A deeper dive into primitive polylactate polymerization and microdroplet assembly under restrictive early Earth conditions

Abstract

Alpha hydroxy acids (AHAs) play various roles in modern biology such as in metabolism, but could have played different roles as “non-biomolecules” at the origins of life. Specifically, lactic acid (LA) and other AHAs can polymerize into polyesters and subsequently assemble into membraneless microdroplets (MMDs) upon dehydration-rehydration cycles, suggesting that polyester MMDs could have been relevant protocell models on early Earth. However, in particular, how the harsh conditions of early Earth, namely salinity and decreased reactant volume/concentration, could have affected primitive LA polymerization and subsequent MMD assembly is unclear. In this study, we found that while monovalent salts such as NaCl and KCl did not inhibit LA polymerization and MMD assembly even at high concentrations (up to 1 M), divalent salts such as MgCl2 and CaCl2 were inhibitory at much lower concentrations. Additionally, we found that lower reaction volumes and concentrations, potentially simulating confined microenvironments on early Earth, still supported LA polymerization and MMD assembly to some extent, albeit with decreased efficiency. These results suggest that early Earth’s environmental conditions could have still supported the polymerization of LA and the formation of MMD-based protocells in a variety of settings despite being quite harsh and variable. Lactic acid (LA), likely abundant on early Earth, can polymerize into polyesters and assemble into membraneless microdroplets (MMDs). However, how the harsh conditions of early Earth could have affected primitive LA polymerization and subsequent MMD assembly is unclear. In this study, we probed the effect of salinity, reaction volume, and reaction concentration on primitive LA polymerization and subsequent MMD assembly, and find that early Earth’s harsh environmental conditions could have still supported the formation of LA-based MMD protocells in a variety of settings.