Abstract
Abstract. The origin and development of the primary autotrophic metabolism on early Earth were influenced by the two main regimes of degassing of the Earth – reducing (predominance CH4) and oxidative (CO2). Among the existing theories of the autotrophic origin of life in hydrothermal environments, CO2 is usually considered to be the carbon source for nascent autotrophic metabolism. However, the ancestral carbon used in metabolism may have been derived from CH4 if the outflow of magma fluid to the surface of the Earth consisted mainly of methane. In such an environment, the primary autotrophic metabolic systems had to be methanotrophic. Due to the absence of molecular oxygen in the Archean conditions, this metabolism would have been anaerobic; i.e., oxidation of methane must be realized by inorganic high-potential electron acceptors. In light of the primacy and prevalence of CH4-dependent metabolism in hydrothermal systems of the ancient Earth, we propose a model of carbon fixation where the methane is fixed or transformed in a sequence of reactions in an autocatalytic methane–fumarate cycle. Nitrogen oxides are thermodynamically the most favorable among possible oxidants of methane; however, even the activity of oxygen created by mineral buffers of iron in hydrothermal conditions is sufficient for methanotrophic acetogenesis. The hydrothermal system model is considered in the form of a phase diagram, which demonstrates the area of redox and P and T conditions favorable for the development of the primary methanotrophic metabolism.
Highlights
Belonogova: Fluid degassing on early Earth redox buffer from −8 to +4 in the Hadean time and from −7 to +7 in the early Archean eon
Impulses of CO2 and CH4 degassing on our planet must have determined the preference of specific autotrophic carbon fixation metabolism development
It is widely accepted that autotrophic metabolism is the fixation of inorganic carbon solely in the form of CO2, but the origin of methane, both on the ancient Earth and on the planets and satellites, is clearly inorganic; carbon fixation from methane is a manifestation of autotrophic metabolism
Summary
The deep hydrocarbon generation in seismically active satellites manifested as the significant concentrations of hydrocarbons, including methane on their surface. The hydrogen in a reaction like 4H2 + H2CO3=3H2O + CH4 destroys the acid components in fluids, and this determines the alkaline slope in the development of magmatism This is a two-stage model of the development of the C–H–O system (I ↔ II), which depends on the composition of Earth’s core fluids and their transformations in magma chambers. We assume that in geodynamic regime II (CH4 + H2O paragenesis), carbon ancestral metabolism could use methane as a carbon source if the flow of free energy from the geochemical environment is coupled with biomass formation reactions Perhaps these different regimes of fluid degassing determined the physicochemical conditions of the ambient environment, which, in turn, provided an opportunity for the emergence and development of various systems of ancient autotrophic metabolism. Lateral gene transfer and subsequent phylogenetic divergence erased most evolutionary information recorded in ancestral prokaryotic genomes (Martin et al, 2016)
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