Clarifying Microbial Nitrous Oxide Reduction under Aerobic Conditions: Tolerant, Intolerant, and Sensitive

One of the major challenges for the bioremediation application of microbial nitrous oxide (N₂O) reduction is its oxygen sensitivity. While a few strains were reported capable of reducing N₂O under aerobic conditions, the N₂O reduction kinetics of phylogenetically diverse N₂O reducers are not well understood. Here, we analyzed and compared the kinetics of clade I and clade II N₂O-reducing bacteria in the presence or absence of oxygen (O₂) by using a whole-cell assay with N₂O and O₂ microsensors. Among the seven strains tested, N₂O reduction of Stutzerimonas stutzeri TR2 and ZoBell was not inhibited by oxygen (i.e., oxygen tolerant). Paracoccus denitrificans, Azospirillum brasilense, and Gemmatimonas aurantiaca reduced N₂O in the presence of O₂ but slower than in the absence of O₂ (i.e., oxygen sensitive). N₂O reduction of Pseudomonas aeruginosa and Dechloromonas aromatica did not occur when O₂ was present (i.e., oxygen intolerant). Amino acid sequences and predicted structures of NosZ were highly similar among these strains, whereas oxygen-tolerant N₂O reducers had higher oxygen consumption rates. The results suggest that the mechanism of O₂ tolerance is not directly related to NosZ structure but is rather related to the scavenging of O₂ in the cells and/or accessory proteins encoded by the nos cluster. IMPORTANCE Some bacteria can reduce N₂O in the presence of O₂, whereas others cannot. It is unclear whether this trait of aerobic N₂O reduction is related to the phylogeny and structure of N₂O reductase. The understanding of aerobic N₂O reduction is critical for guiding emission control, due to the common concurrence of N₂O and O₂ in natural and engineered systems. This study provided the N₂O reduction kinetics of various bacteria under aerobic and anaerobic conditions and classified the bacteria into oxygen-tolerant, -sensitive, and -intolerant N₂O reducers. Oxygen-tolerant N₂O reducers rapidly consumed O₂, which could help maintain the low O₂ concentration in the cells and keep their N₂O reductase active. These findings are important and useful when selecting N₂O reducers for bioremediation applications.