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 (N(2)O) reduction is its oxygen sensitivity. While a few strains were reported capable of reducing N(2)O under aerobic conditions, the N(2)O reduction kinetics of phylogenetically diverse N(2)O reducers are not well understood. Here, we analyzed and compared the kinetics of clade I and clade II N(2)O-reducing bacteria in the presence or absence of oxygen (O(2)) by using a whole-cell assay with N(2)O and O(2) microsensors. Among the seven strains tested, N(2)O reduction of Stutzerimonas stutzeri TR2 and ZoBell was not inhibited by oxygen (i.e., oxygen tolerant). Paracoccus denitrificans, Azospirillum brasilense, and Gemmatimonas aurantiaca reduced N(2)O in the presence of O(2) but slower than in the absence of O(2) (i.e., oxygen sensitive). N(2)O reduction of Pseudomonas aeruginosa and Dechloromonas aromatica did not occur when O(2) was present (i.e., oxygen intolerant). Amino acid sequences and predicted structures of NosZ were highly similar among these strains, whereas oxygen-tolerant N(2)O reducers had higher oxygen consumption rates. The results suggest that the mechanism of O(2) tolerance is not directly related to NosZ structure but is rather related to the scavenging of O(2) in the cells and/or accessory proteins encoded by the nos cluster. IMPORTANCE Some bacteria can reduce N(2)O in the presence of O(2), whereas others cannot. It is unclear whether this trait of aerobic N(2)O reduction is related to the phylogeny and structure of N(2)O reductase. The understanding of aerobic N(2)O reduction is critical for guiding emission control, due to the common concurrence of N(2)O and O(2) in natural and engineered systems. This study provided the N(2)O reduction kinetics of various bacteria under aerobic and anaerobic conditions and classified the bacteria into oxygen-tolerant, -sensitive, and -intolerant N(2)O reducers. Oxygen-tolerant N(2)O reducers rapidly consumed O(2), which could help maintain the low O(2) concentration in the cells and keep their N(2)O reductase active. These findings are important and useful when selecting N(2)O reducers for bioremediation applications.