“
Ca.
Nitrotoga” is a NOB of high environmental relevance, but physiological data exist for only a few representatives. Initially, it was detected in specialized niches of low temperature and low nitrite concentrations, but later on, its ubiquitous distribution revealed its critical role for N removal in engineered systems like WWTPs. In this study, we analyzed the competition between
Nitrotoga
and
Nitrospira
in bioreactors and identified conditions where the
K
strategist
Ns. defluvii
was almost replaced by
Nitrotoga
sp. BS. We show that the pH value is an important factor that regulates the composition of the nitrite-oxidizing enrichment with a dominance of
Nitrotoga
sp. BS versus
Ns. defluvii
at a neutral pH of 7.4 in combination with a temperature of 17°C. The physiological diversity of novel
Nitrotoga
cultures improves our knowledge about niche differentiation of NOB with regard to functional nitrification under suboptimal conditions.