Dissimilatory Reduction of NO 2 − to NH 4 + and N 2 O by a Soil Citrobacter sp

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Abstract
Dissimilatory reduction of NO 2 − to N 2 O and NH 4 + by a soil Citrobacter sp. was studied in an attempt to elucidate the physiological and ecological significance of N 2 O production by this mechanism. In batch cultures with defined media, NO 2 − reduction to NH 4 + was favored by high glucose and low NO 3 − concentrations. Nitrous oxide production was greatest at high glucose and intermediate NO 3 − concentrations. With succinate as the energy source, little or no NO 2 − was reduced to NH 4 + but N 2 O was produced. Resting cell suspensions reduced NO 2 − simultaneously to N 2 O and free extracellular NH 4 + . Chloramphenicol prevented the induction of N 2 O-producing activity. The K m for NO 2 − reduction to N 2 O was estimated to be 0.9 mM NO 2 − , yet the apparent K m for overall NO 2 − reduction was considerably lower, no greater than 0.04 mM NO 2 − . Activities for N 2 O and NH 4 + production increased markedly after depletion of NO 3 − from the media. Amendment with NO 3 − inhibited N 2 O and NH 4 + production by molybdate-grown cells but not by tungstate-grown cells. Sulfite inhibited production of NH 4 + but not of N 2 O. In a related experiment, three Escherichia coli mutants lacking NADH-dependent nitrite reductase produced N 2 O at rates equal to the wild type. These observations suggest that N 2 O is produced enzymatically but not by the same enzyme system responsible for dissimilatory reduction of NO 2 − to NH 4 + .