Competition between oxygen and nitrate respirations in continuous culture of <i>Pseudomonas aeruginosa</i> performing aerobic denitrification

Continuous culture of P. aeruginosa was conducted with nitrate-containing media under the dilution rates (D) of 0.026, 0.06, and 0.13/h and the dissolved oxygen concentrations (DO) of 0-2.2 mg/L. The bacterium performed simultaneous O(2) and nitrate respiration in all of the systems studied. For each D, the (apparent) cell yield from glucose (Y(X/S)) was lower at zero DO, but did not change substantially with non-zero DO. In non-zero DO systems, Y(X/S) increased with increasing D, and when fit with a model considering cell death, gave the following parameters: maximum cell yield Y(X/S) (m) = 0.49, maintenance coefficient M(S) = 0.029 (/h), and cell decay constant k(d) = 0.014/h. The same model failed to describe the behaviors of zero-DO systems, where neither glucose nor nitrate was limiting and the limiting factor(s) remained unknown. The cell yield from accepted electron (Y(X/e)) was however relatively constant in all systems, and the energy yield per electron accepted via denitrification was estimated at approximately 69% of that via O(2) respiration. A closer examination revealed that increasing DO enhanced O(2) respiration only at extremely low DO ( <0.05 mg/L), beyond which the increasing DO only slightly increased its weak inhibition on denitrification. While O(2) was the preferred electron acceptor, the fraction of electrons accepted via denitrification increased with increasing D.