Physiological and Genomic Characterization of Oligotrophic <i>Nitrobacter</i> Isolated from a Forest Soil in Japan

Nitrite is a key intermediate in global nitrogen cycles. It has been widely recognized that the accumulation of nitrite is often not appreciable in environments, and nitrite concentrations in canonical media for the cultivation of nitrite-oxidizing bacteria (NOB) in laboratories may not be low enough to recover oligotrophic NOB. We herein report the isolation, physiology, and genomics of oligotrophic NOB from a Japanese forest soil. NOB in soil samples were enumerated using the most probable number method with a medium containing urea for enriching oligotrophic NOB. Urea was completely converted into nitrate, and nitrite was not detected in any nitrifier-positive tubes cultivated after 9‍ ‍weeks of incubation. After subculturing NOB several times in a medium supplemented with 1‍ ‍mM nitrite and performing the extinction-dilution procedure, a novel strain oxidizing nitrite to nitrate was obtained and designated as strain CN101, which was affiliated with the genus Nitrobacter at the 16S rRNA gene level. The half-saturation constant of strain CN101 was lower than other known Nitrobacter strains, suggesting that Nitrobacter strains do not always exhibit low affinity for nitrite. The complete genome of strain CN101 included a larger number of nitrite/nitrate transporters than other Nitrobacter strains, which may serve as tools for flexibly adapting to varying nitrite concentrations in soils. Therefore, the physiological and genomic characteristics of strain CN101 will expand knowledge of the ecologically important but understudied genus Nitrobacter.