NxrB encoding the beta subunit of nitrite oxidoreductase as functional and phylogenetic marker for nitrite-oxidizing Nitrospira

Nitrospira are the most widespread and diverse known nitrite-oxidizing bacteria and key nitrifiers in natural and engineered ecosystems. Nevertheless, their ecophysiology and environmental distribution are understudied because of the recalcitrance of Nitrospira to cultivation and the lack of a molecular functional marker, which would allow the detection of Nitrospira in the environment. Here we introduce nxrB, the gene encoding subunit beta of nitrite oxidoreductase, as a functional and phylogenetic marker for Nitrospira. Phylogenetic trees based on nxrB of Nitrospira were largely congruent to 16S ribosomal RNA-based phylogenies. By using new nxrB-selective polymerase chain reaction primers, we obtained almost full-length nxrB sequences from Nitrospira cultures, two activated sludge samples, and several geographically and climatically distinct soils. Amplicon pyrosequencing of nxrB fragments from 16 soils revealed a previously unrecognized diversity of terrestrial Nitrospira with 1801 detected species-level operational taxonomic units (OTUs) (using an inferred species threshold of 95% nxrB identity). Richness estimates ranged from 10 to 946 coexisting Nitrospira species per soil. Comparison with an archaeal amoA dataset obtained from the same soils [Environ. Microbiol. 14: 525-539 (2012)] uncovered that ammonia-oxidizing archaea and Nitrospira communities were highly correlated across the soil samples, possibly indicating shared habitat preferences or specific biological interactions among members of these nitrifier groups.