Nitrosomonas is a genus of Gram-negative bacteria belonging to the class Betaproteobacteria. It is one of the five genera of ammonia-oxidizing bacteria and, as an obligate chemolithoautotroph, uses ammonia (NH₃) as an energy source and carbon dioxide (CO₂) as a carbon source in the presence of oxygen.

Nitrosomonas species are important in the global biogeochemical nitrogen cycle because they increase the bioavailability of nitrogen to plants and play a role in denitrification, a process important for the release of nitrous oxide, a powerful greenhouse gas. These microbes are photophobic and usually generate a biofilm matrix, or form clumps with other microbes, to avoid light. Nitrosomonas can be divided into six lineages:

• The first includes Nitrosomonas europaea, Nitrosomonas eutropha, Nitrosomonas halophila, and Nitrosomonas mobilis.
• The second presents Nitrosomonas communis, N. sp. I, and N. sp. II.
• The third includes only Nitrosomonas nitrosa.
• The fourth includes Nitrosomonas ureae and Nitrosomonas oligotropha.
• The fifth and sixth lineages include Nitrosomonas marina, N. sp. III, Nitrosomonas aestuarii, and Nitrosomonas cryotolerans.

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