Nitrite oxidation kinetics of two Nitrospira strains: The quest for competition and ecological niche differentiation

Nitrite oxidation is an aerobic process of the nitrogen cycle in natural ecosystems, and is performed by nitrite-oxidizing bacteria (NOB). Also, nitrite oxidation is a rate-limiting step of nitrogen removal in wastewater treatment plants (WWTPs). Although Nitrospira is known as dominant NOB in WWTPs, information on their physiological properties and kinetic parameters is limited. Here, we report the kinetic parameters and inhibition of nitrite oxidation by free ammonia in pure cultures of Nitrospira sp. strain ND1 and Nitrospira japonica strain NJ1, which were previously isolated from activated sludge in a WWTP. The maximum nitrite uptake rate (Vmax_NO2) and the half-saturation constant for nitrite uptake (Km_NO2) of strains ND1 and NJ1 were 45 ± 7 and 31 ± 5 (μmol NO2−/mg protein/h), and 6 ± 1 and 10 ± 2 (μM NO2−), respectively. The Vmax_NO2 and Km_NO2 of two strains indicated that they adapt to low-nitrite-concentration environments like activated sludge. The half-saturation constants for oxygen uptake (Km_O2) of the two strains were 4.0 ± 2.5 and 2.6 ± 1.1 (μM O2), respectively. The Km_O2 values of the two strains were lower than those of other NOB, suggesting that Nitrospira in activated sludge could oxidize nitrite in the hypoxic environments often found in the interiors of biofilms and flocs. The inhibition thresholds of the two strains by free ammonia were 0.85 and 4.3 (mg-NH3 l−1), respectively. Comparing the physiological properties of the two strains, we suggest that tolerance for free ammonia determines competition and partitioning into ecological niches among Nitrospira populations.