Selective Organic Carbon Enrichment Influences Nitrous Oxide Reduction by Denitrifiers: Electron Competition Insights

A promising approach to reduce nitrous oxide (N2O) emission from wastewater treatment plants (WWTPs) would be to enrich highly efficient heterotrophic denitrifiers that are capable of regulating N2O reduction. The specific denitrifying community structure could be influenced by the types of organic carbon fed, while their corresponding N2O reduction capacity and electron competition mechanism remain unclear. In this study, the N2O reduction capacity by methanol-, ethanol-, and sodium acetate-enriched denitrifiers was systematically explored at varying carbon-loading rates. The effect of electron competition on N2O reduction was investigated by the combination of the microbial community and biochemical experiments. The results indicated that sodium acetate-enriched denitrifiers harboring abundant N2O reductase (Nos) genes invariably maintained the highest N2O reduction rate (rN2O). However, the concurrent presence of multiple electron acceptors (i.e., NO3– and/or NO2–) resulted in the higher susceptibility of rN2O for sodium acetate-enriched denitrifiers, which could explain the decrease in the N2O reduction rate under sufficient carbon supply. However, under insufficient organic carbon supply, Nos was confronted with fiercer electron competition from the upstream electron pool, causing more N2O accumulation. Moreover, a quantitative investigation of the electron distribution corroborated the aforementioned N2O reduction data. Further studies are warranted to optimize organic carbon-feeding strategies to utilize denitrifiers as an effective N2O sink at WWTPs.