Nitrous oxide inhibition of nitrite reduction to ammonium following oxic-to-anoxic transition

Abstract

Dissimilatory nitrate/nitrite reduction to ammonium (DNRA) has recently attracted attention as a nitrogen retention pathway that can mitigate nitrogen loss due to denitrification. DNRA bacteria with nosZ, such as $\textit{Bacillus vireti}$ and $\textit{Shwanella loihica}$, have reduced N2O and NH4+ production capabilities, which are at the core of the "nitrogen dilemma", but research on these bacteria is limited and the reason for the existence of nosZ in DNRA bacteria remains unknown. In this research, we mainly focus on the physiology of $\textit{Bacillus sp.}$, the DNRA bacteria harboring nosZ. NO2-, NO3-, NH4+, and N2O were monitored under four partially oxic-conditions: 1)100% N2, 2)1000ppm N2O, 3)10% C2H2 and 4)1000ppm N2O and 10% C2H2. About 7–9% of N2O was released from acetylene-groups, and N2O was not detected at 100% N2 group. 1000 ppm of N2O was consumed within 10 hours of complete oxygen depletion. About 0.2 to 0.25 mM of nitrite was reduced to ammonium in the acetylene-group compared to the non-acetylene group that all nitrite was reduced to ammonium within 70hour. Both N2O and C2H2 were possible inhibitors in culture experiments, but C2H2 was excluded from the possible inhibitors through the results of the fed-batch reactor. However, slow nitrite reduction was observed when cultures were exposed to oxygen. As a result of sequential oxygen exposure, both N2O and oxygen can deactivate the DNRA pathway. We proposed that NrfA can be compromised by oxygen and we proposed three possible assumptions for NrfA activation based on the results,