NosZ-II-type NO-reducing bacteria play dominant roles in determining the release potential of NO from sediments in the Pearl River Estuary, China.

The microbial reduction of NO serves as a "gatekeeper" for NO emissions, determining the flux of NO release into the atmosphere. Estuaries are active regions for NO emissions, but the microbial functions of NO-reducing bacteria in estuarine ecosystems are not well understood. In this study, the N isotope tracer method, qPCR, and high-throughput sequencing were used to analyze NO production, reduction, and emission processes in surface sediments of the Pearl River Estuary. The N isotope tracer experiment showed that the NO production rates declined and the NO reduction potential (R, the ratio of NO reduction rates to NO production rates) increased from upstream to downstream of the Pearl River Estuary, leading to a corresponding decrease of the NO emission rates from upstream to downstream. The gene abundance ratio of nosZ/nir gradually increased from upstream to downstream and was negatively correlated with the water NO saturation. The gene abundance of nosZ II was significantly higher than that of nosZ I in the estuary, and the nosZ II/nosZ I abundance ratio was positively correlated with NO reduction potential. Furthermore, the community composition of NosZ-I- and NosZ-II-type NO-reducing bacteria shifted from upstream to downstream. NosZ-II-type NO-reducing bacteria, especially Myxococcales, Thiotrichales, and Gemmatimonadetes species, contributed to the high NO reduction potential in the downstream. Our results suggest that NosZ-II-type NO-reducing bacteria play a dominant role in determining the release potential of NO from sediments in the Pearl River Estuary. This study provides a new insight into the function of microbial NO reduction in estuarine ecosystems.