The Xiamen Symposium on Marine Environmental Sciences (XMAS 2025)

Nitrogen cycle is an important biogeochemical cycle in the ocean, controlling the productivity of the oceans. Particle-attached (PA) and free-living (FL) microorganisms play unique ecological roles in regulating biogeochemical cycles in estuaries. The seawater in the Yangtze River Estuary (YRE) is characterized with high turbidity. The large number of particles provide microorganisms niches with diverse environmental features, potentially influencing nitrogen cycling. However, the potentials of PA and FL in nitrogen cycle in this region and the mechanisms leading to the distinction have not been well understood. To fill this knowledge gap, we collected 142 water samples along the salinity gradient of the YRE in four seasons from the years 2019 to 2020. We investigated the spatio-temporal variations of microbial communities and gene abundances in PA and FL based on 16S rRNA genes and metagenomic sequences, and conducted a Weighted Gene Co-Expression Network Analysis (WGCNA) for exploring the association among distributions of key genes involved in nitrogen cycling, microbial assemblies of PA and FL, and environmental variables. We identified the key taxa affiliated with genes for revealing the role of marine microorganisms on nitrogen cycling. The predominant classes Gammaproteobacteria and Alphaproteobacteria was lower in PA compared to FL, Cyanophyceae and Planctomycetia showed opposite trend. Through WGCNA analysis, the distribution of PA and FL was primarily influenced by salinity and temperature, respectively. Salinity and nitrate were identified affecting the spatial distribution of nitrogen-cycling genes, while temperature and chlorophyll-a influenced the temporal distribution. Denitrification was revealed as the most predominant pathway in the YRE. Nitrification in PA and FL had high potential in marine waters, while other processes (e.g., DNRA) had high potential in freshwater and estuarine areas. In PA, the potential of nitrogen cycle typically peaked in July or October, whereas in FL, it was lowest in July, but highest in May. WGCNA analysis indicated gene distribution was strongly associated with microbial composition. Nitrososphaeria and Nitrospiria were the key encoding classes in nitrification process. Betaproteobacteria and Gammaproteobacteria were associated with denitrification and assimilatory nitrate reduction. DNRA was affiliated with unclassified Acidobacteriota class and Gammaproteobacteria, while nitrogen fixation was mostly related to Cyanophyceae. This study revealed distributions, compositions, abiotic influences and interactions of nitrogen-cycling genes and microbial communities in different niches of the YRE and adjacent regions, enhancing the understanding of nitrogen cycle processes and their regulation potentials in the estuary.

nitrogen cycle, particle-attached microorganisms, free-living microorganisms, Yangtze River Estuary, metagenome, spatio-temporal distribution