Ammonium removal through anaerobic ammonium oxidation coupled to iron(III) reduction along the Yangtze river–estuary continuum

Anaerobic ammonium oxidation coupled to iron(III) reduction (Feammox) process has recently been recognized as an important pathway for removing ammonium in various natural habitats. However, our understanding for Feammox in river–estuary continuum is limited. In this study, stable isotope tracers and high-throughput amplicon sequencing were employed to determine Feammox rates and identify associated microbial communities in sediments along the Yangtze river–estuary continuum. Feammox rates averaged 0.0058 ± 0.0069 mg N/(kg·d) and accounted for approximately 22.3% of the ammonium removed from the sediments. Sediment Fe(III), ammonium (NH₄⁺), total organic carbon (TOC), and pH were identified as important factors influencing Feammox rates. Additionally, Spirochaeta, Caldilineaceae_uncultured, and Ignavibacterium were found potentially associated with Feammox, which had not been documented as Feammox-associated microbial taxa previously. This study demonstrates that Feammox plays a vital role in ammonium removal within the Yangtze river–estuary continuum, providing greater insight into nitrogen removal and cycling in aquatic ecosystems.