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 mol min ؊1 unit of optical density at 540 nm ؊1 at dissolved oxygen (O 2 ) (DO) concentrations of 39 and 38 mol 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 ␤ 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.Nitrous oxide (N 2 O) is a gaseous nitrogen oxide that is present at a concentration of about 350 ppb in the atmosphere. The concentration of this compound was maintained below 300 ppb in the global nitrogen cycle before the 20th century. However, recent reports suggest that the atmospheric concentration of N 2 O is now increasing at a rate as high as 0.3% per year (1). N 2 O has a 200-to 300-fold-stronger greenhouse effect than carbon dioxide (CO 2 ) and has the potential to destroy the ozone layer (17). Therefore, the N 2 O balance is critical to the natural environment. The proposed sources of N 2 O are chemical industries, burning fossil fuels, and biomass, as well as soil denitrification of nitrogenous compounds resulting from excess agricultural fertilizer (3,6,25). Another critical source of N 2 O is wastewater treatment plants, in which considerable amounts of nitrogen pollutants removed from treated water are released into the atmosphere as N 2 O, as well as dinitrogen (N 2 ).Currently, nitrogen removal in wastewater treatment plants is essentially based on the activity of nitrifying and denitrifying microorganisms, both of which are inhabitants of activated sludge. Nitrifying bacteria aerobically oxidize ammonium contaminants to nitrite (NO 2 Ϫ ) and nitrate (NO 3 Ϫ ), which are then reduced by denitrifying bacteria to gaseous nitrogen forms such as N 2 O and N 2 . Efficient wastewater treatment relies on successively exposing water to aerobic and anaerobic conditions, since nitrification and denitrification are aerobic and anaerobic processes, respectively (4, 18). These properties represent a shortcoming of current systems since both denitrification and nitrification produce N 2 O as a by-product in the absen...

No abstract