THE PRODUCTION AND UTILIZATION OF NITRIC-OXIDE BY A NEW, DENITRIFYING STRAIN OF PSEUDOMONAS-AERUGINOSA

When a new strain of Pseudomonas aeruginosa was grown aerobically and then transferred to anaerobic conditions, cells reduced NO3- quantitatively to NO2- in NO3-respiration. In the absence of nitrate, NO2- was immediately reduced to NO or N2O but not to N2 indicating that NO2--reductase but not N2O-reductase was active. The formation of the products NO or N2O depended on the pH in the medium and the concentration of NO2- present. When P. aeruginosa was grown anaerobically for at least three days N2O-reductase was also active. Such cells reduced NO to N2 via N2O. The new strain generated a H+-gradient and grew by reducing N2O to N2 but not by converting NO to N2O. For comparison, Azospirillum brasilense Sp7 showed the same pattern of NO-reduction. In contrast, Paracoccus denitrificans formed 3.5 H+/NO during the reduction of NO to N2O in oxidant pulse experiments but could not grow in the presence of NO. Thus the NO-reduction pattern in P. denitrificans on one side and P. aeruginosa and A. brasilense on the other was very different. The mechanistic implications of such differences are discussed.