Investigators have reported various results for the influence of oxygen on the biochemistry of the denitrification process. Dissimilatory nitrate reduction (denitrification) is inhibited by oxygen, whereas assimilatory nitrate reduction is unaffected.
Payne (1973) explains that oxygen either represses the formation of the enzyme nitrate reductase or acts just as an electron acceptor, thereby preventing the reduction of nitrate.
Beneficial effects of oxygen in the denitrification process have been observed by Ide etal. (1972). The activity of denitrifying organisms seems to be enhanced after exposure to oxygen. This effect could be explained by the presence of haem in the electron transport system, as some organisms need oxygen in order to synthesize haem (Porra and Lascelles 1965; Tanaiguchi 1961).
The exact control mechanism exerted by oxygen on denitrifying enzyme synthesis, has not been clearly demonstrated yet, and may very well vary among species of denitrifiers.
When using attached cultures, it is especially important to distinguish between oxygen tension within the micro-environment around the bacteria, and oxygen tension within the macro environment.
It appears that 1-2 mg Cyi does not influence denitrification in filters; but in suspended cultures the oxygen concentration should be below 0,5 mg 02/l. Table 4.7 show the oxygen concentration in various denitrifying experiments.
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