Prebiotic Vesicles Retain Solutes and Grow by Micelle Addition after Brief Cooling below the Membrane Melting Temperature

Abstract

Replication of RNA genomes within membrane vesicles may have been a critical step in the development of protocells on the early Earth. Cold temperatures near 0 °C improve the stability of RNA and allow efficient copying, while some climate models suggest a cold early Earth, so the first protocells may have arisen in cold-temperature environments. However, at cold temperatures, saturated fatty acids, which would have been available on the early Earth, form gel-phase membranes that are rigid and restrict mobility within the bilayer. Two primary roles of protocell membranes are to encapsulate solutes and to grow by incorporating additional fatty acids from the environment. We test here whether fatty acid membranes in the gel phase accomplish these roles. We find that gel-phase membranes of 10carbon amphiphiles near 0 °C encapsulate aqueous dye molecules as efficiently as fluid-phase membranes do, but the contents are released if the aqueous solution is frozen at −20 °C. Gelphase membranes do not grow measurably by micelle addition, but growth resumes when membranes are warmed above the gel−liquid transition temperature. We find that longer, 12carbon amphiphiles do not retain encapsulated contents near 0 °C. Together, our results suggest that protocells could have developed within environments that experience temporary cooling below the membrane melting temperature, and that membranes composed of relatively short-chain fatty acids would encapsulate solutes more efficiently as temperatures approached 0 °C.

Evidence of Heritability in Prebiotically Realistic Membrane-Bound Systems

“…One potential source of noise is variation in the duration of freezing. However, although the time that the samples spent in a frozen state varied among generations, all samples that were compared to each other (e.g., EPS samples from two different generations) were thawed an equal number of times prior to analysis, which is important because freezing and thawing change the membrane dynamics [ 37 ]. Moreover, the TR and NTC samples from a given generation were always frozen and thawed at the same time.…”