Source-specific organic matter modulates Anammox via humic-like DOM in mangrove sediments: Implications for...

Anaerobic ammonium oxidation (Anammox) is a pivotal nitrogen loss pathway in coastal ecosystems, but the regulatory roles of organic matter (OM) sources and composition in mangrove sediments remain poorly understood. Here, we combined δ¹³C and C/N ratios, excitation-emission matrix (EEM) fluorescence, the ¹⁵N-tracer technique, 16S rRNA gene sequencing, hzsB gene quantification, and multivariate statistical analyses to investigate the Anammox process in Dongzhai Harbor mangrove wetland (China). Sediments were classified into three OM sources: mangrove/terrestrial, mixed, and marine/local. Mangrove-derived OM exhibited the highest potential Anammox rates (42.64-70.38 nmol N g^-1 h^-1) and hzsB gene abundance (7.80 × 10⁶-5.01 × 10⁷ copies g^-1), dominated by Candidatus Anammoximicrobium, while mixed and marine/local OM sediments showed lower activity. Anammox was primarily promoted by total organic carbon (TOC), total organic nitrogen (TON), and water content (WC) (which sustain anoxic microsites) and inhibited by salinity and NO₂^--N. Crucially, dissolved organic matter (DOM) molecular composition emerged as a key regulator: humic-like components (with high aromaticity and humification) correlated more strongly with Anammox rates (r = 0.79, p < 0.001) than protein-like DOM (r = 0.68, p < 0.01), implying that humic substances may act as electron shuttles. Anammox contributed 22.1 to 39.7% to total nitrogen loss, highlighting its substantial contribution to mangrove nitrogen cycling. Our findings highlight the primacy of OM sources and quality (rather than quantity) in regulating Anammox. Specifically, mangrove-derived humic-rich OM enhances Anammox via substrate supply and microenvironmental optimization. This study provides novel mechanistic insights into nitrogen cycling and highlights mangrove conservation and restoration as effective nature-based solutions for coastal eutrophication mitigation.