Mechanisms of nitrous oxide emission during photoelectrotrophic denitrification by self-photosensitized Thiobacillus denitrificans

Photoelectrotrophic denitrification (PEDeN) using bio-hybrids has the potential to remove nitrate (NO₃^- ) from wastewater in an economical and sustainable way. As a gas of global concern, the mechanisms of nitrous oxide (N₂O) emissions during this novel process remain unclear. Herein, a self-photosensitized bio-hybrid, i. e., Thiobacillus denitrificans-cadmium sulfide, was constructed and the factors affecting N₂O emissions during PEDeN by the bio-hybrids were investigated. The system was sensitive to the input NO₃^- -N and NO₂^- -N, resulting in changes in the N₂O/(N₂+N₂O) ratio from 1% to 95%. In addition to free nitrous acid (FNA), reactive oxidative species (ROS) were a unique factor affecting N₂O emission during PEDeN. Importantly, the N₂O reduction step exhibited greater susceptibility to the ROS than nitrate reduction step. The contributions of hydrogen peroxide (H₂O₂), superoxides (O₂^-• ), hydroxyl radicals (•OH) and FNA to the inhibition of N₂O reduction were >15.0%, >5.4%, 1.3%, and <70.2%, respectively for a reduction of 13.5 mg/L NO₃^- -N. A significant down-regulation of the relative transcription of the gene nosZ demonstrated that the inhibition of N₂O reductase occurred at the gene level. This finding has important implications not only for mitigating N₂O emissions during the PEDeN process but also for encouraging a reexamination process of N₂O emissions in nature, particularly in systems in which ROS are present during the denitrification process.