5:15 PM - 6:45 PM
[HCG24-P01] Photosynthetic methane production achieved by strictly anaerobic co-culturing
★Invited Papers
Keywords:Photosynthesis, Green sulfur bacteria, Hydrogenase, Hydrogen production, Methanogen, Co-culture
Green sulfur bacteria are obligatory anaerobic photoautotrophs, which grow by non-oxygen evolving photosynthesis using reduced sulfur compounds as an electron source. Chlorobaculum tepidum, a model species, is genetically amenable to any gene modification and heterologous expression, thereby expected to be a promising host for over-production of anaerobic proteins that become non-functional under oxygenic conditions. One of the well-known oxygen-sensitive proteins is [FeFe]-type hydrogenase, which is an enzyme catalyzing proton reduction to hydrogen. Its unique iron-containing complex, named “H-cluster”, on the catalytic center is easily and irreversibly degraded by oxygen exposure. [FeFe]-type hydrogenases thus require a completely non-oxygen environment from biosynthesis to its function. In a previous study, we have successfully introduced and expressed HydA1, a [FeFe]-type hydrogenase from a green alga, together with its specific maturation protein, in the C. tepidum cell and produced a large amount of hydrogen gas from its photosynthetic culture.
The C. tepidum strain expressing the holo-type HydA1 is a photosynthetic hydrogen-producing system that can achieve a high yield of hydrogen evolution. However, since hydrogen gas has a low energy density and is not a carbon compound, the photosynthetic hydrogen production system of C. tepidum cannot directly bed used for carbon storage or atmospheric CO2 reduction. For this reason, we conceived to convert the large amount of hydrogen gas produced by the C. tepidum system to methane by the dissimilatory CO2 reduction system, which is a unique metabolic pathway of methanogens, a group of archaea. The dissimilatory CO2 reduction system of methanogens oxidizes hydrogen as an electron source and catabolically reduces CO2 to methane. However, this unique system of methanogens is a massive and complex metabolic pathway consisting of many specific enzymes and cofactors; and heterologous expression of all the genes in the C. tepidum cell is not practical and close to be impossible at present. To address this issue, we propose autotrophically co-culturing the hydrogen-evolving C. tepidum and the methane-evolving Methanococcus maripaludis, a model species of methanogens, in a same closed system to conjugate the unique metabolic pathways of each bacterial strain via hydrogen gas.
The C. tepidum strain expressing the holo-type HydA1 is a photosynthetic hydrogen-producing system that can achieve a high yield of hydrogen evolution. However, since hydrogen gas has a low energy density and is not a carbon compound, the photosynthetic hydrogen production system of C. tepidum cannot directly bed used for carbon storage or atmospheric CO2 reduction. For this reason, we conceived to convert the large amount of hydrogen gas produced by the C. tepidum system to methane by the dissimilatory CO2 reduction system, which is a unique metabolic pathway of methanogens, a group of archaea. The dissimilatory CO2 reduction system of methanogens oxidizes hydrogen as an electron source and catabolically reduces CO2 to methane. However, this unique system of methanogens is a massive and complex metabolic pathway consisting of many specific enzymes and cofactors; and heterologous expression of all the genes in the C. tepidum cell is not practical and close to be impossible at present. To address this issue, we propose autotrophically co-culturing the hydrogen-evolving C. tepidum and the methane-evolving Methanococcus maripaludis, a model species of methanogens, in a same closed system to conjugate the unique metabolic pathways of each bacterial strain via hydrogen gas.