As potential CO2 geological storage site of Carbon dioxide Capture & Storage (CCS), the use of depleted oil-gas reservoirs and aquifer has been proposed. The long-term aim of this research is to establish a biotechnological system to convert geologically-stored CO2 into methane, an energy resource. To develop a means for the conversion, we focus on technological application of a bio-electrochemical system using microbially-catalyzed electrode (biocathode). On the biocathode surface, methanogenic microorganisms utilize electrons to convert CO2 (CO2 + 8H~+ + 8e~- → CH4 + 2H2O). Such 'electromethanogenic' system is an attractive option for energy conversion, as the biocathode yields methane from electrical current, which can be provided by renewable energy sources. In other words, intermittent electrical energy provided by, for instance, wind turbines and solar cells can be stored in a stable energy form, methane. Toward technological application of the electromethanogenic system, we examined electromethanogenic activity of subsurface microorganisms. Indigenous microorganisms originated from a domestic oil reservoir were inoculated into bio-electrochemical reactor cells. Upon application of constant voltage of -0.75 V, the reactors produced methane at a rate of 386 mmol day(-1)m~(-2) (cathode surface area), which was the highest electromethanogenic production rate so far documented. Moreover, current-to-methane conversion efficiency was as high as 98%. Thus, we concluded that microorganisms indigenous to the subsurface reservoir are highly capable of electromethanogenic conversion of CO2. Electrochemical and microbial analyses suggested a reaction mechanism, in which electron-releasing bacteria mediated electron transfer from the electrode to methanogenic archaea. These outcomes imply the possibility of electromethanogenesis in underground CO2 storage reservoirs. For further enhancement of the electromethanogenic activity, we are currently on the way to biotechnologically improving electromethanogenic biocatalysts as well as to optimizing configuration of the reactor system for subsurface reservoirs.
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