| Summary: | Microorganisms acting as sinks for the greenhouse gas nitrous oxide (N<sub>2</sub>O) are gaining increasing attention in the development of strategies to control N<sub>2</sub>O emissions. Non-denitrifying N<sub>2</sub>O reducers are of particular interest because they can provide a real sink without contributing to N<sub>2</sub>O release. The bacterial strain under investigation (IGB 4-14<sup>T</sup>), isolated in a mesocosm experiment to study the litter decomposition of <i>Phragmites australis</i> (Cav.), is such an organism. It carries only a <i>nos</i> gene cluster with the sec-dependent Clade II <i>nosZ</i> and is able to consume significant amounts of N<sub>2</sub>O under anoxic conditions. However, consumption activity is considerably affected by the O<sub>2</sub> level. The reduction of N<sub>2</sub>O was not associated with cell growth, suggesting that no energy is conserved by anaerobic respiration. Therefore, the N<sub>2</sub>O consumption of strain IGB 4-14<sup>T</sup> rather serves as an electron sink for metabolism to sustain viability during transient anoxia and/or to detoxify high N<sub>2</sub>O concentrations. Phylogenetic analysis of 16S rRNA gene similarity revealed that the strain belongs to the genus <i>Flavobacterium</i>. It shares a high similarity in the <i>nos</i> gene cluster composition and the amino acid similarity of the <i>nosZ</i> gene with various type strains of the genus. However, phylogenomic analysis and comparison of overall genome relatedness indices clearly demonstrated a novel species status of strain IGB 4-14<sup>T</sup>, with <i>Flavobacterium lacus</i> being the most closely related species. Various phenotypic differences supported a demarcation from this species. Based on these results, we proposed a novel species <i>Flavobacterium azooxidireducens</i> sp. nov. (type strain IGB 4-14<sup>T</sup> = LMG 29709<sup>T</sup> = DSM 103580<sup>T</sup>).
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