Sahai Nita, Adebayo Segun, Schoonen Martin A
School of Polymer Science and Polymer Engineering and University of Akron, Akron, Ohio, USA.
Department of Geoscience, University of Akron, Akron, Ohio, USA.
Astrobiology. 2022 Jun;22(6):641-671. doi: 10.1089/ast.2020.2396. Epub 2022 Apr 21.
The chemical composition of aqueous solutions during the Hadean era determined the availability of essential elements for prebiotic synthesis of the molecular building blocks of life. Here we conducted quantitative reaction path modeling of atmosphere-water-rock interactions over a range of environmental conditions to estimate freshwater and evaporite brine compositions. We then evaluated the solution chemistries for their potential to influence ribonucleotide synthesis and polymerization as well as protocell membrane stability. Specifically, solutions formed by komatiite and tonalite (primitive crustal rocks) weathering and evaporation-rehydration (drying-wetting) cycles were studied assuming neutral atmospheric composition over a wide range of values of atmospheric partial pressure of CO (P) and temperatures (T). Solution pH decreased and total dissolved concentrations of inorganic P, Mg, Ca, Fe, and C (P, Mg, Ca, Fe, and C) increased with increasing P. The P and T dictated how the solution evolved with regard to minerals precipitated and ions left in solution. At T = 75°C and P < 0.05 atm, the concentration ratio of magnesium to calcium ion concentrations (Mg/Ca) was < 1 and predominantly metal aluminosilicates (including clays), dolomite, gibbsite, and pyrite (FeS) precipitated, whereas at P > 0.05 atm, Mg/Ca was > 1 and mainly magnesite, dolomite, pyrite, chalcedony (SiO), and kaolinite (AlSiO) precipitated. At T = 75°C and P > 0.05 atm, hydroxyapatite (HAP) precipitated during weathering but not during evaporation, and so, P increased with each evaporation-rehydration cycle, while Mg, Ca, and Fe decreased as other minerals precipitated. At T = 75°C and P ∼5 atm, reactions with komatiite provided with high enough Mg concentrations to promote RNA-template directed and montmorillonite-promoted nonenzymatic RNA polymerization, but incompatible with protocell membranes; however, montmorillonite-promoted RNA polymerization could proceed with little or no Mg present. from komatiite weathering at T = 75°C and P ∼5 atm yielded the following: (1) high P values that could promote ribonucleotide synthesis, and (2) low divalent cation concentrations compatible with amino acid-promoted, montmorillonite-catalyzed RNA polymerization and with protocell membranes, but too low for template-directed nonenzymatic RNA polymerization. For all P values, Mg and P concentrations decreased, whereas the HCO concentration increased within increasing temperature, due to the retrograde solubility of the minerals controlling these ions' concentrations; Fe concentration increased because of prograde pyrite solubility. Tonalite weathering and cyclical wetting-drying reactions did not produce solution compositions favorable for promoting prebiotic RNA formation. Conversely, the ion concentrations compatible with protocell emergence, placed constraints on P of early Earth's atmosphere. In summary: (1) prebiotic RNA synthesis and membrane self-assembly could have been achieved even under neutral atmosphere conditions by atmosphere-water-komatiite rock interactions; and (2) constraints on element availability for the origins of life and early P were addressed by a single, globally operating mechanism of atmosphere-water-rock interactions without invoking special microenvironments. The present results support a facile origins-of-life hypothesis even under a neutral atmosphere as long as other favorable geophysical and planetary conditions are also met.
冥古宙时期水溶液的化学成分决定了生命分子构建块的益生元合成所需基本元素的可利用性。在此，我们针对一系列环境条件开展了大气 - 水 - 岩石相互作用的定量反应路径建模，以估算淡水和蒸发岩盐水的成分。然后，我们评估了这些溶液化学性质对核糖核苷酸合成与聚合以及原始细胞膜稳定性的潜在影响。具体而言，假设大气成分呈中性，在广泛的二氧化碳大气分压（P）和温度（T）值范围内，研究了由科马提岩和英云闪长岩（原始地壳岩石）风化以及蒸发 - 再水化（干湿）循环形成的溶液。随着P的增加，溶液pH值降低，无机磷、镁、钙、铁和碳（P、Mg、Ca、Fe和C）的总溶解浓度增加。P和T决定了溶液在沉淀的矿物质和留在溶液中的离子方面如何演化。在T = 75°C且P < 0.05 atm时，镁与钙离子浓度之比（Mg/Ca）< 1，主要沉淀出金属铝硅酸盐（包括粘土）、白云石、三水铝石和黄铁矿（FeS），而在P > 0.05 atm时，Mg/Ca > 1，主要沉淀出菱镁矿、白云石、黄铁矿、玉髓（SiO）和高岭石（AlSiO）。在T = 75°C且P > 0.05 atm时，风化过程中会沉淀出羟基磷灰石（HAP），但蒸发过程中不会，因此，随着每个蒸发 - 再水化循环，P增加，而随着其他矿物质沉淀，Mg、Ca和Fe减少。在T = 75°C且P ∼ 5 atm时，与科马提岩的反应提供了足够高的Mg浓度以促进RNA模板导向和蒙脱石促进的非酶促RNA聚合，但与原始细胞膜不相容；然而，蒙脱石促进的RNA聚合在几乎没有Mg或没有Mg的情况下也可以进行。来自T = 75°C且P ∼ 5 atm时科马提岩风化的结果如下：（1）高P值可促进核糖核苷酸合成，以及（2）低二价阳离子浓度与氨基酸促进的、蒙脱石催化的RNA聚合以及原始细胞膜相容，但对于模板导向的非酶促RNA聚合来说太低。对于所有P值，由于控制这些离子浓度的矿物质的逆向溶解度，Mg和P浓度降低，而HCO浓度随着温度升高而增加；由于黄铁矿的正向溶解度，Fe浓度增加。英云闪长岩风化和周期性干湿反应并未产生有利于促进益生元RNA形成的溶液成分。相反，与原始细胞出现相容的离子浓度对早期地球大气的P施加了限制。总之：（1）即使在中性大气条件下，通过大气 - 水 - 科马提岩岩石相互作用也可能实现益生元RNA合成和膜自组装；（2）生命起源和早期P的元素可利用性限制通过大气 - 水 - 岩石相互作用这一单一的全球作用机制得以解决，而无需引入特殊的微环境。只要满足其他有利的地球物理和行星条件，目前的结果支持即使在中性大气下生命起源也很容易的假说。
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