Metabolic division of labour drives estuarine-coastal N2O emissions

Estuarine and coastal systems are global hotspots of marine nitrous oxide (N2O) emissions, where microbial nitrification and denitrification are the primary processes regulating N2O dynamics. However, how interactions among different N2O-associated microorganisms influence ecosystem-scale N2O emissions remains poorly understood. This study combined in situ N2O concentrations, 15N-based potential rates, metagenomics, metatranscriptomics, and genome-scale metabolic model analysis to explore N2O production and reduction processes in estuarine and coastal systems. Potential N2O production and reduction rates, together with in situ concentrations, the relative abundance, and the transcriptional activity of associated genes, were significantly higher at low salinity and declined toward coastal regions. Based on the gene content of 974 recovered N2O-associated genomes, microorganisms were classified into three functional groups: net N2O producers, net N2O consumers, and self-sustaining N2O players. The abundance, composition, and activity of these functional groups shifted along estuarine-coastal gradients. A larger NO/N2O exchange gap, reflecting the imbalance between model-inferred NO and N2O handoff potentials, was found at low salinity and was associated with elevated bottom-water N2O concentrations. Together, community-level division of labour and the associated exchange gap provide a conceptual framework for linking N2O-related functional groups to N2O accumulation in estuarine-coastal ecosystems.