Effects of Amino Acids on Phosphate Adsorption Onto Iron (Oxy)hydroxide Minerals under Early Earth Conditions

Abstract

Phosphate, an essential molecule in biochemistry, is not abundant in modern oceans and would have been even less abundant in early Earth’s oceans. One possible mechanism for concentrating phosphate for prebiotic reactions is adsorption onto iron (oxy)hydroxide minerals, which would have precipitated from interactions between iron-rich oceans of the early Earth and near neutral-alkaline hydrothermal fluids. In this work, we synthesized ferrous and ferric (oxy)hydroxides to test their adsorptivity toward phosphate under early Earth oceanic conditions (anoxic, dissolved Fe²⁺, pH 6–9, and low phosphate levels). Prebiotically relevant amino acids (cysteine, histidine, and arginine) were added to test their effect on phosphate adsorption. Colorimetry techniques coupled with nuclear magnetic resonance and statistical analysis were utilized to determine how experimental conditions influenced the adsorption reaction. We observed an 80–90% reduction of ferric to ferrous iron minerals in the presence of cysteine; we hypothesize that iron and cysteine underwent a redox reaction to produce cystine. In addition, phosphate was readily adsorbed onto iron (oxy)hydroxide minerals, but their efficacy depended on the iron redox state and pH at which the minerals were precipitated. Phosphate adsorption was the greatest with ferrous (oxy)hydroxide minerals precipitated at pH 9, reaching a maximum average adsorption of 45%. Under these conditions, the addition of organics significantly enhanced phosphate adsorption by an additional ∼30%; differences due to the amino acid side chain were not statistically significant. This work shows how environmental conditions (redox state, pH, and presence of organics) influenced adsorption in a simulated mineral system; such systems merit further study under increasingly complex conditions in order to better understand phosphate dynamics on wet-rocky worlds such as early Earth or Mars.