Abstract: One-step nitrification whereby the whole process of nitrification from NH₃ to NO3- is completed by one microorganism (complete ammonia oxidizers; comammox), was first reported in late 2015. The discovery of one-step nitrification and comammox, ended a 100-year-old belief that biological nitrification was necessarily a two-step process, and raised many important scientific questions about the implications for the global nitrogen cycle, such as the ecological niche of these microorganisms in the environment and their relative contribution to nitrification. This study briefly reviews the progress in understanding one-step nitrification and comammox during the past two years. The reported comammox are all members of the genus Nitrospira spp. Linage Ⅱ, including two clades (A and B). They are widely distributed in agricultural soils, forest soils, paddy and fresh water environments. The comammox Nitrospira inopinata has a much higher affinity for ammonia than most cultured ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA), which confers them a competitive advantage in ammonia-limited environments. This finding changed our view of the potential role of complete ammonia oxidizers in nitrification. Rather than being rare organisms inhabiting peripheral habitats, it turns out that comammox bacteria are well adapted to the low ammonia environments that characterize most of the world’s biosphere. Additionally, the comammox members of Nitrospira possess different C and N metabolism pathways compared to AOA and AOB, and thus may also have different ecological roles or niches in the environment. The relative contribution of one-step and two-step nitrification pathways in different environments and the niche differentiation of comammox will be important fields for future study. So far, only a few ecosystems have been studied for the abundance of comammox, but the studies to date demonstrate that these microbes are widely distributed in low-ammonia environments. There is now an urgent need to determine their contribution to nitrification in other ecosystems. Their abundance in some ammonia-depleted environments is probably due not only to their high affinity for ammonia, but also to other factors, including their ability to grow on substrates other than ammonia, and their low energy requirements.

Key words: Nitrospira, nitrite oxidizing bacteria, ammonia oxidizing microorganism, nitrogen cycle