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.