AEROBIC DENITRIFICATION

DENITRIFICAITION is the reduction of nitrates to gaseous nitrogen, sometimes mixed with oxides of nitrogen: it is the only process which certainly causes a loss of nitrogen in nature, because the nitrogen gas escapes into the atmosphere. It is known that nitrite certainly, and probably hyponitrite, are formed as intermediates in the course of the reaction; it is obvious that nitrate (or nitrite) must be present if denitrification is to take place; and it is also known that a compound to act as a hydrogen donator is necessary for the process. Karlsen (1938) published a detailed study of the effect of p H and of iron on denitrification by Pseudomonas aeruginosa ; and Elema (1932) and Elema et al. (1934) studied the changes in oxidation‐reduction potential during denitrification by Micrococcus denitrificans. Elema's paper includes a set of equations which represents the most probable course of the reaction. Several details of the process remain obscure, among them being the effect of aeration. The generally accepted view is that denitrification is an essentially anaerobic process, and that it is diminished or stopped by the admission of air. This supposition is probably accepted mainly because it provides a neat teleological explanation for the fact of denitrification; oxygen is made available by the reduction of nitrate; so under anaerobic conditions the bacteria reduce the nitrate in order to supply themselves with oxygen. Under aerobic conditions they need not reduce nitrate to obtain oxygen, and it is therefore assumed that they do not reduce it. This supposition is based on very little experimental evidence; though Weissenberg (1902) described a strain of Pseudomonas aerugimsa , which grew on nitrite under aerobic conditions, but did not reduce it, while in anaerobic cultures the nitrite was all reduced with much gas production. There is no reason to suppose, however, that every denitrifying species is affected by aeration in the same way. Seiser & Walz (1925) found that nitrogen was lost from both aerobic and anaerobic cultures of Ps. putida. The effect of aeration on Ps. deniirofIuolescetrs was studied by Korochkina (1936), who remarks “a relatively high rH value does not exclude denitrification. Therefore it is very difficult to eliminate denitrification by means of increased aeration.” The experiments described in the present paper were mainly concerned with the effect of aeration on denitrification; they were performed with pure cultures of two denitrifying species in simple synthetic liquid media.