Biokinetic Characterization and Activities of NO-Reducing Bacteria in Response to Various Oxygen Levels

Nitrous oxide (NO)-reducing bacteria, which reduce NO to nitrogen in the absence of oxygen, are phylogenetically spread throughout various taxa and have a potential role as NO 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 NO reduction, remain poorly understood. We used an O and NO micro-respirometric system to compare the NO reduction kinetics of four strains, i.e., two strains of an sp., harboring clade II type , and and , harboring clade I type , in the presence and absence of oxygen. In the absence of oxygen, the highest NO-reducing activity, , was 5.80 ± 1.78 × 10 pmol/h/cell of sp. I13, and the highest and lowest half-saturation constants were 34.8 ± 10.2 μM for and 0.866 ± 0.29 μM for sp. I09. Only sp. I09 showed NO-reducing activity under microaerophilic conditions at oxygen concentrations below 110 μM, although the activity was low (10% of ). This trait is represented by the higher O inhibition coefficient than those of the other strains. The activation rates of NO reductase, which describe the resilience of the NO reduction activity after O exposure, differ for the two strains of sp. (0.319 ± 0.028 h for strain I09 and 0.397 ± 0.064 h for strain I13) and (0.200 ± 0.013 h), suggesting that sp. has a potential for rapid recovery of NO reduction and tolerance against O inhibition. These physiological characteristics of sp. can be of promise for mitigation of NO emission in industrial applications.