The overall goal of this study was to determine the molecular and metabolic responses of chemostat cultures of model nitrifying bacteria to imposition of and recovery from transient anoxic conditions. Based on the study, a specific directionality in
nitrous oxide (N(2)O) and
nitric oxide (NO) production was demonstrated. N(2)O production was only observed during recovery to aerobic conditions after a period of
anoxia and correlated positively with the degree of
ammonia accumulation during
anoxia. NO, on the other hand, was emitted mainly under
anoxia. The production of NO was linked to a major imbalance in the expression of the
nitrite reductase gene, which was overexpressed during transient
anoxia. In contrast, genes coding for
ammonia and
hydroxylamine oxidation and
nitric oxide reduction were generally under-expressed during transient
anoxia. These results are different from the observed parallel expression and activity of
nitrite and
nitric oxide reductase in heterotrophic bacteria subjected to transient
oxygen cycling. Unlike NO, the production of N(2)O could not be solely correlated to gene expression patterns and likely involved responses at the
enzyme activity or metabolic levels. Based on experimental data, the propensity of the nitrifying cultures for N(2)O production is related to a shift in their metabolism from a low specific activity (q < q(max)) toward the maximum specific activity (q(max)).