FeO might be more suitable than Fe²⁺ for the construction of anammox-dominated Fe-N coupling system: Based on ¹⁵N...

Iron not only influences the activity of anammox bacteria (AnAOB) but also participates in complex Fe-N cycles. In this study, the advanced ¹⁵N isotope tracing method was set up to quantify the potential rates of full nitrogen metabolic pathways under different ferrous iron (Fe²⁺ and FeO) within two identical anammox granular reactors. The results indicated that both Fe²⁺ and FeO enhanced AnAOB activity. However, compared to Fe²⁺, which readily precipitates and oxidizes, the system supplemented with FeO exhibited higher Fe-N metabolic activity and greater metabolic diversity. This is attributed to the gradual release of Fe²⁺ from FeO, providing a sustainable and stable supply of Fe²⁺ for microorganisms. Furthermore, Subgroup_10 and Paludibaculum were identified as potential functional bacteria for feammox, while Denitratisoma, I-8 and Arenimonas were for NDFO. These results suggest that FeO addition is more beneficial for the construction of a Fe-N coupling system. Overall, this study enhances our understanding of how with exogenous iron can strengthen the anammox system, laying a theoretical foundation for the development of anammox-dominant Fe-N coupling systems.