SUBSTRATE OXIDATION AND NITROUS OXIDE UTILIZATION IN DENITRIFICATION

It is generally assumed that during denitrification bacteria oxidize organic substances completely to carbon dioxide concomitant with the reduction of nitrate or nitrite to nitrogen and nitrous oxide. This assumption is based upon the early experiments of Gayon and Dupetit (1886) which demonstrated that the ratio of carbon dioxide to nitrogen in denitrifying cultures corresponded to the theoretical ratio to be expected for complete oxidation of the organic substrate. This evidence is not conclusive because a given ratio of carbon dioxide to nitrogen could result from either complete or incomplete oxidation of the substrate. In order to obtain unequivocal evidence on this point we have made complete carbon and nitrogen balances on a denitrifying culture in which succinate and glutamate were provided as organic substrates. The role of nitrous oxide in denitrification is not clear, partly as a result of the use of inadequate techniques by earlier workers. Beijerinck and Minkman (1910) and Suzuki (1912) maintained that nitrous oxide was always present in the gaseous products of denitrification, whereas Gayon and Dupetit (1886) and others claimed that nitrous oxide was entirely absent in some of their experiments. Beijerinck and Minkman (1910) and Kluyver and Donker (1926) postulated that nitrous oxide is a normal precursor of nitrogen in denitrification, whereas later, Elema (1932) suggested a mechanism not involving nitrous oxide. We are reporting here a few observations which demonstrate that nitrous oxide is not always formed in denitrification, nor is it an obligatory intermediate in the conversion of nitrite to nitrogen.