**6. Strategy of atmospheric carbon dioxide fixation using the solar energy**

In global ecosystem, land plants, aquatic plants, and photoautotrophic microorganisms produce biomass that is original source of organic compounds (O'Leary, 1988). Autotrophs that are growing naturally or cultivating artificially have fixed the atmospheric carbon dioxide generated by heterotrophs, by which the atmospheric carbon dioxide may be balanced ecologically. However, the carbon dioxide generated from the combustion of organic compounds (petroleum and coal) that are not originated from biomass may be accumulated additionally in the atmosphere, inland water, and sea water. The solar radiation that reaches to the earth may not be limited for photosynthesis of phototrophs or electric generation of solar cells; however, the general habitats for growth of the phototrophs have been decreased by various human activities and the places for installation of the solar cells are limited to the habitats for human. If the solar cells were installed in the natural habitats, phototrophic fixation of carbon dioxide may be decreased in proportion to the electricity generation by the solar cells. The constructions of new cities, farmlands, golf courses, ski resorts, and sport grounds cause to convert the forests to grass field whose ability for carbon dioxide fixation is greatly lower than the forest. Consequently, the plantation of trees and grasses in the habitable lands or cultivation of algae and cyanobacteria in the habitable waters can't be the way to decrease additionally the atmospheric carbon dioxide.

Carbon dioxide has been fixed biologically by photoautotrophic, chemoautotrophic and mixotrophic organisms. The photoautotrophic bacteria assimilate carbon dioxide into organic compounds for cell structures with reducing power regenerated by the solar radiation under atmospheric condition (Kresge et al., 2005). The chemoautotrophs assimilate carbon dioxide into cell structure in coupling with production of methane or acetic acid with reducing power regenerated by hydrogenase under strict anaerobic hydrogen atmosphere (Perreault et al., 2007). The mixotrophs assimilate carbon dioxide into biomolecules with reducing power regenerated in coupling with metabolic oxidation of organic or inorganic compounds (Eiler, 2006). The photoautotrophs, chemoautotrophs, and mixotrophs can reduce metabolically carbon dioxide to organic carbon with the common reducing power (NADH or NADPH), which, however, are regenerated by

The *Achromobacter* sp. described in previous research was a facultative chemoautotroph (Hamilton *et al*., 1965; Romanov *et al*., 1977); however, it grew autotrophically with electrochemical reducing power under a CO2 atmosphere and consumed CO2 in this study. This result demonstrates that *Achromobacter* sp. grown in the electrochemical bioreactor may be a chemoautotroph capable of fixing CO2 with the electrochemical reducing power. Meanwhile, various articles have reported that *Alcaligenes* sp. grew autotrophically (Frete and Bowien, 1994; Doyle and Arp. 1987; Leadbeater and Bowien, 1984) or heterotrophically (Reutz *et al*., 1982). According to these articles, *Alcaligenes* spp. are capable of growing autotrophically with a gas mixture of H2, CO2, and O2, as well as heterotrophically under air on a broad variety of organic substrates. *Alcaligenes* spp. metabolically oxidize H2 to regenerate the reducing power during autotrophic growth under H2-CO2 atmosphere (Hogrefe *et al*., 1984). The essential requirement for the autotrophic growth of both *Achromobacter* spp. and *Alcaligenes* spp. under CO2 atmosphere is to regenerate reducing power in conjunction with metabolic H2 oxidation, which may be replaced by the electrochemical reducing power on the basis of the results obtained in this research. The electrochemical reducing power required for the cultivation of carbon-dioxide fixing bacteria can be produced completely by the solar panel, by which atmospheric carbon

**6. Strategy of atmospheric carbon dioxide fixation using the solar energy** 

In global ecosystem, land plants, aquatic plants, and photoautotrophic microorganisms produce biomass that is original source of organic compounds (O'Leary, 1988). Autotrophs that are growing naturally or cultivating artificially have fixed the atmospheric carbon dioxide generated by heterotrophs, by which the atmospheric carbon dioxide may be balanced ecologically. However, the carbon dioxide generated from the combustion of organic compounds (petroleum and coal) that are not originated from biomass may be accumulated additionally in the atmosphere, inland water, and sea water. The solar radiation that reaches to the earth may not be limited for photosynthesis of phototrophs or electric generation of solar cells; however, the general habitats for growth of the phototrophs have been decreased by various human activities and the places for installation of the solar cells are limited to the habitats for human. If the solar cells were installed in the natural habitats, phototrophic fixation of carbon dioxide may be decreased in proportion to the electricity generation by the solar cells. The constructions of new cities, farmlands, golf courses, ski resorts, and sport grounds cause to convert the forests to grass field whose ability for carbon dioxide fixation is greatly lower than the forest. Consequently, the plantation of trees and grasses in the habitable lands or cultivation of algae and cyanobacteria in the habitable waters can't be the way to

Carbon dioxide has been fixed biologically by photoautotrophic, chemoautotrophic and mixotrophic organisms. The photoautotrophic bacteria assimilate carbon dioxide into organic compounds for cell structures with reducing power regenerated by the solar radiation under atmospheric condition (Kresge et al., 2005). The chemoautotrophs assimilate carbon dioxide into cell structure in coupling with production of methane or acetic acid with reducing power regenerated by hydrogenase under strict anaerobic hydrogen atmosphere (Perreault et al., 2007). The mixotrophs assimilate carbon dioxide into biomolecules with reducing power regenerated in coupling with metabolic oxidation of organic or inorganic compounds (Eiler, 2006). The photoautotrophs, chemoautotrophs, and mixotrophs can reduce metabolically carbon dioxide to organic carbon with the common reducing power (NADH or NADPH), which, however, are regenerated by

dioxide may be fixed by same system to the photosynthesis.

decrease additionally the atmospheric carbon dioxide.

different metabolisms. The photoautotrophs, especially cyanobacteria that fix carbon dioxide by completely same metabolism (Calvin cycle) with plants, appear as if they are ideal organism to fix biologically carbon dioxide without chemical energy; however, they are unfavorable to be cultivated in the tank-type bioreactor owing to the limitation of reachable distance of solar radiation in aquatic condition. The chemoautotrophs may be useful to produce methane and acetic acid from carbon dioxide; however, they can grow only in the limit condition of the lower redox potential than -300 mV (vs. NHE) and with hydrogen. The mixotrophs can grow in the condition with electron donors, which are regardless of organic or inorganic compounds, for regeneration of reducing power under aerobic and anaerobic condition. This is the reason why the facultative anaerobic mixotrophs may be more effective than others to fix the atmospheric carbon dioxide directly by simple process. Especially, the cylinder-type electrochemical bioreactor equipped with the built-in anode compartment (Fig 9) is an optimal system for the cultivation or enrichment of facultative anaerobic mixotrophs. Basements of buildings or villages are used generally for maintenances or facilities for wastewater collection, electricity distribution, tap water distribution, and garage. The basements can't be the habitats for cultivation of plants with the natural sun light but can be utilized for cultivation of the carbon dioxide-fixing bacteria with electric energy generated from the solar cells that can be installed on the rooftop as shown in Fig 12.

Fig. 12. Schematic structure of the electrochemical bioreactors installed in the building basement. The carbon dioxide-fixing bacteria can be cultivated using the electric energy generated by the solar cells.

Bioelectrochemical Fixation of Carbon Dioxide with Electric Energy Generated by Solar Cell 205

biochemical reducing power electrochemically regenerated. The electrochemical reducing power can induce regeneration of NADH and ATP, by which both the assimilation of organic carbon and carbon dioxide into bacterial structure compounds can be activated. The goal of cultivation of bacterial cells using the cylinder-type electrochemical is to assimilate the atmospheric carbon dioxide to the organic compounds for bacterial structure without the combustion of fossil fuel and without production of metabolites. Some metabolites that are methane and acetic acid can be generated by the strict anaerobic bacteria under anaerobic hydrogen-carbon dioxide atmosphere but not useful for industrial utility owing to the cost for production. Meanwhile, the methane and acetic acid produced from the organic compounds in the process for treatment of wastewater or waste materials may be useful as the by-product for the industrial utility. The cell size and structural character of bacteria permits to put directly the bacterial cells in the empty petroleum well without any process, by which the atmospheric carbon dioxides are

Writing of this chapter was supported by the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the

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**8. Acknowledgement** 

**9. References** 

The facultative anaerobic mixotrophs assimilate heterotrophically organic compounds contained in the wastewater into the structural compounds of bacterial cells under oxidation condition but autotrophically carbon dioxide into the biomass under condition with high balance of biochemical reducing power (NADH/NAD+). DC electricity generated from the solar cells can be transferred very conveniently to the cylinder-type electrochemical bioreactor without conversion, which is the energy source for increase of biochemical reducing power balance. A part of the atmospheric carbon dioxide has been generated from the combustion system of fossil fuel, which may be required to be return to the empty petroleum well. To store the bacterial cells in the empty petroleum well is to return the carbon dioxide generated from petroleum combustion to the original place. The peptidoglycans, phospholipids, proteins, and nucleic acids that are major ingredients of bacterial cell structures are stable chemically to be stored in the empty petroleum well owing to the non-oxygenic condition. Conclusively, what the atmospheric carbon dioxide originated from the petroleum and coal is returned to the original place again may be best way to decrease the greenhouse effect.
