Nitrobacter laanbroekii sp. nov.

Acidotolerant, chemolithoautotrophic nitrite-oxidizing bacterium. Cells are rod-shaped and possess typical Nitrobacter stacked intracytoplasmic membranes arranged as a polar cap of paired membranes. Oxidizes nitrite (NO₂⁻) stoichiometrically to nitrate (NO₃⁻). Acidotolerant, with growth occurring over a pH range of 3.5–7.5; the pH optimum is substrate-dependent, being pH 6.0 at an initial NO₂⁻ concentration of 500 μM but decreasing to pH 5.0 at lower NO₂⁻ concentrations (20–50 μM), with growth extending down to pH 3.5 at the lowest concentration. Growth occurs with initial NO₂⁻ concentrations up to 5 mM and is inhibited at ~10 mM. Growth occurs between 10°C and 30°C, with optimal growth at 25°C (maximum specific growth rate μmax of 1.17 d⁻¹ at 25°C and 0.5 mM NO₂⁻). Exhibits a relatively high affinity for nitrite, with an apparent half-saturation constant [Km(app)] of 54 μM NO₂⁻ (at 25°C, pH 6), approximately one order of magnitude lower than Nitrobacter hamburgensis X14. Capable of forming nitrifying consortia with acidophilic and neutrophilic ammonia oxidizers, supporting their growth under acidic conditions through continuous removal of nitrite. Possesses a cyanase, enabling production of ammonium from cyanate and reciprocal cross-feeding with ammonia oxidizers. Genome comprises a 3.3 Mb chromosome and two plasmids (0.28 and 0.21 Mb), with an overall G+C content of 62.3 mol%. Encodes the nitrite oxidoreductase subunit genes nxrA (three copies), nxrB (two copies), and nxrC, accessory genes (nxrX, nxrD, cytochrome c class I), and nitrite/nitrate transporters. Possesses genes for dissimilatory sulphur oxidation (soxXYZAB), assimilatory nitrite reductase (nirBD), RuBisCO (Calvin cycle CO₂ fixation), PII-like regulatory proteins, four gene clusters encoding molybdopterin-containing carbon monoxide dehydrogenase (Mo-CODH, including coxL/cutL), formate dehydrogenase, the TorD chaperone, and molybdopterin cofactor biosynthesis genes. Lacks urea catabolic genes. Shares an average nucleotide identity (ANI) of 92.4% and average amino acid identity (AAI) of 86.5% with its closest validated relative, N. hamburgensis X14 (below the 95% ANI species threshold). Distributed widely in soils globally across a broad pH range; not restricted to narrow pH conditions. The reference strain is NHB1, originally isolated from pH 3.8 fertilized heathland soil (Hoorneboeg, Netherlands) under Calluna vulgaris and Deschampsia flexuosa vegetation. The genome sequence is available under NCBI BioProject accession number PRJNA1074292.