Comammox Nitrospira clade B contributes to nitrification in soil

Comammox, one nitrifying microorganism carries out the complete oxidation of ammonia to nitrate, have been recently discovered, and are found in a wide range of environments, including soil. However, conditions under which they actually contribute to nitrification in soil have not yet been demonstrated. By 13CO2-based DNA stable isotope probing with real-time quantitative PCR and gene sequence, we reported two uncultured strains, which are closely related to comammox Nitrospira clade B, autotrophically grew in both forest and paddy soils only in the absence of ammonium amendment. Furthermore, all clade B amoA sequences amplified from isotopically enriched genomic DNA in both soils were derived from one or two phylotypes, indicating a low diversity of active comammox strains in soils.