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

Nitrite (NO<sub>2</sub><sup>–</sup>) substrate under certain conditions can cause failure of N-removal processes relying on anaerobic ammonium oxidizing (anammox) bacteria. Detoxification of NO<sub>2</sub><sup>–</sup> can potentially be achieved by using exogenous nitrate (NO<sub>3</sub><sup>–</sup>). In this work, continuous experiments in bioreactors with anammox bacteria closely related to “<i>Candidatus Brocadia caroliniensis</i>” were conducted to evaluate the effectiveness of short NO<sub>3</sub><sup>–</sup> additions to reverse NO<sub>2</sub><sup>–</sup> toxicity. The results show that a timely NO<sub>3</sub><sup>–</sup> addition immediately after a NO<sub>2</sub><sup>–</sup> stress event completely reversed the NO<sub>2</sub><sup>–</sup> inhibition. This reversal occurs without NO<sub>3</sub><sup>–</sup> being metabolized as evidence by lack of any <sup>30</sup>N<sub>2</sub> formation from <sup>15</sup>N-NO<sub>3</sub><sup>–</sup>. The maximum recovery rate was observed with 5 mM NO<sub>3</sub><sup>–</sup> added for 3 days; however, slower but significant recovery was also observed with 5 mM NO<sub>3</sub><sup>–</sup> for 1 day or 2 mM NO<sub>3</sub><sup>–</sup> for 3 days. Without NO<sub>3</sub><sup>–</sup> addition, long-term NO<sub>2</sub><sup>–</sup> inhibition of anammox biomass resulted in irreversible damage of the cells. These results suggest that a short duration dose of NO<sub>3</sub><sup>–</sup> to an anammox bioreactor can rapidly restore the activity of NO<sub>2</sub><sup>–</sup>-stressed anammox cells. On the basis of the results, a hypothesis about the detoxification mechanism related to <i>narK</i> genes in anammox bacteria is proposed and discussed.