Biokinetic Characterization and Activities of N₂O-Reducing Bacteria in Response to Various Oxygen Levels

Nitrous oxide (N₂O)-reducing bacteria, which reduce N₂O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N₂O sinks in the environment. However, research on their physiological traits has been limited. In particular, their activities under microaerophilic and aerobic conditions, which severely inhibit N₂O reduction, remain poorly understood. We used an O₂ and N₂O micro-respirometric system to compare the N₂O reduction kinetics of four strains, i.e., two strains of an Azospira sp., harboring clade II type nosZ, and Pseudomonas stutzeri and Paracoccus denitrificans, harboring clade I type nosZ, in the presence and absence of oxygen. In the absence of oxygen, the highest N₂O-reducing activity, V_m,N2O, was 5.80 ± 1.78 × 10^-3 pmol/h/cell of Azospira sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 μM for Pa. denitirificans and 0.866 ± 0.29 μM for Azospira sp. I09. Only Azospira sp. I09 showed N₂O-reducing activity under microaerophilic conditions at oxygen concentrations below 110 μM, although the activity was low (10% of V_m,N2O). This trait is represented by the higher O₂ inhibition coefficient than those of the other strains. The activation rates of N₂O reductase, which describe the resilience of the N₂O reduction activity after O₂ exposure, differ for the two strains of Azospira sp. (0.319 ± 0.028 h^-1 for strain I09 and 0.397 ± 0.064 h^-1 for strain I13) and Ps. stutzeri (0.200 ± 0.013 h^-1), suggesting that Azospira sp. has a potential for rapid recovery of N₂O reduction and tolerance against O₂ inhibition. These physiological characteristics of Azospira sp. can be of promise for mitigation of N₂O emission in industrial applications.