Insight into a direct carbon dioxide effect on denitrification and denitrifying bacterial communities in estuarine sediment

With the elevation of atmospheric CO ₂ content, the potential effects of CO ₂ on organisms and various environmental processes have gained increasing concern. Most previous studies on denitrification have been conducted on ecosystems comprising plants, soils and microbes, but they have ignored the direct effect of CO ₂ on denitrification and denitrifying bacterial communities. Here, by excluding the effects of plants, we found that both short- and long-term exposure to CO ₂ directly inhibited the denitrification process, and caused the total nitrogen removal efficiency to decrease by up to 37%. Compared with the control, long-term exposure to CO ₂ (30,000 ppm) also caused >276-fold increase in N ₂ O emissions, and significantly inhibited the decomposition process. Enzymatic and qPCR assays showed that CO ₂ decreased the denitrifying enzymes activity (DEA) and the copy numbers of denitrifying genes, which directly resulted in the inhibitory effect of CO ₂ on denitrification process. Further study indicated that adverse effect of CO ₂ on DEA and denitrifying genes were caused by reducing the relative abundance of denitrifying bacteria. Moreover, the relative abundance of fermenting bacteria also decreased as CO ₂ concentration increased, which might result in insufficient liable carbon for the activity of denitrifying bacteria, and ultimately exacerbate the negative denitrification performance. Overall, this study suggests that, in the absence of plants, CO ₂ could directly affect the denitrifying and fermenting bacterial community, and inhibit denitrification and decomposition processes, which is detrimental to sediment nitrogen and carbon cycles.