Aerobic Denitrifying Bacteria That Produce Low Levels of Nitrous Oxide

ABSTRACT Most denitrifiers produce nitrous oxide (N 2 O) instead of dinitrogen (N 2 ) under aerobic conditions. We isolated and characterized novel aerobic denitrifiers that produce low levels of N 2 O 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 (NO 3 − ) to N 2 at rates of 0.9 and 0.03 μmol min −1 unit of optical density at 540 nm −1 at dissolved oxygen (O 2 ) (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 N 2 but evolved more than 10-fold more N 2 O than strains TR2 and K50 evolved. The isolates denitrified NO 3 − with concomitant consumption of O 2 . These results indicated that strains TR2 and K50 are aerobic denitrifiers. These two isolates were taxonomically placed in the β 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 N 2 O emission, the single-stage process for nitrogen removal, and microbial N 2 O emission into the ecosystem.