Effect of NO\nConcentration on the Biological Conversion\nFrom NO to N<sub>2</sub>O under Thermophilic Conditions

Recently, some research has explored the production of\nnitrous\noxide (N<sub>2</sub>O) through biological denitrification of NO derived\nfrom flue gas under both mesophilic and thermophilic conditions. However,\nthe effects of the NO concentration on N<sub>2</sub>O production and\nits optimal range for thermophilic conditions remain unclear. In this\nstudy, we explored the effects of the NO concentration on the biological\nconversion of NO to N<sub>2</sub>O at 45 °C using flask tests.\nThe highest conversion efficiency from NO to N<sub>2</sub>O was 92%,\nwith 1.3 × 10<sup>5</sup> mg/m<sup>3</sup> N<sub>2</sub>O detected\nin the headspace at an initial NO concentration of 20 mM in the solution.\nThe ratio of NO reductase to N<sub>2</sub>O reductase (NOR/N<sub>2</sub>OR) peaked at a NO concentration of 20 mM. 16S rRNA gene sequencing\nanalysis highlighted a positive correlation between <i>Escherichia-Shigella</i> and <i>Propionicicella</i> with N<sub>2</sub>O accumulation.\nMetagenome analysis results further indicate that <i>Escherichia</i> possesses <i>norVWR</i> genes but not <i>nos</i> genes, enabling conversion of NO to N<sub>2</sub>O rather than N<sub>2</sub>, which is distinct from the typical denitrifying genes <i>norBC</i>. This study demonstrated that the optimal NO concentration\nrange is 10–20 mM under thermophilic conditions and identified\na unique denitrifier with special functional genes that contribute\nto N<sub>2</sub>O accumulation. These findings could deepen our understanding\nof the mechanism of biological conversion from NO to N<sub>2</sub>O and help to develop a biological N<sub>2</sub>O production process\nfor flue gas emission control and reclamation.