Abstract
<p><strong>Abstract.</strong> Strategies for the origin and development of primary metabolism on early Earth were determined by the two main regimes of degassing of Earth in the form of CO<sub>2</sub> or CH<sub>4</sub> fluid impulses. Among the existing theories of the autotrophic origin of the life, CO<sub>2</sub> is usually considered the carbon source for nascent autotrophic metabolism. However, the ancestral carbon used in metabolism may have been derived from CH<sub>4</sub> if the outflow of magma fluid to the surface of the Earth consisted mainly of methane. Primary biochemical systems are present in methane degassing regimes developed in an environment of high partial pressure of methane, which is a source of carbon for nascent metabolic systems. Due to the absence of molecular oxygen in the Archaean 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 predominance of CH<sub>4</sub>-dependent metabolism in hydrothermal systems of the ancient Earth, we propose a model of carbon fixation, which is a sequence of reactions in a hypothetical methane-fumarate (MF) cycle. Thermodynamics calculations showed a high efficiency of oxidation of methane to acetate (methanotrophic acetogenesis) by oxidized nitrogen compounds in hydrothermal systems. Thermodynamically favorable were also reactions involving the introduction of carbon methane into the intermediates of the proposed MF cycle. The methane oxidation reactions with the use of oxygen of iron mineral buffers are closer to the equilibrium state, which apparently determines the possibilities of primordial cycle flow in the forward or reverse directions.</p>
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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