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Astrobiology Science Conference 2017 (LPI Contrib. No. 1965)
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CONSTRAINING PHOSPHATE, MAGNESIUM AND CALCIUM CONCENTRATIONS FOR
PREBIOTIC SYNTHESIS AND SELF-ASSEMBLY OF THE BUILDING BLOCKS OF LIFE. N. Sahai1 and
M. A. Schoonen2, 3, 1Department of Polymer Science, 170 Univeristy Ave., University of Akron, Akron, OH 443253909 ([email protected].); 2Environmental and Climate Sciences, BNL Building 460, P. O. Box 5000, Upton, NY
11973, USA; 3Dept. Geosciences, Stony Brook University, Stony Brook, NY 11794, USA ([email protected]).
Introduction: The availability of total dissolved inorganic phosphate (PT) in modern environments is orders
of magnitude lower than the concentrations found
within cells and those required for the non-enzymatic
(model prebiotic) synthesis of the most important molecular units of life, namely, nucleic acid monomers,
adenosine phophates and phospholipids. This has lead
to the paradox known as the “phosphate problem” in
understanding the origins of life. Similar hurdles exist
in comparing the modern geochemical availability of
dissolved Mg and Ca with the concentrations required
for non-enzymatic nucleotide polymerization and stability of model protocell membranes. Ad hoc geological conditions are often invoked to address these problems or the problem is not addressed at all. Here, using
a thermodynamic modeling approach of atmospherewater-rock interactions, we determined the solution
compositions obtained by the globally-occurring geological processes of rock weathering and evaporation
under a wide range of partial pressures of atmospheric
CO2 (PCO2) and temperatures [1]. We examined weathering of komatiite (primitive oceanic crust) and tonalite
(primitive continental crust) [1, 2].
Methods: Atmosphere-water-rock interactions were
determined based on the princples of mass balance and
electroneutrality for all elements in the system using
Geochemist’s Workbench [3]. Atmospheric SO2/H2S
ratio and PN2 was set constant. PCO2 was varied from
present atmospheric level (PAL) to 30 atm. Dissolved
sulfate and carbonate in rain water produced acidic
solution which was reacted with komatiite or tonalite
with various primary hydroxyapatite contents as the P
source. Partial weathering (5%) of the rock released
dissolved phosphate, Mg, Na, K, Ca, Al, Si, and Fe,
which reached equilibrium by precipitation of secondary minerals. Weathering was carried out either subaerially (open carbonate system), or isolated from the
atmosphere (closed carbonate system) in sub-surface
weathering. The “end-of-weathering” (EW) solution
was evaporated to 5% of the original volume and new
equilibria were established in this solution. Upto ten
cycles of evaporation and rehydration with fresh aliquots of EW solution were performed. Temperature
was varied from 75 to 25 0 [1].
Results: Sub-serial weathering of komatiite followed
by evaporation at PCO2 > 5 atm provided total dissolved
concentrations of P, Mg and Ca required biologically
for nucleotide synthesis and polymerization, and protocell-membrane stability. The PCO2 determined on
which of a “geochemical divide” between Mg and Ca
the system evolved as the solution evaporated [1, 4].
At PCO2 = PAL, hydroxyapatite precipitated upon
evaporation, thus depleting PT. At PCO2 ≥ 1 atm, carbonates precipitated, resulting in higher PT. Concentrations required for non-enzynamtic nucleotide synthesis
were not obtained up to PCO2 = 30 atm and total dissolved Mg concentration required for non-enzymatic
nucleotide polymerization were approached at high
PCO2 [1]. Final solution pH was ~5-6. Tonalite did not
yield relevant desired P or Mg concentrations.
Our results provide realistic constraints on the
plausible ion concentrationns on early Earth for prebiotic synthesis of the molecular building blocks of life
and for experiments attempting to model prebiotic synthesis and polymerization. Conversely, the biologically-required concentrations of PT, Mg and Ca provided
constraints on the PCO2 levels on early Earth. Previous
estimates for PCO2 on Hadean and early Archean Earth
have ranged over five orders of magnitude! The present results also provide the detailed, quantitative speciation of thirteen elements (H, C, N, O, S, Mg, Ca, Al,
Si, Fe, K, Na, Cl) on early Earth distributed between
various phases. Our study is the first to provide a single, global mechanism to simultaneously constrain
solution concentrations and PCO2 on early Earth, thus
setting the stage for prebiotic organic molecule synthesis and self-assembly leading to the emergence of life.
This approach brings together the geochemical, biochemical and organic synthesis fields, which often
operate independently in the origins of life field.
References:
[1] Sahai N. and Schoonen M.A. (2017) Nat. Geosci. in review. [2] Schoonen M., Smirnov A. and Cohen C. A. (2004) Ambio 33, 539-551. [3] Bethke C.
(2008) Geochemical and biogeochemical reaction
modeling. Cambridge University Press, NY. [4] Hardie
L. and Eugster H. (1970) Mineral. Soc. Amer. Spec.
Paper 3, 273-290.