Nitrite reduction by Bacterium denitrificans in relation to oxidation-reduction potential and oxygen tension.

Gaseous losses of nitrogen (N2 oIr N20) fr'Om bacterial culture media and soils maintained under conditions commonly considered aerobic have been observed frequently (Meiklejohn, 1940; Corbet and Wooldridge, 1940; Korsakova, 1941; Broadbent, 1951; Broadbent and Stojanovie, 1952; Marshall et al., 1953; and Collins, 1955). Not all workers, however, have considered such observations as proof that denitrification can occur under aerobic conditions, since the dissolved oxygen contents of the media were not determined. Furthermore, a low paitial pressure of oxygen will often markedly inhibit denitrification by young cultures of bacteria, as has been shown by Sacks and Barker (1949), Wijler and Delwiche (1954), and repeatedly confirmed in this laboratory. Skerman et al. (1951) are probably the only workers who have measured the oxygen content of cultures. They observed no nitrate reduction in cultures of Pseudomonas if dissolved oxygen was present. Nitrite reduction was not studied. Arnold (1954) observed no evolution of nitrous oxide from well aerated soils but did obtain it where moisture was present in excess. Undoubtedly the varying results can be explained in part by variations among bacterial species and strains, and by the conditions under which the organisms were grown prior to study. When oxygen is present not only does it compete with nitrate or nitrite, and thus reduces denitrification, but also it maintains the medium at a higher oxidation-reduction potential than would otherwise occur. This potential is also affected by the metabolic products excreted by the bacterial cells. The question arises, therefore, as to whether the inhibiting effect of oxygen on denitrification may not be due in part to the raising of the potential to a level that may be too high to permit nitrite reduction to occur. Very few studies of this type have been reported. Elema et al. (1934) found that Micrococcus denitrificans, when grown anaerobically, excreted metabolic products into thc me(ldlm that estab