| Study Abstract |
Nitrification is a two-step process where ammonia was considered to first be oxidized to nitrite by ammonia-oxidizing bacteria (AOB) and/or archaea (AOA), and subsequently to nitrate by nitrite-oxidizing bacteria (NOB). Described by Winogradsky already in 1890, this division of labour between the two functional groups is a central dogmagenerally accepted characteristic of the biogeochemical nitrogen cycle. Complete oxidation of ammonia to nitrate in one organism (complete ammonia oxidation; comammox) is energetically feasible and it was postulated that this process could occur under conditions selecting for organisms with lower growth-rates but higher growth-yields than canonical ammonia-oxidizing microorganisms (AOM). Still, the organisms catalysing this process have not yet been discovered. Here, we report the enrichment and initial characterization of two Nitrospira species that encode all enzymes necessary for ammonia oxidation via nitrite to nitrate in their genomes, and indeed completely oxidize ammonium to nitrate to conserve energy. Their ammonia monooxygenase (AMO) enzymes are phylogenetically distinct from their counterparts in AOM, rendering recent acquisition by horizontal gene transfer from known AOM unlikely. We also found highly similar amoA sequences (encoding the AMO subunit A) in public sequence databases, which were apparently misclassified as methane monooxygenases. The recognition of a novel amoA sequence group and the discovery of the long-sought-after comammox process will not only shed new light on the environmental abundance and distribution of AOM, but also change our perception of the nitrogen cycle. |
