Low Temperature and Neutral pH Define " Candidatus Nitrotoga sp." as a Competitive Nitrite Oxidizer in Coculture with Nitrospira defluvii

Nitrification is an essential process for N removal in activated sludge to avoid toxicity of ammonium and nitrite. Besides , " Nitrotoga" has been identified as a key nitrite-oxidizing bacterium (NOB) performing the second step of nitrification, nitrite oxidation to nitrate, in wastewater treatment plants (WWTPs). However, the driving forces for the dominance of in certain plants have often remained unclear and could not be explained solely by temperature effects. In this study, we characterized the physiology of the ammonium-dependent sp. BS with regard to temperature and pH variations and evaluated its competitiveness against Both NOB originated from the same WWTP and shared a comparable pH optimum of 7.3. Based on these results, coculturing experiments with these NOB were performed in batch reactors operated at either 17°C or 22°C to compare their abundances under optimal (pH 7.4) or suboptimal (pH 6.4) conditions using 1 mM nitrite. As revealed by quantitative PCR (qPCR), fluorescence hybridization (FISH), and 16S amplicon sequencing, sp. BS was clearly favored by its optimal growth parameters and dominated over at pH 7.4 and 17°C, whereas a pH of 6.4 was more selective for Our synthetic communities revealed that niche differentiation of NOB is influenced by a complex interaction of environmental parameters and has to be evaluated for single species. " 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 and in bioreactors and identified conditions where the strategist was almost replaced by 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 sp. BS versus at a neutral pH of 7.4 in combination with a temperature of 17°C. The physiological diversity of novel cultures improves our knowledge about niche differentiation of NOB with regard to functional nitrification under suboptimal conditions.