Nitrogen Transformation Survival Strategies of Ammonia-Oxidizing Bacterium N.eA1 Under High Nitrite Stress

Ammonia-oxidizing bacteria (AOB) are key to the nitrogen cycle, but their resistance to nitrite (NO2−-N) accumulation is unclear. This study examined N.eA1, an AOB from the completely autotrophic nitrogen removal over nitrite (CANON) process, assessing its adaptive responses to NO2−-N. The ammonia oxidation and N2O emission were evaluated at varying NO2−-N levels, and 3D fluorescence, extracellular polymeric substances (EPS), and soluble microbial products (SMP) analysis were used to probe stress responses. Cellular respiration and key enzyme activities were measured, and proteomics was applied to study protein expression changes. Results showed that higher NO2−-N levels boosted N2O production, inhibited nitrification, and stimulated denitrification in N.eA1. At 100 mg·L−1 NO2−-N, EPS rose and SMP fell, with ammonia monooxygenase (AMO) suppressed and nitrite reductase (NIR) as well as nitric oxide reductase (NOR) enhanced. Gene expression analysis revealed decreased AMO, hydroxylamine oxidoreductase (HAO), and energy transport-related enzymes, but increased NIR and NOR genes. The downregulation of electron transport complex genes offered insights into molecular adaptation to nitrite stress of N.eA1, highlighting the interplay between metabolic and genetic responses, which is essential for developing sustainable and efficient nitrogen management strategies.