Salinity-driven shifts in potential ammonia oxidation rates and microbial community dynamics in estuarine mangrove...

Estuarine wetlands are dynamic interfaces where salinity gradients critically influence sediment nitrogen cycling and ammonia oxidation is particularly sensitive to salinity. This study quantitatively analyzed the spatiotemporal variations in potential ammonia oxidation rate (PAR) and microbial communities under different salinity gradients (0-40) in mangrove wetland sediments from the Zhenhai Bay, located east of Guanghai Bay in eastern China. Twelve sampling sites, distributed from upstream to downstream along a natural salinity gradient, were included. The results showed that PAR was generally higher in summer than in winter, with the highest ammonia oxidation rates occurring in medium-salinity regions, while both low- and high-salinity regions exhibited inhibition on ammonia oxidation rates to varying degrees. Additionally, the results of salinity gradient experiments showed a unimodal pattern, where the optimal salinity was positively correlated with in situ salinity. In general, AOA dominated over AOB, whereas AOB were relatively more abundant in low-salinity regions. With the increase in salinity, Nitrosomonas and Nitrospira decreased in relative abundance, while Nitrosopumilus increased in abundance. The results highlighted the complex interplay between salinity variations, seasonal changes, and ammonia oxidation processes. The findings have important implications for wetland management and pollution remediation amid ongoing global changes in coastal ecosystems.