Effect of CO₂ on Microbial Denitrification via Inhibiting Electron Transport and Consumption

Increasing anthropogenic CO₂ emissions have been reported to influence global biogeochemical processes; however, in the literature the effects of CO₂ on denitrification have mainly been attributed to the changes it causes in environmental factors, while the direct effects of CO₂ on denitrification remain unknown. In this study, increasing CO₂ from 0 to 30000 ppm under constant environmental conditions decreased total nitrogen removal efficiency from 97% to 54%, but increased N₂O generation by 240 fold. A subsequent mechanistic study revealed that CO₂ damaged the bacterial membrane and directly inhibited the transport and consumption of intracellular electrons by causing intracellular reactive nitrogen species (RNS) accumulation, suppressing the expression of key electron transfer proteins (flavoprotein, succinate dehydrogenase, and cytochrome c) and the synthesis and activity of key denitrifying enzymes. Further study indicated that the inhibitory effects of CO₂ on the transport and consumption of electrons were caused by the decrease of intracellular iron due to key iron transporters (AfuA, FhuC, and FhuD) being down-regulated. Overall, this study suggests that the direct effect of CO₂ on denitrifying microbes via inhibition of intracellular electron transport and consumption is an important reason for its negative influence on denitrification.