Denitrification in Azospirillum brasilense

Several nitrogen fixers were isolated from the roots and rhizosphere of Quebec crops. Many of these nitrogen fixers were discarded when the production of N(,2)O in the presence of C(,2)H(,2), as a test for denitrifying ability, was included as a selected characteristic. Further characterization of the Nir('+) strains showed that they were Azospirillum lipoferum.The cytochrome composition of Azospirillum brasilense (a denitrifier) grown under various conditions in a defined medium was investigated. Optical absorbance difference spectra of the particulate fraction of cells grown under aerated conditions indicated the presence of cytochromes of type b, c and a+a(,3). Under low aeration there was a quantitative increase in cytochromes b and c with a concomitant decrease in the a+a(,3)-type cytochrome. At high aeration, a CO spectrum indicated the possible participation of an o-type cytochrome.At both high and low oxygen concentrations, the supernatant fraction revealed only one c-type cytochrome. Its abundance was increased at low oxygen concentrations.Cytochrome spectra of anaerobically grown cells using different nitrogen oxides (NO(,3)('-), NO(,2)('-) and N(,2)O) as final electron acceptors revealed the presence of the different cytochromes involved in anaerobic respiration. The reduction of NO(,2)('-) was associated with the cytochrome cd (peak at 620 nm) found only in the supernatant fraction of NO(,2)('-)-grown cells.Growth on NO(,3)('-) was characterized by a diauxic type of curve in which the first logarithmic phase corresponded to the reduction of NO(,3)('-). The second logarithmic phase corresponded to the reduction of NO(,2)('-).Growth of Azospirillum brasilense with NO(,2)('-) and N(,2)O as final electron acceptor was possible only when a small amount of NO(,3)('-) was present initially. In contrast with other bacteria, growth of Azospirillum brasilense with tungstate instead of molybdate did not result in NO(,3)('-) reductase-deficient cells.The NO(,2)('-) accumulation observed with NO(,3)('-)-grown cells possibly resulted from the different NO(,3)('-)- and N(,2)('-)-reductase specific activities. However, the longer lag in the NO(,2)('-) reduction when higher concentrations of NO(,3)('-) were used may be due to a direct effect of NO(,3)('-) on the synthesis or activity of the NO(,2)('-) reductase