Effect of CO<sub>2</sub> on Microbial Denitrification via Inhibiting Electron Transport and Consumption

Abstract

Increasing anthropogenic CO<sub>2</sub> emissions have been reported to influence global biogeochemical processes; however, in the literature the effects of CO<sub>2</sub> on denitrification have mainly been attributed to the changes it causes in environmental factors, while the direct effects of CO<sub>2</sub> on denitrification remain unknown. In this study, increasing CO<sub>2</sub> from 0 to 30 000 ppm under constant environmental conditions decreased total nitrogen removal efficiency from 97% to 54%, but increased N<sub>2</sub>O generation by 240 fold. A subsequent mechanistic study revealed that CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> on denitrifying microbes via inhibition of intracellular electron transport and consumption is an important reason for its negative influence on denitrification

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