Membrane hybridizations showed that the biomass fraction of Nitrosomonas and N. mobilis far exceeded that of Nitrosospira in all reactors, suggesting that the former were responsible for
most of the ammonia oxidation. These results concur with a number of previous studies that have suggested that Nitrosomonas can outcompete Nitrosospira in environments with high nitrogen loads (14, 17, 21). amoA DGGE profiles showed that a close relative to Nitrosomonas sp. strain Nm107 corresponded to the dominant band in virtually all samples analyzed.Nitrosomonas sp. strain Nm107 was first isolated from an activated sludge rendering plant and is believed to be a strain
of Nitrosococcus mobilis based on its 16S rRNA (35). In addition,the amoA sequences of Nitrosomonas sp. strain Nm107 and Nitrosococcus mobilis were shown to be identical (35).
Nitrosococcus mobilis was isolated from a sample of brackish water and is characterized by an obligate salt requirement,being considered a moderate halophilic AOB (24). The maximum growth rate of Nitrosococcus mobilis reported for pure cultures is close to that of N. europaea/N. oligotropha (14, 24).Juretschko et al. (21) used fluorescent in situ hybridization to confirm that N. mobilis was the numerically dominant AOB in the nitrifying activated sludge of an industrial wastewater treatment plant receiving sewage with high ammonia concentrations.Gieseke et al. (14) detected coexistence of Nitrosococcus mobilis and Nitrosomonas europaea/N. oligotropha in a biofilm from a nitrifying pilot-scale sequencing batch reactor treating
nitrogen-rich wastewater loads. Although Nitrosomonas sp. strain Nm107 corresponded to the dominant band in practically all amoA DGGE profiles, it may not have been the numerically
dominant or the most active AOB in the reactors due to the inherent qualitative rather than quantitative feature of PCR-DGGE.