Nitrate Reverses Severe Nitrite Inhibition of Anaerobic Ammonium Oxidation (Anammox) Activity in Continuously-Fed Bioreactors

Nitrite (NO₂^–) substrate under certain conditions can cause failure of N-removal processes relying on anaerobic ammonium oxidizing (anammox) bacteria. Detoxification of NO₂^– can potentially be achieved by using exogenous nitrate (NO₃^–). In this work, continuous experiments in bioreactors with anammox bacteria closely related to “Candidatus Brocadia caroliniensis” were conducted to evaluate the effectiveness of short NO₃^– additions to reverse NO₂^– toxicity. The results show that a timely NO₃^– addition immediately after a NO₂^– stress event completely reversed the NO₂^– inhibition. This reversal occurs without NO₃^– being metabolized as evidence by lack of any ³⁰N₂ formation from ¹⁵N-NO₃^–. The maximum recovery rate was observed with 5 mM NO₃^– added for 3 days; however, slower but significant recovery was also observed with 5 mM NO₃^– for 1 day or 2 mM NO₃^– for 3 days. Without NO₃^– addition, long-term NO₂^– inhibition of anammox biomass resulted in irreversible damage of the cells. These results suggest that a short duration dose of NO₃^– to an anammox bioreactor can rapidly restore the activity of NO₂^–-stressed anammox cells. On the basis of the results, a hypothesis about the detoxification mechanism related to narK genes in anammox bacteria is proposed and discussed.