| Summary: | Knowledge of the interactions among different microorganisms is important to understand how ecological function transformation is affected by elevated CO<sub>2</sub> levels in CO<sub>2</sub>-enhanced oil recovery (CO<sub>2</sub>-EOR) sites. Molecular ecological networks were established to reveal the interactions among different microbes of the soil bacterial community with the high-throughput sequencing data of 16S rRNA genes. The results showed that these networks are a powerful tool to identify and explain the interactions and keystone species in the communities under elevated CO<sub>2</sub> pressure. The structures of networks under different CO<sub>2</sub> leakage concentrations were different as a result of the networks’ topology properties, such as node numbers, topological roles of individual nodes, and network hubs. These indicators imply that the interactions among different groups were obviously changed. Moreover, changes in the network structure were significantly correlated with soil pH value, which might suggest that the large CO<sub>2</sub> leakage affected the soil ecosystem functions by changing the network interactions. Additionally, the key microbial populations such as <i>Bacteroidetes</i> and <i>Proteobacteria</i> were distinguished based on network topology to reveal community structure and ecosystem functioning. The work developed in this study could help microbiologists to address some research questions that could not be approached previously, and, hence, might represent a new area of research for microbial ecology.
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