Model Atmospheric Aerosols Convert to Vesicles upon Entry into Aqueous Solution

Abstract

Aerosols are abundant on the Earth and likely played a role in prebiotic chemistry. Aerosol particles coagulate, divide, and sample a wide variety of conditions conducive to synthesis. While much work has centered on the generation of aerosols and their chemistry, little effort has been expended on their fate after settling. Here, using a laboratory model, we show that aqueous aerosols transform into cell-sized protocellular structures upon entry into aqueous solution containing lipid. Such processes provide for a heretofore unexplored pathway for the assembly of the building blocks of life from disparate geochemical regions within cell-like vesicles with a lipid bilayer in a manner that does not lead to dilution. The efficiency of aerosol to vesicle transformation is high with prebiotically plausible lipids, such as decanoic acid and decanol, that were previously shown to be capable of forming growing and dividing vesicles. The high transformation efficiency with 10-carbon lipids in landing solutions is consistent with the surface properties and dynamics of short-chain lipids. Similar processes may be operative today as fatty acids are common constituents of both contemporary aerosols and the sea. Our work highlights a new pathway that may have facilitated the emergence of the Earth's first cells.

Figure 1

(A) Schematic representation of aerosol to vesicle transformation. (B) Experimental apparatus and workflow. Compressed N₂ gas (a) was fed through a drying column (b) to a pressure regulator (c) connected to a reservoir of spray solution (d). Simultaneously, the dry gas fed two mass flow controllers (e) used to regulate the relative humidity of the downstream carrier gas with a water bubbler (f) and a humidity sensor (g). The spray solution and the carrier gas met at the nozzle of a concentric borosilicate glass nebulizer (h). The generated aerosols passed through a flow tube (i) which removed larger droplets due to gravitational settling. The aerosols were conveyed by polyurethane antistatic tubing (j) to a collection chamber (k) containing the landing solution. A transmission electron microscopy (TEM) photograph showing aerosols transformed into vesicles. The spray and landing solutions were 20 mM 2:1 oleic acid/octadecenol with 10 mg·mL^–1 ferritin, and 20 mM 2:1 decanoic acid/decanol, respectively. Panel (A) used modified templates from Servier Medical Art, licensed under a Creative Commons Attribution 3.0 Unported License. Panel (B) was made with BioRender.

Figure 2

Effect of lipid composition on aerosol to vesicle transformation efficiency. (A) Aerosol to vesicle transformation efficiency as a function of lipid concentration in the spray solution for lipids of different chain lengths. The composition of the landing solutions was 20 mM C10:1, 1.0 mM C14:1, and 0.5 mM C16:1. Data were fit with a second-order polynomial. (B) Impact of chain length when all landing solutions contained 20 mM lipid. (C) Effect of aerosol lipid composition when landing in 2:1 decanoic acid/decanol. The concentration of lipid in the spray and landing solutions was 20 mM. For all panels, landing and spray solutions always contained 0.2 M bicine, pH 8.0. C7:0, C10:0, C14:1, and C16:1 indicate heptanoic acid, 2:1 decanoic acid/decanol, 2:1 myristoleic acid/tetradecenol, and 2:1 palmitoleic acid/hexadecenol, respectively. Spray solutions contained 10 mM HPTS. Data are average ± standard deviation (SD) of three independent experiments.

Figure 3

Negative-stained TEM images at 2200× magnification of a landing solution purified by size exclusion chromatography before (A) and after (B) contact with aerosols. The landing solution contained 20 mM 2:1 decanoic acid/decanol. Aerosols possessed 20 mM 2:1 oleic acid/octadecenol and 10 mg·mL^–1 ferritin. Both solutions were in 0.2 M bicine, pH 8.0.