Shifts in the Community Dynamics and Activity of Ammonia-Oxidizing Prokaryotes Along the Yangtze Estuarine Salinity Gradient

Ammonia oxidation, the first and rate-limiting step in nitrification, plays a critical role in the nitrogen cycle. However, the links between the dynamics of ammonia-oxidizing communities and ecosystem processes along the estuarine salinity gradient remain uncertain. In this study, we examined the diversity, abundance, and community structure of ammonia-oxidizing prokaryotes, and the potential nitrification rates along the Yangtze estuarine salinity gradient. Phylogenetic analysis showed that the predominant ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) fell within the Nitrosospira and Nitrosopumilus clusters, respectively. The AOB amoA gene abundance (4.67 × 10⁵ to 3.90 × 10⁷ copies per gram of dry sediment) outnumbered AOA (5.14 × 10⁴ to 8.88 × 10⁶ copies per gram of dry sediment). The potential nitrification rates varied between 0.13 and 0.63 μg N·g⁻¹·day⁻¹ and related only to AOA amoA gene abundance. Salinity had significant effects on AOA amoA gene abundance, nitrification rates, and the community structure of ammonia-oxidizing prokaryotes. Principal coordinate analysis showed that the AOB amoA gene clones derived from the middle- and high-salinity regions behaved as a cohesive group, while all the low-salinity clone libraries were grouped together. Moreover, the distribution of AOA communities showed a distinct salinity differentiation. Overall, this study improves the understanding of the dynamic shifts in ammonia-oxidizing microorganisms in the Yangtze Estuary.