Kinetics of denitrification by Pseudomonas fluorescens: Oxygen effects

To determine the inhibitory effect of O 2 on denitrification in pure cultures of Pseudomonas fluorescens, measurements of net NO and N 2O production rates, f NO and f N 2O , under anaerobic conditions were compared to rates under partially aerobic conditions. Nitrate or NO 2 − solutions were injected into buffered suspensions and gaseous NO and N 2O were sparged by constant flows of N 2 carrier gas, or mixtures of N 2 with measured small quantities of O 2 ( ca 6–60 Pa). Apparent Michaelis-Menten parameters were calculated from saturation kinetic plots obtained by monitoring net NO or N 2O production following periodic addition of NO 3 −, NO 2 − or NO to suspensions of cells cultured aerobically in the absence of NO 3 −. With NO 3 − substrate added under anaerobic conditions the apparent K M=3,74± l.80mM, and under partially aerobic conditions K M =2.10 ± 0.54 mM; with NO 2 − substrate (anaerobic conditions) K M = 5.28±0.76 nM (partially aerobic conditions), K M=6.05± 1.30 μM, and with NO substrate, anaerobic conditions, K M = 7.3 ± 7.4 mu. The addition of O 2 resulted in short-term transient effects indicating rapid and reversible inhibition of NO and N 2O production. When O 2 was present, NO 2 −, accumulated in greater amounts than observed under completely anaerobic conditions. The transient effects suggest that the reduction of NO 2 −, NO and N 2O are each inhibited to increasingly greater extents by O 2.