Effects of Organic Compounds on Dissolution of the Phosphate Minerals Chlorapatite, Whitlockite, Merrillite, and Fluorapatite: Implications for Interpreting Past Signatures of Organic Compounds in Rocks, Soils and Sediments

Abstract

Phosphate is an essential nutrient for life on Earth, present in adenosine triphosphate (ATP), deoxyribonucleic acid (DNA), ribonucleic acid (RNA), and phospholipid membranes. Phosphorus does not have a significant volatile phase, and its release from minerals is therefore critical to its bioavailability. Organic ligands can enhance phosphate release from minerals relative to release in inorganic solutions, and phosphorus depletion in paleosols has consequently been used as a signature of the presence of ligands secreted by terrestrial organisms on early Earth. We performed batch dissolution experiments of the Mars-relevant phosphate minerals merrillite, whitlockite, chlorapatite, and fluorapatite in solutions containing organic compounds relevant to Mars. We also analyzed these phosphate minerals using the ChemCam laboratory instrument at Los Alamos, providing spectra of end-member phosphate phases that are likely present on the surface of Mars. Phosphate release rates from chlorapatite, whitlockite, and merrillite were enhanced by mellitic, oxalic, succinic, and acetic acids relative to inorganic controls by as much as >35 × . The effects of the organic compounds could be explained by the denticity of the ligand, the strength of the complex formed with calcium, and the solution saturation state. Merrillite, whitlockite, and chlorapatite dissolution rates were more strongly enhanced by acetic and succinic acids relative to inorganic controls (as much as >10 ×) than were fluorapatite dissolution rates (≲2 ×). These results suggest that depletion of phosphate in soils, rocks or sediments on Mars could be a sensitive indicator of the presence of organic compounds.

The Role of Sulfuric Acid, Abiotic–Organic Acids, and Biotic Acids on Serpentinite Dissolution and Trace Metal Release

“…Results from Model 1, designed to test differences in serpentinite reactivity in inorganic, abiotic-organic, and biotic systems, show that across a wide range of temperatures, Mg and Si release from serpentinite in the presence of biotic acids is significantly enhanced compared with Mg and Si release in the presence of nitric acid (Figure 1A,B; Table 2). This is consistent with previous laboratory and terrestrial field studies that show that organic acids accelerate mineral weathering [28][29][30][31][32][33][34][35][36][37][38][39].…”

“…On Earth, studies have shown that organic acids produced by microorganisms and plants enhance mineral dissolution rates in weathering environments [28][29][30][31][32][33][34][35][36][37][38][39]. In addition to accelerated dissolution, some trace elements may be preferentially released in the presence of organic acids during water-rock interaction [36,[40][41][42], which may result in a preserved record of mineral-microbe interactions.…”

Abiotic formation of ribose 5'-phosphate from ribose and apatite with carbonate- and formate-rich solutions

“…1). The same has been observed with acetate in a previous study (Bartlett et al, 2018). Formates and carbonates generated more concentrated phosphate solutions than the experiments in the absence of these chemicals at pH 8.…”

Phosphates on Mars and Their Importance as Igneous, Aqueous, and Astrobiological Indicators

“…Laboratory experiments have demonstrated the effect of organic acids on phosphatebearing whole rocks [7,[239][240][241] as well as individual minerals in the presence of organic compounds [242][243][244][245][246][247] including the Mars-relevant Ca-phosphate-bearing minerals fluorapatite, merrillite, and whitlockite [248]. The dissolution of apatite-bearing rocks in the presence of organic compounds has shown enhancement of phosphate release in the presence of acetate, benzoate, citrate, formate, fumarate, gallate, glutarate, lactate, malonate, oxalate, phthalate, salicylate, and succinate [7,[239][240][241].…”

“…The dissolution of apatite-bearing rocks in the presence of organic compounds has shown enhancement of phosphate release in the presence of acetate, benzoate, citrate, formate, fumarate, gallate, glutarate, lactate, malonate, oxalate, phthalate, salicylate, and succinate [7,[239][240][241]. The dissolution of fluorapatite in experiments containing organic compounds has indicated enhanced dissolution in the presence of acetate, citrate, oxalate, phthalate, and salicylate [243,246], and the dissolution of multiple Ca phosphate minerals relevant to Mars in experiments containing organic compounds relevant to Mars has indicated that the enhancement of dissolution by the organic compounds is likely due to the ligand denticity, the strength of the complex formed between the organic compound and calcium, and the saturation state of the solution [248]. The dissolution of hydroxyapatite in the presence of amino acids has indicated enhanced release of phosphate in the presence of aspartic acid but not alanine or lysine, which is attributed to the charge on the amino acids [244].…”