Isolation and Characterization of Ammonia-Oxidizing Bacterium N.eA1: Insights into Nitrogen Conversion and N₂O Emissions in Varied Environmental Conditions

While temperature, pH, DO, and ammonia nitrogen concentration are known to affect nitrous oxide (N₂O) emissions from ammonia-oxidizing bacteria (AOB), the specific responses of individual AOB species to these environmental variables have yet to be fully elucidated. The present study reports the isolation and pure culture of a new AOB strain, designated as N.eA1, from a stable CANON bioreactor. The strain’s denitrification and N₂O emission were systematically evaluated through a comprehensive analysis of growth kinetics, morphological characteristics, genetic composition, and nitrogen transformation under various environmental processes. Our results indicated that N.eA1 shares 95.33% sequence homology with Nitrosomonas europaea H1 AOB3, and exhibited higher nitrite (NO₂⁻-N) conversion efficiency. Morphological examination revealed white, semi-transparent spherical colonies. The bacterial growth kinetics included adaptation phase (0–12 h), exponential growth phase (12–36 h), stationary phase (36–72 h) and decline phase (after 72 h). Under optimal cultivation conditions (30 °C, DO concentration of 7.3 mg∙L⁻¹, pH 8.0, and NH₄⁺-N concentration of 260 mg∙L⁻¹), the culture achieved a maximum growth rate of 0.0723 h⁻¹, a maximum ammonia oxidation rate (AOR) of 10.74 mg∙(MLVSS∙h)⁻¹, and a minimum doubling time of 9.59 h. The peak time of nitrogen conversion was earlier than that of N₂O emission, with a maximum N₂O-N conversion from NH₄⁺-N of 1.039%.