[U-12] CO環境の生命惑星化学

[U12-P08] H2-driven pyrite/pyrrhotite redox cycle: a sustained electron generator for the autotrophic origin of life

*北台 紀夫¹ (1.海洋研究開発機構)

キーワード：生命の起源、アストロバイオロジー、深海熱水噴出孔

Electrochemical survey of Okinawa trough hydrothermal fields and the subsequent laboratory simulations have suggested that the electricity generation in deep-sea hydrothermal vent environments has served as a crucial source of autotrophic energy through the origin and evolution of life. However, although thermodynamic calculation has predicted that H₂ oxidation in alkaline hydrothermal fluids (H₂ => 2H⁺ + 2e^–) is a powerful process for generating strongly negative electric potentials, the factors controlling the potentials and current available in realistic geochemical systems remain largely unknown: there should be a gap between the thermodynamic potentials and the potentials provided to the mineral–seawater interface. Here I show preliminary results of potential measurement and linear sweep voltammetry of iron sulfide under HS^– and H₂-containing alkaline hydrothermal fluids. It was found that, despite of the presence of H2, iron sulfide controls the electric potential through the mackinawite-pyrite equilibrium (FeS + HS^– => FeS₂ + H⁺ + 2e^–). H₂ chemically reduces the resultant pyrite, realizing a sustained electricity generation. With increasing temperature, the monitored electric potential approached to the thermodynamic potential for the mackinawite/pyrite equilibrium, reaching less than –650 mV versus the standard hydrogen electrode (at 25°C) under naturally realistic alkaline hydrothermal conditions. Additionally, higher temperatures led to greater current generation under oxidative potentials. These results indicate that in deep-sea systems, iron sulfide flexibly changes its oxidation state in response to the local electric potentials, serving as a natural battery between the reductive hydrothermal fluids and oxidative seawater.