Tidal action alters nitrogen removal processes through affecting microbiome in Liaohe estuarine sediments

Tidal action shapes the highly dynamic environment of the estuary, profoundly impacting the transformation and fate of nitrogen in estuarine sediments. However, how tidal action regulates the rates and pathways of microbially mediated nitrogen removal from estuarine sediments remain poorly understood. Here, the impacts of macro-tidal on denitrification, anaerobic ammonia oxidation and nitrification coupled nitrogen removal processes in sediments of the Liaohe Estuary were investigated through continuous-flow experiments combined with 15N tracing techniques and molecular methods. Results showed that denitrification was the main nitrogen removal processes in estuarine sediments, and its proportion increased to 94% during the macro-tidal, while the community composition of denitrifying bacteria also changed significantly. As the macro-tidal elevated the salinity level in the estuary, it significantly increased the abundance of AOB-type nitrifying bacteria and Nitrospina, and contributed to the emergence of nitrification coupled nitrogen removal processes. Furthermore, the process of nitrite production appears to be significantly enhanced during the mega-tide, however, the dilution of nitrite by seawater brought by the mega-tide is more severe, contributing to the reduction of nitrite content in offshore areas and the suppression of sediment denitrification rate. Overall, this work highlights that macro-tidal regulate the form and rate of nitrogen removal processes mainly by altering environmental physicochemical factors, functional gene abundance and microbial community characteristics in estuarine. Our results give a comprehensive view of how tides regulate nitrogen removal processes in estuarine sediments.