Revisiting Darwin’s Little Pond as a Method to Liberate Phosphorus from Apatite Under Prebiotic Hadean Earth Conditions
Doctor of Philosophy (Ph.D.)
Degree Granting Department
Matthew Pasek, Ph.D.
Arthur Omran, Ph.D.
Theresa Evans-Nguyen, Ph.D.
Aurélie Germa, Ph.D.
Zachary Atlas, Ph.D.
Alternative Solvents, Origins of life, Prebiotic minerals
Phosphorus is an essential element for life as we know it. Phosphorus, mainly in the form of phosphate, is key to biologic functions such as genetic material, energy production, and cellular framework. As phosphorus is key to so many important biological functions it is of no wonder the question of how phosphorus was incorporated into life initially is a fundamental question in how life began.
During this time a prebiotic phosphorus source would need to have originated in rock, as phosphorus has not volatile source on Earth. The most prevalent mineral source on a prebiotic Earth would likely have been the apatite mineral group. Apatites are known for being insoluble in a neutral water solvent. Therefore, the question arises how was apatite solubilized in order for phosphorylation to occur? In this dissertation I explore methods to solubilize apatite to liberate phosphorus to allow for phosphorylation reactions to occur.
The focus of chapter 1 and 2 of this dissertation is a literature review. Previous efforts to phosphorylate nucleosides typically involved the use of soluble phosphates, high temperatures, condensing agents, and/or alternative solvents. These studies seldom were performed within the constraints of a prebiotic Earth.
Chapter 3 of this dissertation focuses on my first delve into the use of alternative solvents. We utilized apatite as the phosphorus source for these studies and our solvents were low water activity solvents comprised of 1-2 carbon compounds, and urea. We studied five different solvents: UAFW (urea, ammonium formate, water), UAAW (urea, ammonium acetate, water), UAcW (urea, acetamide, water), UAFAAW (urea, ammonium formate, ammonium acetate, water), and the deep eutectic solvent of UAcAN (urea, acetamide, ammonium nitrate). The evolution of these solvents over time and mild heating (65 & 85 °C) were determined by H-NMR. This showed the solvents containing ammonium formate/acetate evolved to produce formamide/acetamide over time. We also studied the ability of these solvents to liberate phosphorus from lab grade hydroxylapatite by mixing hydroxylapatite in with the solvents and determining the free phosphorus in the solution via ICP-OES. This study demonstrated the ability of the UAFW solvent to liberate phosphorus in appreciable yields from a typically insoluble mineral.
Chapter four of this dissertation explores the question “how robust is this solvent”? The success of the UAFW solvent had previously been demonstrated in published works but was questioned on the robustness of the solvent. We utilized thermodynamic calculations to justify the prebiotic nature of the components within this milieu and the formation of formamide. We then explored the UAFW in multiple ratios to determine the ability of it to remain a liquid state and its ability to liberate phosphorus from lab grade hydroxylapatite. The results of this work revealed that the ability to solubilize apatite was dependent of the ability of the system to remain a liquid and for the system to remain in a liquid state the urea needed to be in a lower molar ratio than ammonium formate. We determined that the molar ratios could vary, but a “goldilocks zone” occurred when the urea in the system had a lower molar ratio than the ammonium formate and when water was not more than 75% (molar) of the total composition. Within the “goldilocks zone” free phosphorus could reach up to ~70-90%.
Chapter five of this dissertation is the final work performed. The idea of chapter five came from a desire to move the system from a “laboratory” to the Hadean Earth. We had previously validated the prebiotic nature of the UAFW, the ability of this system to tolerate various conditions, and the ability of this solvent to solubilize lab grade hydroxylapatite in large percentages. Could this solvent be as effective on natural phosphorus-rich rock samples? For this work I purchased apatite rich rocks, determined the validity of these rocks (via XRD) and then solubilized these rocks in the UAFW (1:2:4 molar ratio) in the same methods as performed previously with the hydroxylapatite. The rocks were heated to remove any biogenic phosphorus from the surface and then crushed and sieved into three different grain sizes (coarse grain sand (0.63-2.0 mm), fine grain sand (0.063-0.2 mm), and silt (< 0.063 mm)). The results from the ICP-OES were then compared to those from the previous studies. We determined that the ability of the UAFW to liberate the phosphorus from the rocks was comparable to that of the lab grade hydroxylapatite and was dependent upon the grain size/surface area of the apatite itself. As the surface area increased, the solubility of the mineral increased, thus liberating more phosphorus. These results were then modeled into a “warm little pond” scenario to demonstrate that in less than 300 years a 2 meter radius pond, half filled with phosphorus-rich silt grains, could reach a molarity of ~1M phosphorus.
The results of this dissertation are helpful to the discovery of how life began. Overcoming the “phosphate problem” has been attempted for decades within this field of research. Although many problems still exist in determining the origin of life, this dissertation provides a sound, prebiotic pathway to liberating the needed phosphorus from an insoluble mineral.
Scholar Commons Citation
Lago, Jennifer, "Revisiting Darwin’s Little Pond as a Method to Liberate Phosphorus from Apatite Under Prebiotic Hadean Earth Conditions" (2021). USF Tampa Graduate Theses and Dissertations.