Shifts in the high-resolution spatial distribution of dissolved NO and the underlying microbial communities and processes in the Pearl River Estuary.

Estuaries are significant sources of the ozone-depleting greenhouse gas NO. However, owing to large spatial heterogeneity and discrete measurements, NO emissions from estuaries are considerably uncertain. Microbial processes are disputed in terms of the dominant NO production under severe human disturbance. Herein, combining real-time and high-resolution measurements with bioinformatics analysis, we accurately mapped the consecutive two-dimensional NO distribution in the Pearl River Estuary (PRE), China, and revealed its underlying microbial mechanisms. Both the horizontal and vertical distributions of NO concentrations varied greatly at fine scales. Supersaturated NO concentrations (9.1 to 132.2 nmol/L) in the surface water decreased along the estuarine salinity gradient, with several emission hotspots scattering upstream. The vertical NO distribution showed marked differences from complete mixing upstream to incomplete mixing downstream, with constant or changeable concentrations with increasing depth. Furthermore, spatially varied denitrifying and nitrifying microorganisms controlled the NO production and distribution in the PRE, with denitrification playing the dominant role. The nirK-type and nirS-type denitrifying bacteria were the primary producers of NO in the water and sediment columns, respectively. In addition, substrate concentration (NO and DOC) regulated NO production by affecting key microbial processes, while physical influences (water-mass mixing and salt wedges) reshaped NO distribution. With these information, a conceptual model of estuarine NO production and distribution was constructed to generalize the possible biochemical processes under environmental constraints, which could provide insights into the NO biogeochemical cycle and emission mitigation from a mechanistic perspective.