EMERGING PRINCIPLES OF INORGANIC NITROGEN-METABOLISM IN PARACOCCUS-DENITRIFICANS AND RELATED BACTERIA

The taxonomy of Paracoccus denitrificans and related bacteria is discu ssed. Evidence is given which shows that the physiological differences between P. denitrificans and Thiosphaera pantotropha are less fundame ntal than previously thought. A proposal to consider a species P. pant otropha is mentioned. The properties of the denitrifying enzymes and t he genes involved in their formation in P. denitrificans is discussed. The synthesis of the membrane-bound nitrate reductase is regulated by FNR, that of the nitrite- and nitric oxide reductase by NNR. Evidence is given that FNR acts as a redox sensor rather than an oxygen sensor . The occurrence of aerobic denitrification and coupled heterotrophic nitrification-denitrification in the original strain of Thiosphaera pa ntotropha are explained by a limiting respiratory activity which activ ates FNR. Aerobic denitrification leads to a lower growth yield and an increase in mu(max) in batch culture when a limiting respiratory acti vity is assumed and when excess substrate is present. Coupled heterotr ophic nitrification-denitrification gives a Smaller increase in mu(max ),, and a more drastic reduction in yield. Both processes are thus adv antageous to the organism. In a chemostat with limiting substrate thes e processes are disadvantageous. I: pantotropha has lost the ability f or aerobic denitrification during extended cultivation. Possibly the s ubstrate concentration was limiting during extended cultivation giving a selective advantage to variants which have lost these properties. T he calculations predict that P. denitrificans should be able to grow c hemolithotrophically with hydroxylamine.