Transition from Anammox to Feammox metabolic modes: Regulation strategies for nitrite in Anammox enrichment cultures.

This studyachievedthe metabolic transition from anaerobic ammonium oxidation (Anammox) to Fe(III)-mediated ammonium oxidation (Feammox)usingiron-carbon micro-electrolytic spheres as a slow-release iron sourcethrougha stepwise reductionin influent NO2--N concentration. The results demonstrated that sustained Feammox activitywas governed bynitrate-dependent ferrous oxidation (NDFO) metabolismcombined withoxygen-regulated Fe(III) regeneration,resulting ina peak total nitrogen removal efficiency of 91.6 %at40 mg/L NO2--N.Whileexclusive NH4+-N feeding inhibited Feammox activity,this suppression was reversible upon NO2--N supplementation. Intriguingly, Anammox activityremained robustdespite decreasing NO2--N levelsand showedsignificant positive correlation with Feammox activity,suggesting shared metabolic modules. Metagenomic profilingfurther identifiedCa. Brocadia as the core functional genus driving NH4+-N oxidation,highlighting its niche adaptationin iron-mediated systems.These mechanistic insights establish a framework fordesigning energy-efficient nitrogen removal processesleveraging iron-redox cycling.