Biological ammonia (NH₃) oxidation to nitrate (NO₃^–)––nitrification––is a critical pathway of the biogeochemical nitrogen cycle. Additional products and by-products of this pathway include nitrite (NO₂^–), nitric oxide (NO), nitrous oxide (N₂O), and nitrogen dioxide (NO₂), several of which are pollutants. How these species are generated during nitrification is not entirely clear, but pathways toward their generation have drawn substantial research effort. The cumulative evidence shows several parallel biological pathways comprising the net nitrification process. Bacteria were long thought to mediate all nitrification transformations; however, archaeal nitrifiers are now recognized. Furthermore, nitrification was thought to require two distinct microbial classes: NH₃ oxidizers to oxidize NH₃ to NO₂^–, and NO₂^– oxidizers that oxidize NO₂^– to NO₃^–. Comammox bacteria, which effect complete oxidation of NH₃ to NO₃^–, were recently discovered. This Perspective summarizes the current understanding of nitrification biochemistry and highlights exciting opportunities for future research.

生物氨（NH ₃）氧化为硝酸盐（NO ₃ ^-）-硝化-是生物地球化学氮循环的关键途径。此途径的其他产品和副产品包括亚硝酸盐（NO ₂ ^–），一氧化氮（NO），一氧化二氮（N ₂ O）和二氧化氮（NO ₂），其中几种是污染物。这些物种在硝化过程中如何产生尚不完全清楚，但是产生这些物种的途径已经引起了巨大的研究努力。累积的证据显示了构成净硝化过程的几种平行生物学途径。长期以来，细菌一直被认为可以介导所有硝化转化。但是，古硝化器现已被认可。此外，硝化被认为需要两种不同的微生物类：NH ₃氧化剂氧化NH ₃为NO ₂ ^-和NO ₂ ^-氧化剂该NO氧化₂ ^-为NO ₃ ^-。Comammox细菌，NH的这效果完全氧化₃至NO ₃ ^- ，最近发现的。该观点概述了对硝化生物化学的当前理解，并突出了未来研究的令人兴奋的机会。