Elucidating the Feammox nitrogen transformation pathway: Key intermediates and putative multi-species metabolic...

The emerging Fe(Ⅲ) reduction coupled to anaerobic ammonia oxidation (Feammox) process offers a promising approach toward carbon neutrality in wastewater treatment. However, its nitrogen transformation pathway and metabolic mechanism remain unclear. This study established a Feammox-dominant sequencing batch reactor (Fe-SBR) and operated it for 515 days, achieving an ammonia removal efficiency of 97.9 ± 4.5% during the stable phase. Feammox was confirmed as the dominant process for NH₄⁺-N conversion, accounting for 83.2% of ammonia transformation. NH₂OH, NO, and N₂O were identified as key intermediates in the Feammox nitrogen transformation pathway. By integrating metagenomic analysis of functional gene dynamics with metagenome-assembled genomes (MAGs), a potential coupled iron-nitrogen (Fe-N) metabolic pathway was proposed. This pathway suggested that the Feammox process might be accomplished through multi-species metabolic cooperation, with MtrC-mediated extracellular electron transfer potentially serving as the key link coupling nitrogen transformation to the iron redox cycle. These findings provide novel insights into the Feammox metabolic pathway and lay a theoretical foundation for the future precise control and optimization of this process.