Temperature Variations Shape Niche Occupation of Nitrotoga-like Bacteria in Activated Sludge

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 occupation of Nitrotoga-like bacteria. However, the classification of the species-level community showed that Candidatus Nitrotoga fabula-like populations remain less competitive at low temperatures, whereas two Nitrospira species belonging to sublineage I are adaptive over a wide temperature range of 4-34 °C. In addition, a temperature coefficient (θ) for the Nitrotoga-containing activated sludge was estimated to be 1.042 between 4 and 22 °C.