The effect of oxygen on denitrification in Paracoccus denitrificans and Pseudomonas aeruginosa

Denitrification by Paracoccus denitrificans and Pseudomonas aeruginosa was studied using quadrupole membrane-inlet mass spectrometry to measure simultaneously and continuously dissolved gases. Evidence was provided for aerobic denitrification by both species: in the presence of O2, N2O production increased in Pa. denitrificans, while that of N2 decreased; with Ps. aeruginosa, the concentrations of both N2 and N2O increased on introducing O2 into the gas phase. Disappearance of NO-3 was monitored in anaerobically and aerobically grown cells which were maintained either anaerobically or aerobically: the rate and extent of NO-3 utilization by both species depended on growth and maintenance conditions. The initial rate of disappearance was most rapid under completely anaerobic conditions, and lowest rates occurred when cells were grown anaerobically and maintained aerobically. In nitrogen balance experiments both species converted over 87% of the added NO-3 to N2 and N2O under both anaerobic and aerobic maintenance conditions.