A single microorganism, Nitrospira sp.1 and sp.2, was identified as capable of complete nitrification, oxidizing ammonia to nitrate in one step. This process, known as complete ammonia oxidation (comammox), was previously thought to require two distinct microbial groups. The study found that these Nitrospira species encode all necessary enzymes for ammonia oxidation via nitrite to nitrate, and they completely oxidize ammonium to nitrate to conserve energy. Their ammonia monooxygenase (AMO) enzymes are phylogenetically distinct from known AMOs, suggesting recent horizontal gene transfer is unlikely. The discovery of these organisms changes our understanding of the nitrogen cycle, showing that a single organism can perform the entire nitrification process. The study also found that these Nitrospira species can use urea as an alternative ammonium source, enabling them to thrive in environments where urea is present. The results indicate that these organisms were previously overlooked in nitrification studies and that the long-held assumption of two distinct functional groups for nitrification is incorrect. The study also highlights the importance of these organisms in various environments, including wastewater treatment plants and aquatic systems. The findings have significant implications for understanding the biogeochemical nitrogen cycle and the role of microorganisms in environmental processes.A single microorganism, Nitrospira sp.1 and sp.2, was identified as capable of complete nitrification, oxidizing ammonia to nitrate in one step. This process, known as complete ammonia oxidation (comammox), was previously thought to require two distinct microbial groups. The study found that these Nitrospira species encode all necessary enzymes for ammonia oxidation via nitrite to nitrate, and they completely oxidize ammonium to nitrate to conserve energy. Their ammonia monooxygenase (AMO) enzymes are phylogenetically distinct from known AMOs, suggesting recent horizontal gene transfer is unlikely. The discovery of these organisms changes our understanding of the nitrogen cycle, showing that a single organism can perform the entire nitrification process. The study also found that these Nitrospira species can use urea as an alternative ammonium source, enabling them to thrive in environments where urea is present. The results indicate that these organisms were previously overlooked in nitrification studies and that the long-held assumption of two distinct functional groups for nitrification is incorrect. The study also highlights the importance of these organisms in various environments, including wastewater treatment plants and aquatic systems. The findings have significant implications for understanding the biogeochemical nitrogen cycle and the role of microorganisms in environmental processes.