Anaerobic ammonium oxidation coupled to iron(Ⅲ) reduction (termed Feammox) with either dinitrogen,nitrite or nitrate as end-product,is a newly discovered nitrogen (N) cycling process.However,previous studies related to Feammox generally lacked information about the microorganisms responsible for thisprocess,thus hinder the elucidation of the mechanism underpinning the process.In this study,we obtained an enrichment capable of Feammox from an anammox inoculum amended with ferrihydrite.15N-labelled isotopic tracing technique in combination with acetylene inhibition methods were employed to reveal the direct evidence for Feammox and to determine the relative contribution of the three potential Feammox pathways.Theenrichment exhibits robust ammonium oxidizing ability coupled to iron reduction under anaerobic conditions,in which direct dinitrogen production was demonstrated to be the dominant Feammox pathway,contributing maximumly 67% of total N2 generation.It was estimated that 26.9-53.9% of Fe(Ⅲ) reduction was linked to Feammox in our enrichment.Illuminahigh-throughputsequencing of the 16S rRNA suggested that members of Geobacter and Pseudomonasmight be the key microbial players involved in Feammox process.Overall,these findings suggest that Feammox could be a potentially promising pathway for ammonium removal in anaerobic iron rich environments.