Hink, Linda

Abstract Nitrobacter strain NHB1 is a nitrite-oxidising bacterium previously demonstrated to form a consortium capable of nitrification under acidic conditions when co-cultivated with a neutrophilic ammonia-oxidising bacterium. Here, we characterize the growth of isolated NHB1 under different pH and nitrite (NO2-) concentrations, as well as its influence on the activity of obligately acidophilic soil ammonia-oxidising archaea (AOA) isolated from acidic soils when grown in co-culture. NHB1 is acidotolerant with optimal growth at pH 6.0 (range: 5.0–7.5) at an initial NO2- concentration of 500 μM. However, at lower NO2- concentrations, closer to those found in soil, its pH optimum decreases to 5.0 and with detectable growth extended to pH 3.5. In co-culture, NHB1 enhances the growth of the acidophilic AOA Nitrosotalea devaniterrae Nd1 and Nitrosotalea sinensis Nd2, which are highly sensitive to NO2--derived compounds and typically oxidise only ~200–300 μM ammonia (NH3) when grown in batch cultures as isolates. However, in co-culture with NHB1, both strains oxidised up to ~3 mM NH3, limited only by the buffering capacity of the medium, and their pH range was also extended downward by ~0.5 units. NHB1 also possesses a cyanase, enabling reciprocal cross-feeding through cyanate-derived NH3 production while utilizing AOA-derived NO2-. These findings suggest that NO2- removal is essential for ammonia oxidiser growth in acidic soils and emphasize the importance of considering substrate and metabolic product concentrations when characterising ecophysiology. Genome analysis reveals that NHB1 is distinct from validated species, and we propose the name ‘Nitrobacter laanbroekii’.

Nitrobacterstrain NHB1 is a nitrite-oxidising bacterium previously co-enriched with the neutrophilic ammonia-oxidising bacteriumNitrosospiraAHB1, a consortium that nitrifies in acidic conditions in co-culture. Here we characterise the growth of the isolateNitrobacterstrain NHB1 as a function of pH and nitrite (NO2-) concentration, and its influence on the activity of acidophilic soil ammonia-oxidising archaea (AOA). NHB1 is acidotolerant and grows optimally at pH 6.0 (range 5.0 - 7.5) at initial NO2-concentrations of 500 μM. However, the optimum decreases to pH 5.0 at lower initial NO2-concentrations typically found in soil, with detectable growth down to pH 3.5. NHB1 has a comparatively high affinity for NO2-with an apparent-half-saturation constant (54 μM) one order of magnitude lower than its closest relative, the neutrophilic strainNitrobacter hamburgensisX14. In co-culture, NHB1 enhances the growth of acidophilic AOA. Specifically,Nitrosotalea devaniterraeNd1 andNitrosotalea sinensisNd2 are sensitive to NO2--derived compounds and only oxidise ~200-300 μM ammonia (NH3) in batch cultures. However, in co-culture with NHB1, pH ranges were lowered by ~0.5 pH units and both strains could oxidise up to 3 mM NH3, only limited by buffering capacity. NHB1 possesses a cyanase facilitating reciprocal cross-feeding via generating cyanate-derived NH3and utilising AOA-derived NO2-. Removal of NO2-is likely crucial for the growth of nitrifiers in acidic soils and this study highlights the importance of considering substrate and metabolic product concentrations when characterising physiology. Genome analysis reveals NHB1 is distinct from validated species and the name "Nitrobacter laanbroekii" is proposed.