Biokinetic Characterization and Activities of N(2)O-Reducing Bacteria in Response to Various Oxygen Levels

Nitrous oxide (N(2)O)-reducing bacteria, which reduce N(2)O to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as N(2)O 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 N(2)O reduction, remain poorly understood. We used an O(2) and N(2)O micro-respirometric system to compare the N(2)O 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 N(2)O-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 N(2)O-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 O(2) inhibition coefficient than those of the other strains. The activation rates of N(2)O reductase, which describe the resilience of the N(2)O reduction activity after O(2) 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 N(2)O reduction and tolerance against O(2) inhibition. These physiological characteristics of Azospira sp. can be of promise for mitigation of N(2)O emission in industrial applications.