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Oxidation of nitrite to nitrate is an essential process in wastewater treatment systems. Previous culturing studies have revealed that a new nitrite-oxidizing Candidatus Nitrotoga genus is particularly adaptive in cold habitats. This study provides the first observation of the influence of temperature on the competition and adaptation behavior of Nitrotoga-like bacteria in activated sludge nitrifying communities. With Nitrotoga-dominated sludge as the inoculum, two laboratory nitrifying reactors were fed with real domestic wastewater and operated at reduced temperatures from 14 to 4 °C and increased temperatures from 22 to 34 °C. Within 180 days of operation, activated sludge samples were regularly collected for 16S rRNA gene-based quantitative polymerase chain reaction, Illumina, and third-generation PacBio sequencing analyses. The results demonstrated that temperature is a deciding factor affecting niche …