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.

A bacterium, strain BC, was isolated from a benzene-degrading chlorate-reducing enrichment culture. Strain BC degrades benzene in conjunction with chlorate reduction. Cells of strain BC are short rods that are 0.6 m wide and 1 to 2 m long, are motile, and stain gram negative. Strain BC grows on benzene and some other aromatic compounds with oxygen or in the absence of oxygen with chlorate as the electron acceptor. Strain BC is a denitrifying bacterium, but it is not able to grow on benzene with nitrate. The closest cultured relative is Alicycliphilus denitrificans type strain K601, a cyclohexanol-degrading nitrate-reducing betaproteobacterium. Chlorate reductase (0.4 U/mg protein) and chlorite dismutase (5.7 U/mg protein) activities in cell extracts of strain BC were determined. Gene sequences encoding a known chlorite dismutase (cld) were not detected in strain BC by using the PCR primers described in previous studies. As physiological and biochemical data indicated that there was oxygenation of benzene during growth with chlorate, a strategy was developed to detect genes encoding monooxygenase and dioxygenase enzymes potentially involved in benzene degradation in strain BC. Using primer sets designed to amplify members of distinct evolutionary branches in the catabolic families involved in benzene biodegradation, two oxygenase genes putatively encoding the enzymes performing the initial successive monooxygenations (BC-BMOa) and the cleavage of catechol (BC-C23O) were detected. Our findings suggest that oxygen formed by dismutation of chlorite can be used to attack organic molecules by means of oxygenases, as exemplified with benzene. Thus, aerobic pathways can be employed under conditions in which no external oxygen is supplied.

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.

Complete biodegradation of azo dyes requires an anaerobic and aerobic step, in the anaerobic step sulfonated azo dyes (SADs) are reduced, yielding (sulfonated) aromatic amines ((S)AAs) which can be degraded aerobically. The complete biodegradation of the SAD Mordant Yellow 10 (MY10) was studied in a sequential anaerobic and aerobic bioreactor. Anaerobically, MY10 was reductively cleaved and the resulting aromatic amines, 5-aminosalicylic acid (5-ASA) and sulfanilic acid (SA), were both recovered in high stoichiometric yields. One of the AAs, 5-ASA, was readily degraded under aerobic conditions. However, SA was not degraded aerobically in the continuous experiment because no SA-degrading bacterial activity was present in the system. Therefore, a SA-degrading enrichment culture derived from Rhine sediment was used as an inoculum source. This enrichment culture was bioaugmented into the aerobic reactor by increasing the hydraulic retention time (HRT), thus enabling SA-degrading activity to develop and maintain in the aerobic reactor. After decreasing the HRT, the SA-degrading activity remained in the bioreactor and the stoichiometric recovery of sulfate (a SA biodegradation product) indicated the mineralization of SA after bioaugmentation. Batch experiments with aerobic reactor sludge confirmed the biodegradation of SA and 5-ASA. The sequential anaerobic and aerobic bioreactor was able to completely remove the sulfonated azo dye MY10 at a maximum loading rate of 210 mg MY10 (lreactor d)-1 after the appropriate microorganisms for aerobic degradation of SA were bioaugmented into the aerobic bioreactor.