Effect of NO Concentration on the Biological Conversion From NO to N2O under Thermophilic Conditions

Recently, some research has explored the production of nitrous oxide (N₂O) through biological denitrification of NO derived from flue gas under both mesophilic and thermophilic conditions. However, the effects of the NO concentration on N₂O production and its optimal range for thermophilic conditions remain unclear. In this study, we explored the effects of the NO concentration on the biological conversion of NO to N₂O at 45 °C using flask tests. The highest conversion efficiency from NO to N₂O was 92%, with 1.3 × 10⁵ mg/m³ N₂O detected in the headspace at an initial NO concentration of 20 mM in the solution. The ratio of NO reductase to N₂O reductase (NOR/N₂OR) peaked at a NO concentration of 20 mM. 16S rRNA gene sequencing analysis highlighted a positive correlation between Escherichia-Shigella and Propionicicella with N₂O accumulation. Metagenome analysis results further indicate that Escherichia possesses norVWR genes but not nos genes, enabling conversion of NO to N₂O rather than N₂, which is distinct from the typical denitrifying genes norBC. This study demonstrated that the optimal NO concentration range is 10–20 mM under thermophilic conditions and identified a unique denitrifier with special functional genes that contribute to N₂O accumulation. These findings could deepen our understanding of the mechanism of biological conversion from NO to N₂O and help to develop a biological N₂O production process for flue gas emission control and reclamation.