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 30 000 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.