Abstract: Nitrification is the process of microbial oxidation of ammonia to nitrate, completed by the ammonia oxidizers (AOB and AOA) and nitrite oxidizing bacteria (NOB), sequentially. For a century, we have been learning and studying the established two-step process as the unique way for nitrification. According to the kinetic theory, there should be a one-step nitrification in the environment, that is, the whole process of nitrification (from NH₃ to NO₃^-) was completed by one microorganism, but no study could give a direct proof for the existence of this microorganism. Until the end of 2015, three different cultivated bacteria (Candidatus Nitrospira nitrosa, Candidatus Nitrospira nitrificans and Candidatus Nitrospira inopinata) and an uncultivated bacterium (Nitrospira-like orga-nism) respectively found in different environments by three research teams, were defined as complete ammonia oxidizing microorganisms (Comammox), because all of them could carry out the complete oxidation of ammonia to nitrate. The discovery of one-step nitrification and Comammox ended the 100-year-old dogma, and triggered many important scientific issues of the global nitrogen cycle, such as the ecological niche of these microorganisms in the environment and their relative contribution in nitrification. This paper made a brief overview of the discovery of one-step nitrification and Comammox.

Key words: ammonia oxidizing microorganism, nitrogen cycle, Nitrospira, kinetic theory