MASS-SPECTROMETRIC STUDIES OF THE EFFECT OF PH ON THE ACCUMULATION OFINTERMEDIATES IN DENITRIFICATION BY PARACOCCUS-DENITRIFICANS

We have used a quadrupole mass spectrometer with a gas-permeable membr ane inlet for continuous measurements of the production of N2O and N-2 from nitrate or nitrite by cell suspensions of Paracoccus denitrifica ns. The use of nitrate and nitrite labeled with N-15 was shown to simp lify the interpretation of the results when these gases were measured. This approach was used to study the effect of pH on the production of denitrification intermediates from nitrate and nitrite under anoxic c onditions. The kinetic patterns observed were quite different at acidi c and alkaline pK values. At pH 5.5, first nitrate was converted to ni trite, then nitrite was converted to N2O, and finally N2O was converte d to N-2. At pH 8.5, nitrate was converted directly to N-2, and the in termediates accumulated to only low steady-state concentrations. The s equential usage of nitrate, nitrite, and nitrous oxide observed at pH 5.5 was simulated by using a kinetic model of a branched electron tran sport chain in which alternative terminal reductases compete for a com mon reductant.