Criteria and methodology for identifying respiratory denitrifiers

Abstract

Respiratory denitrification is not always adequately established when bacteria are characterized. We have tested a simple method that allows one to evaluate whether the two necessary criteria to claim denitrification have been met, namely, that N2 or N2O is produced from nitrate or nitrite and that this reduction is coupled to a growth yield increase. Microorganisms were cultured in sealed tubes under a helium headspace and in the presence of 0, 2, 4, 7, and 10 mM nitrate or nitrite. After growth had ceased, N2 and N2O were quantified by gas chromatography and the final protein concentration was measured. Net protein production was linearly related to nitrate concentration for all denitrifiers tested and ranged from 2 to 6 g of protein per tool of electron equivalent reduced. Nitrogen recovery as N2 plus N2O from nitrate and nitrite transformed exceeded 80% for all denitrifiers. We also suggest that a rate of N gas production of >10 μmol/min/g of protein can be used as an additional characteristic definitive of denitrification since this process produces gas more rapidly than other processes. These characteristics were established after evaluation of a variety of well-characterized respiratory denitrifiers and other N2O- producing nitrate reducers. Several poorly characterized denitrifiers were also tested and confirmed as respiratory denitrifiers, including Aquaspirillum itersonii, Aquaspirillum fasciculus, Bacillus azotoformans, and Corynebacterium nephridii. These criteria distinguished respiratory denitrifiers from other groups that reduce nitrate or produce N2O. Furthermore, they correctly identified respiratory denitrification in weak denitrifiers, a group in which the existence of this process may be overlooked.