes in a large estuary (Luanhe Estuary = 'LHE') and a small estuary (Shuanglong Estuary = 'SLE') to characterize how estuarine scale influences microbial ecosystems and nitrogen cycling. Results showed significant spatial heterogeneity in environmental factors (e.g., organic matter, total nitrogen, total phosphorus) in the LHE, which drove spatial differentiation in microbial communities and functional gene profiles. In contrast, the SLE exhibited lower environmental variability and homogenized microbial compositional patterns. Microbial community assembly in the SLE was dominated by stochastic processes, leading to higher species mobility and compositional uniformity. Network analysis revealed the LHE harbored more complex, modular microbial networks with greater cohesion and functional redundancy, enhancing resilience to environmental perturbations, while the SLE's simpler linear networks were more vulnerable to stressors. Total salinity (TS), organic matter (OM), total nitrogen (TN), and total phosphorus (TP) were key drivers of microbial composition and nitrogen cycling, but their regulatory mechanisms differed: salinity indirectly influenced nitrogen cycling in the LHE via nutrient modulation, whereas direct salinity effects on microbial composition dominated the SLE's simpler metabolic networks. These findings highlight the critical role of estuarine size and hydrological dynamics in shaping microbial diversity, network stability, and biogeochemical processes, providing insights into predicting microbial responses to environmental changes in estuarine ecosystems.