Unveiling Fe reduction-induced synergy between anaerobic ammonium oxidation and Fe-Reducing/Oxidizing bacteria for high efficiency nitrogen removal.

Anammox (anaerobic ammonium oxidation) provides an energy-efficient approach to wastewater treatment but is limited by unstable nitrite supply and nitrate accumulation. This study developed an innovative iron-nitrogen (Fe-N) coupled system that achieved 97 % total nitrogen removal using a low NO2--N/NH4+-N molar ratio of 0.75 and 3.69 mM Fe2O3 addition. Isotopic tracing and molecular analyses revealed a synergistic nitrogen removal network: Anammox contributed 67 % of NH4+-N oxidation, while Feammox (Fe(III) reduction coupled with anaerobic ammonium oxidation) played a complementary role under NO2--N-limiting conditions. The Fe(II) generated through Feammox facilitated NO3--N reduction to NO2--N via nitrate-dependent Fe(II) oxidation, effectively recycling substrates and maintaining iron cycling. Transcriptomic profiling further confirmed Feammox activity in Candidatus Brocadia and Candidatus Kuenenia, highlighting enhanced collaboration between anammox and iron-cycling bacteria. This iron-mediated microbial synergy offers a sustainable and efficient strategy for advanced wastewater treatment.