Abstract

In nitrite-oxidizing bacteria (NOB)-enriched biomass, formate (3 mM) promoted partial denitrification (PD) by NOB- Nitrospira , resulting in a nitrite accumulation rate of 1.02 ± 0.17 mg-N/(g-SS·h). The relationship between nitrite accumulation rate and formate concentration followed Michaelis-Menten kinetics (R 2  = 0.95), with a half-saturation constant of 3.18 mM. The Nitrospira in NOB biomass harbored fdh gene (encoding formate dehydrogenase) and nitrate reduction genes narG and napA , which were upregulated by 137.0%, 5.5%, and 127.0% with formate addition, respectively. This revealed a synergy between formate oxidation (electron donation) via fdh and nitrate reduction, thereby enabling PD. At 3 mM formate, the addition of NOB into anaerobic ammonium oxidizing bacteria (AnAOB) biomass increased the nitrogen removal rate from 1.74 to 3.56 mg-N/(g-SS·h), demonstrating effective coupling between NOB-driven PD and anammox. This study reveals that NOB Nitrospira can shift from a competitor to a collaborator with AnAOB, offering a potential new pathway for nitrogen removal.