Novel Nitrospira-like bacteria as dominant nitrite-oxidizers in biofilms from wastewater treatment plants: diversity and in situ physiology

The frequency and distribution of putatively nitrite-oxidizing, Nitrospira- like bacteria in nitrifying biofilms from two reactors receiving wastewater with different ammonia and salt concentrations were observed by fluorescent in situ hybridization. For this purpose, new 16S rRNA-directed oligonucleotide probes targeting the bacterial phylum Nitrospira and the three main lineages within this phylum were developed and evaluated. The diversity of Nitrospira-like bacteria in the reactors was additionally investigated by retrieval and comparative analysis of full 16S rRNA sequences from the biofilms. We found that, despite of the differences in the influent composition, Nitrospira-like bacteria form dominant populations in both reactors. In addition, first insights into the physiology of these still unculturable bacteria were obtained by the incubation of active biofilm samples with radioactively labeled substrates followed by the combined application of fluorescent in situ hybridization and microautoradiography. The results are discussed in consideration of the frequently observed dominance of Nitrospira-like bacteria in nitrifying bioreactors. Consequently, high priority should be assigned to future studies on the ecology and physiology of these organisms in order to increase our fundamental understanding of nitrogen cycling and to enable knowledge-driven future improvements of nitrifying wastewater treatment plants.