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. The origin and development of the primary autotrophic metabolism onearly Earth were influenced by the two main regimes of degassing of theEarth – reducing (predominance CH4 ) and oxidative ( CO2 ). Amongthe existing theories of the autotrophic origin of life in hydrothermalenvironments, CO2 is usually considered to be the carbon source for nascentautotrophic metabolism. However, the ancestral carbon used in metabolism mayhave been derived from CH4 if the outflow of magma fluid to the surfaceof the Earth consisted mainly of methane. In such an environment, theprimary autotrophic metabolic systems had to be methanotrophic. Due to theabsence of molecular oxygen in the Archean conditions, this metabolism wouldhave been anaerobic; i.e., oxidation of methane must be realized byinorganic high-potential electron acceptors. In light of the primacy andprevalence of CH4 -dependent metabolism in hydrothermal systems of theancient Earth, we propose a model of carbon fixation where the methane isfixed or transformed in a sequence of reactions in an autocatalyticmethane–fumarate cycle. Nitrogen oxides are thermodynamically the most favorableamong possible oxidants of methane; however, even the activity of oxygencreated by mineral buffers of iron in hydrothermal conditions is sufficientfor methanotrophic acetogenesis. The hydrothermal system model is consideredin the form of a phase diagram, which demonstrates the area of redox and P and T conditions favorable for the development of the primary methanotrophicmetabolism.

Ideas and perspectives: Development of nascent autotrophic carbon fixation systems in various redox conditions of the fluid degassing on early Earth