[B-CG03] 地球惑星科学　生命圏フロンティア

[BCG03-07] Mineral-catalyzed hydrothermal CO₂ reduction that generated primordial autotrophic metabolisms

★Invited Papers

*五十嵐 健輔¹ (1.産業技術総合研究所　北海道センター)

キーワード：生命の起源、化学進化、熱水噴出孔、CO2還元、独立栄養代謝

Minerals are thought to play a crucial role in chemical evolution by driving the production of biomolecules via their catalytic activity. Especially in autotrophic theories for the origin of life, mineral catalysts are believed to enhance the continuous generation of organic molecules from inorganics such as CO₂ under the relatively moderate geochemical settings in the Hadean Earth; such as hydrothermal vents. Biochemistry suggests that such primordial CO₂ fixation reactions are preserved as “metabolic fossils” within the extant reductive acetyl-CoA pathway, the most primitive CO₂ fixation pathway, of organisms utilizing CO₂ and H₂, because of the similarity of the key components and reaction flows. In these theories, the most fundamental step that provides the necessary building blocks for autotrophic chemical evolution is reduction of CO₂. Hydrogen is thought to be the primary electron donor for the reaction on the Hadean Earth. Generation of H₂ occurs by water-rock interaction, namely serpentinization, in the Earth’s crust, providing H₂-rich water to hydrothermal vents fueling the catalytic reduction of CO₂ by minerals. Over the past decades, many studies have attempted to find possible chemical routes analogous to the acetyl-CoA pathway assaying CO₂ reduction by H₂ using commonly observed metal sulfides in hydrothermal vents, such as FeS and NiS. However, no efficient reactions have been reported until recently. Recently, a series of new trials have been conducted using minerals formed by serpentinization and/or originated from meteorites rather than conventionally used metal sulfides. The presenter has reconsidered the geochemical settings of the serpentinization reaction and its relevant by-product minerals to assay their potential for CO₂ reduction by H₂ in hydrothermal systems. Greigite (Fe₃S₄), a mixed-valence iron sulfide, was found to catalyze CO₂ reduction by H₂, generating formate, acetate, and CH₄ as products under versatile hydrothermal conditions (pH~10, ~100 °C, and ~100 bar). Similar reaction route catalyzed by other minerals synthesized as serpentinization product, namely magnetite (Fe₃O₄) and awaruite (Ni₃Fe), has also been discovered independently in the different laboratories. Magnetite and awaruite have been shown to catalyze the synthesis of pyruvate and ethanol, in addition to formate and acetate, from CO₂. The product profile of these minerals was similar to those of the intermediates and the end products of the extant reductive acetyl-CoA pathway. These facts suggest the significant involvement of these minerals in the abiotic synthesis of organics from CO₂ with H₂, providing building blocks required for the ancient reductive acetyl-CoA pathway in the chemical evolution. These findings support the theory that the first life could have formed from the primordial catalytic network based on CO₂ fixation catalyzed by geochemical reactions, preceding the enzyme-catalyzed biochemistry. In this presentation, a series of notable progress in mineral-catalyzed CO₂ reduction studies, including the recently discovered reaction route described above, will be introduced. Afterwards, research for ancient metalloenzymes which the presenter is conducting now will also be introduced in the context of the mineral-catalyzed chemical evolution process. Finally, the limitations and future perspectives on the autotrophic origin of life research will be discussed.