METABOLISM OF NITRIC-OXIDE BY PSEUDOMONAS-STUTZERI IN CULTURE AND IN SOIL

Denitrification of nitrate with glucose as electron donor was measured in Pseudomonas stutzeri in batch culture and after inoculation into sterile soil. In closed culture, denitrification of 4 mM nitrate resulted in the intermediate accumulation of nitrite, NO and N2) with maximum concentrations of 2.2 mM, 126 nM and 169 nM, respectively. Denitrification experiments in soil were done in a flow-through system in which the soil was continuously flushed with N2. Denitrification of about 80 mug nitrate-N g-1 dw soil (= 67 mM) resulted in the intermediate accumulation of about 3-6 mug nitrite-N g-1 dw soil (= 2.5-5.0 mM). The produced NO and N2O which were flushed out of the system amounted to 37-61% and 2-6% of the reduced nitrate, respectively. Both glucose and nitrate were required for denitrification and for release of NO and N2O. NO consumption was studied in nitrate-exhausted soil using the flow-through system. NO consumption followed Michaelis-Menten kinetics. The apparent V(max) was about 15-42 amol NO h-1 cell-1, and the apparent K(m) was about 0.92 ppmv NO equivalent to 1.5 nM NO in the soil aqueous phase. At high soil water content (60-75% WHC), O2 mixing ratios between 0 and 20% in the gas phase had no short-term effect on the V a of NO consumption. At low soil water contents (10-30% WHC), however, the aerobic V(max). reached only about 20% of the anaerobic control. At high soil water contents only prolonged incubation (up to 96 h) under 20% O2 resulted also in decreased V(max) being about 30% of the anaerobic control. The K(m) of NO consumption was unaffected by all these treatments. The NO reductase activity of P. stutzeri appeared to be relatively resistant towards O2 when the bacteria had been inoculated into soil, and was sufficient to explain NO uptake in oxic soils.