Shifts in the Abundance and Community Composition of Particle-Associated and Free-Living Nitrospira Across Physicochemical Gradients in the Pearl River Estuary

Nitrospira, a genus of nitrite-oxidizing bacteria, plays an important role in the global nitrogen cycle. However, the distribution patterns of Nitrospira communities and their responses to environmental variables in the estuary are relatively unknown. In this study, Nitrospira communities were collected at two size fractions, 0.2–2.0 μm (free-living) and >2.0 μm (particle-associated), across a physicochemical gradient in the Pearl River Estuary (PRE). The abundance and community composition of Nitrospira bacteria within these two fractions were analysed using qPCR and high-throughput sequencing targeting the nxrB gene. The abundance of particle-associated bacteria decreased dramatically from upstream to downstream, but no significant changes were observed in free-living bacteria, leading to a drastic decrease in the ratio of particle-associated to free-living bacteria (from 36.56 to 0.81). Nitrospira community composition in the PRE also exhibited remarkable spatial variation but no distinct differences in the community composition between particle-associated and free-living fractions. Seven Nitrospira species were detected in water samples from the PRE, of which Nitrospira lenta (N. lenta) and Candidatus Nitrospira defluvii (Ca. N. defluvii) were found to be dominant, and Ca. N. defluvii was observed for the first time in the estuarine environment. A marine Nitrospira, Nitrospira marina (N. marina), was more abundant in winter than in summer, possibly due to strong seawater intrusion in winter. Multivariate statistics revealed that ammonia rather than nitrite was the key factor controlling the community structure of Nitrospira, though different species responded differently to environmental variables. Community network analysis revealed tighter interactions in particle-associated Nitrospira communities than in free-living communities, suggesting that particle association favored interactions. Our analyses indicate that particles play a dominant role in shaping the structure of Nitrospira bacterial communities and imply their major function of the nitrite oxidation in the waters of the PRE.