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Aerobic Denitrifying Bacteria That Produce Low Levels of Nitrous Oxide

Institute of Applied Biochemistry, University of Tsukuba, Tsukuba, Ibaraki 305-8572,¹ Department of Biotechnology, Graduate School of Agricultural and Life Sciences, University of Tokyo, Bunkyo-Ku, Tokyo 113-8657, Japan²

Received 25 November 2002/ Accepted 17 March 2003

Most denitrifiers produce nitrous oxide (N₂O) instead of dinitrogen (N₂) under aerobic conditions. We isolated and characterized novel aerobic denitrifiers that produce low levels of N₂O under aerobic conditions. We monitored the denitrification activities of two of the isolates, strains TR2 and K50, in batch and continuous cultures. Both strains reduced nitrate (NO₃^-) to N₂ at rates of 0.9 and 0.03 µmol min^-1 unit of optical density at 540 nm^-1 at dissolved oxygen (O₂) (DO) concentrations of 39 and 38 µmol liter^-1, respectively. At the same DO level, the typical denitrifier Pseudomonas stutzeri and the previously described aerobic denitrifier Paracoccus denitrificans did not produce N₂ but evolved more than 10-fold more N₂O than strains TR2 and K50 evolved. The isolates denitrified NO₃^- with concomitant consumption of O₂. These results indicated that strains TR2 and K50 are aerobic denitrifiers. These two isolates were taxonomically placed in the ß subclass of the class Proteobacteria and were identified as P. stutzeri TR2 and Pseudomonas sp. strain K50. These strains should be useful for future investigations of the mechanisms of denitrifying bacteria that regulate N₂O emission, the single-stage process for nitrogen removal, and microbial N₂O emission into the ecosystem.