Enrichment of CO2-fixing bacteria in cylinder-type electrochemical bioreactor with built-in anode compartment

Bacterial assimilation of CO2 into stable biomolecules using electrochemical reducing power may be an effective method to reduce atmospheric CO2 without fossil fuel combustion. For the enrichment of the CO2-fixing bacteria using electrochemical reducing power as an energy source, a cylinder-type electrochemical bioreactor with a built-in anode compartment was developed. A graphite felt cathode modified with neutral red (NR-graphite cathode) was used as a solid electron mediator to induce bacterial cells to fix CO2 using electrochemical reducing power. Bacterial CO2 consumption was calculated based on the variation in the ratio of CO2 to N2 in the gas reservoir. CO2 consumed by the bacteria grown in the electrochemical bioreactor (2,000 ml) reached a maximum of approximately 1,500 ml per week. Time-coursed variations in the bacterial community grown with the electrochemical reducing power and CO2 in the mineral-based medium were analyzed via temperature gradient gel electrophoresis (TGGE) of the 16S rDNA variable region. Some of the bacterial community constituents noted at the initial time disappeared completely, but some of them observed as DNA signs at the initial time were clearly enriched in the electrochemical bioreactor during 24 weeks of incubation. Finally, Alcaligenes sp. and Achromobacter sp., which are capable of autotrophically fixing CO2, were enriched to major constituents of the bacterial community in the electrochemical bioreactor.