KINETICS OF DENITRIFICATION BY PSEUDOMONAS-FLUORESCENS - OXYGEN EFFECTS

To determine the inhibitory effect of O2 on denitrification in pure cultures of Pseudomonas fluorescens, measurements of net NO and N2O production rates, f(NO) and f(N2O), under anaerobic conditions were compared to rates under partially aerobic conditions. Nitrate or NO2- solutions were injected into buffered suspensions and gaseous NO and N2O were sparged by constant flows of N2 carrier gas, or mixtures of N, with measured small quantities of O2 (ca 6-60 Pa). Apparent Michaelis-Menten parameters were calculated from saturation kinetic plots obtained by monitoring net NO or N2O production following periodic addition of NO3-, NO2- or NO to suspensions of cells cultured aerobically in the absence of NO3-. With NO- substrate added under anaerobic conditions the apparent K(M) = 3.74 +/- 1.80 mm, and under partially aerobic conditions K(M) = 2.10 +/- 0.54 mm; with NO2- substrate (anaerobic conditions) K(M) = 5.28 +/- 0.76 mum (partially aerobic conditions), K(M) = 6.05 +/- 1.30 mum, and with NO substrate, anaerobic conditions, K(M) = 7.3 +/- 7.4 nm. The addition of O2 resulted in short-term transient effects indicating rapid and reversible inhibition of NO and N2O production. When O2 was present, NO2-, accumulated in greater amounts than observed under completely anaerobic conditions. The transient effects suggest that the reduction of NO2-, NO and N2O are each inhibited to increasingly greater extents by O2.