The Family Nitrosomonadaceae

The Nitrosomonadaceae comprise a monophyletic phylogenetic group within the betaproteobacteria, all of whose cultivated representatives are lithoautotrophic ammonia oxidizers. Ammonia oxidizers generally exert control over nitrification by oxidizing ammonia to nitrite, which is subsequently oxidized by bacterial nitrite oxidizers to nitrate. They therefore play major roles in control of the nitrogen cycle in terrestrial, freshwater, and marine environments and in wastewater treatment processes. They are also of significant economic and environmental importance, leading to loss of ammonium-based fertilizers, nitrous oxide production, and nitrate pollution. Ammonia oxidation is also carried out by thaumarchaea, gammaproteobacteria, and anammox organisms. Phylogeny of the Nitrosomonadaceae is now based on analysis of 16S rRNA genes and amoA genes that encode subunit A of ammonia monooxygenase, which catalyzes the first step in ammonia oxidation. Sequences of cultivated strains and those obtained directly from environmental DNA and RNA suggest two genera, Nitrosomonas and Nitrosospira, each containing seven lineages, although support for these lineages in Nitrosospira is weaker. Two of these lineages are not represented by a laboratory isolate. Genomic analyses are providing insights into the evolution of the Nitrosomonadaceae and into metabolic processes of relevance to ecological studies.