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 fluorescen t in situ hybridization. For this purpose, new 16S rRNA-directed oligonucle otide probes targeting the bacterial phylum Nitrospira and the three main l ineages within this phylum were developed and evaluated. The diversity of N itrospira-like bacteria in the reactors was additionally investigated by re trieval and comparative analysis of full 16S rRNA sequences from the biofil ms. We round that, despite of the differences in the influent composition, Nitrospira-like bacteria form dominant populations in both reactors. in add ition, first insights into the physiology of these still unculturable bacte ria were obtained by the incubation of active biofilm samples with radioact ively labeled substrates followed by the combined application of fluorescen t in situ hybridization and microautoradiography. The results are discussed in consideration of the frequently observed dominance of Nitrospira-like b acteria in nitrifying bioreactors. Consequently, high priority should be as signed to future studies on the ecology and physiology of these organisms i n order to increase our fundamental understanding of nitrogen cycling and t o enable knowledge driven future improvements of nitrifying wastewater trea tment plants.