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.