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Anammox bacteria were exposed to 3,500 ppm nitric oxide (NO) in the gas phase to determine the effects of nitrogen oxides on anammox bacteria. The anammox bacteria were not inhibited by the high NO concentration but instead utilized it to oxidize additional ammonium to dinitrogen gas under conditions relevant to wastewater treatment.

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 (N2O) and nitric oxide (NO) production was demonstrated. N2O 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 N2O 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 N2O production is related to a shift in their metabolism from a low specific activity (q < qmax) toward the maximum specific activity (qmax). [ABSTRACT FROM AUTHOR]

Nitric oxide (NO) and nitrous oxide (N2O) emissions from nitrifying ecosystems are a serious threat to the environment The factors influencing the emission and the responsible microorganisms and pathways were studied using a laboratory-scale nitrifying reactor system. The nitrifying culture was established at growth rates relevant to wastewater treatment plants (WWTPs). During stable ammonia oxidation, 0.03% of ammonium was emitted as NO and 2.8% was emitted as N2O. Although mixed cultures were used, clear responses in emission of ammonia oxidizing bacteria (AOB) could be detected and it was concluded that the denitrification pathway of AOB was the main source of the emissions. Emissions of nitrogen oxides in the system were strongly influenced by oxygen, nitrite, and ammonium concentrations. Steady state emission levels greatly underestimate the total emission, because changes in oxygen, nitrite, and ammonium concentrations induced a dramatic rise in NO and N2O emission. The data presented can be used as an indication for NO and N2O emission by AOB in plug-flow activated sludge systems, which is highly relevant because of the atmospheric impact and potential health risk of these compounds. [ABSTRACT FROM AUTHOR]

The effects of nitrogen oxides on anammox bacteria are not well known. Therefore, anammox bacteria were exposed to 3,500 ppm nitric oxide (NO) in the gas phase. The anammox bacteria were not inhibited by the high NO concentration but rather used it to oxidize additional ammonium to dinitrogen gas under conditions relevant to wastewater treatment. [ABSTRACT FROM AUTHOR]