Acidotolerant soil nitrite oxidiser 'CandidatusNitrobacter laanbroekii' NHB1 alleviates constraints on growth of acidophilic soil ammonia oxidisers

Nitrobacter strain NHB1 is a nitrite-oxidising bacterium previously co-enriched with the neutrophilic ammonia-oxidising bacterium Nitrosospira AHB1, a consortium that nitrifies in acidic conditions in co-culture. Here we characterise the growth of the isolate Nitrobacter strain NHB1 as a function of pH and nitrite (NO 2 - ) 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 NO 2 - concentrations of 500 μM. However, the optimum decreases to pH 5.0 at lower initial NO 2 - concentrations typically found in soil, with detectable growth down to pH 3.5. NHB1 has a comparatively high affinity for NO 2 - with an apparent-half-saturation constant (54 μM) one order of magnitude lower than its closest relative, the neutrophilic strain Nitrobacter hamburgensis X14. In co-culture, NHB1 enhances the growth of acidophilic AOA. Specifically, Nitrosotalea devaniterrae Nd1 and Nitrosotalea sinensis Nd2 are sensitive to NO 2 - -derived compounds and only oxidise ~200-300 μM ammonia (NH 3 ) 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 NH 3 and utilising AOA-derived NO 2 - . Removal of NO 2 - 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.