Neglected Dissimilatory Nitrate Reduction to Ammonium Pathway of Nitrate Reduction in the Chinese Marginal Sea and Its Microbial Community Assembly Driven by Stochastic Processes

Nitrate reduction as a key process of the marine biological nitrogen cycle has received significant attention, yet little is known about the transformation and fate of nitrate in marginal seas, which bear higher loads of nitrogen pollution. In this study, we took the Bohai and Yellow Sea (BYS) as an example and proposed a large-scale measurement of nitrogen transformations in marginal sea, with the aim of exploring the imprint of nitrate reduction and microbial transformation mechanisms in BYS. Our results showed that dissimilatory nitrate reduction to ammonium (DNRA) made a non-negligible contribution to nitrate reduction in BYS, particularly in the Bohai Sea, with an average DNRA rate up to 8.15 +/- 6.30 mu mol N/L/h. Pelobacter and Opitutus played important roles as "module hubs" and "connectors" in the DNRA intraspecific network. The moisture content, the concentration of total nitrogen, total sulfur, and NH4+-N of sediments, as well as the bottom water temperature and conductivity had a significant correlation with DNRA-dominant and keystone species. However, DNRA microbial community assembly was strongly driven by stochastic processes in the BYS. Specifically, homogeneous dispersal and ecological drift contributed 16.3% and 72.1% to DNRA community assembly in the BYS, respectively. To the best of our knowledge, this is the first report on the assembly mechanism of marine DNRA microbial communities. The patterns and drivers of DNRA processes detected in this study may induce novel insights into nitrate release, retention, and removal in marginal seas.