[B-BG01] 地球惑星科学　生命圏フロンティア

[BBG01-09] Aerobic anoxygenic phototrophic methanotrophs were possibly the first organisms on Earth

*大本 洋^1,2、フェリー ジェームス¹ (1.ペンシルヴァニア州立大学、2.東北大学)

キーワード：メタンに富む初期大気、初期生命、光合成触媒作用、メタン酸化細菌、シアノバクテリア

Current theories for the origin and evolution of life and the environments on early Earth are based on the premise that CO2 and N2 have been the principal constituents of the atmosphere since ~4.5 billion years (Ga) ago. Based on thermodynamic analyses of the prebiotic mantle and submarine hydrothermal fluids that supplied the constituents of the oceans and atmosphere on the ocean-covered Earth, we suggest that the prebiotic atmosphere was rich in CH4, H2, and NH3; CO2, CO, N2, and O2 were virtually absent. The oceans had a pH of around 10 and were poor in iron and sulfur (each about 0.1 micro moles/kg H2O). Therefore, CH4, not CO2, was most likely the principal source of carbon for the early organisms, the greenhouse gas that maintained a habitable environment, and the UV shield (via the formation of organic haze) protecting the organisms. We suggest that the first organisms to appear on Earth were aerobic anoxygenic phototrophic methanotrophs or chemolithoautotrophic methanotrophs that evolved in microaerobic environments created on the surfaces of photocatalytic minerals (e.g., platinum (Pt) metal, pyrrhotite (FeS), rutile (TiO₂), and serpentine minerals) exposed to sunlight. These photocatalysts were detrital minerals from ultramafic rocks and accumulated in shallow (<~1 m deep) coastal water bodies on ocean islands.
Plate tectonics played an essential role in the transformation of the CH4/NH3/H2-rich to a CO2/N2/O2-rich atmosphere and in the developments of diverse ecosystems (e.g., oxic oceans, anoxic basins) and organisms (e.g., aerobic- and anaerobic microbes), by ~3.9 Ga. The search for life in the universe should be directed to planets with CH₄-rich atmospheres, as well as those with CO₂-rich atmospheres.