Genomic and physiological characterization of newly isolated nitrous oxide-reducing bacterium Stutzerimonas frequens strain E49 from landfill leachate-treating activated sludge.

Nitrous oxide (N2O) is a potent greenhouse gas and ozone-depleting substance, with a global warming potential 273 times greater than CO2 over a 100-year horizon. Microbial reduction of N2O to dinitrogen represents a key pathway for mitigating emissions under diverse environmental conditions. Here, we report the first comprehensive genomic and physiological characterization of Stutzerimonas frequens strain E49, a newly isolated N2O-reducing bacterium obtained from activated sludge treating landfill leachate. Whole-genome sequencing revealed a 4.51-Mbp circular chromosome and a 35.3-kbp plasmid with high completeness. Functional annotation identified a complete denitrification gene set, including nosZ, as well as the ectABCD-ask gene cluster associated with ectoine biosynthesis, suggesting adaptation to osmotic stress. Strain E49, a uniform rod-shaped bacterium (1.5-2.5 μm), efficiently reduced N2O under anaerobic conditions in the absence of an externally supplied organic carbon source, achieving a biomass-specific rate of 0.70 ± 0.02 µmol-N2O/mg-biomass/h and a cell-specific rate of 7.93 ± 0.23 × 10-10 µmol-N2O/cell/h. Among the cultivation regimens tested, nitrate-free DSMZ 1180 medium supplemented with NH4Cl yielded the highest activity, indicating medium-dependent regulation of N2O respiration. Comparative analysis showed that strain E49 outperformed several reported N2O-reducing isolates under carbon-limited conditions. These findings demonstrate the metabolic versatility of strain E49 and highlight its potential role as a biological sink for N2O in low-nutrient environments.IMPORTANCENitrous oxide is a powerful greenhouse gas that contributes to climate change and ozone depletion. Microorganisms that convert nitrous oxide into nitrogen gas play an essential role in reducing these emissions. In this study, we investigated Stutzerimonas frequens strain E49, a bacterium isolated from wastewater treatment sludge. We found that this organism can efficiently reduce nitrous oxide even without an external supply of organic carbon, which is typically required by most bacteria. This suggests that the bacterium can rely on internal energy reserves to carry out this process. We also identified the genetic basis for its nitrous oxide reduction and its ability to adapt to environmental stress. These findings improve our understanding of how nitrous oxide-reducing bacteria function in nutrient-limited environments and may support the development of strategies to mitigate emissions in wastewater treatment and other engineered systems.