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One leading theory on the origins of life on Earth proposes that simple chemical molecules gradually became more complex, ultimately forming protocells—primitive, non-living structures that were precursors of modern cells.

A promising candidate for protocells is polyester microdroplets, which form through the simple polymerization of alpha-hydroxy acids (αHAs), compounds believed to have accumulated on early Earth possibly formed by lightning strikes or delivered via meteorites, into protocells, followed by simple rehydration in aqueous medium.

A recent study by the Earth-Life Science Institute (ELSI) at the Institute of Science Tokyo provides new evidence supporting the formation of polyester microdroplets under a wider range of realistic prebiotic conditions than previously thought.

The findings are in the journal ACS Bio & Med Chem Au.

Led by Ph.D. student Mahendran Sithamparam of the Space Science Center (ANGKASA), Institute of Climate Change, National University of Malaysia as the first author and co-supervised by ELSI's Specially Appointed Associate Professor Tony Z. Jia and ANGKASA Research Scientist Kuhan Chandru, the study explored the formation of these microdroplets under conditions more reflective of early Earth.

The team found that polyester microdroplets could form even in salt-rich environments, at low αHA concentrations, and in small reaction volumes.

This expands on previous research, which primarily considered their formation at high concentrations or in larger bodies of water such as coastal areas of lakes or hot springs.

The findings suggest instead that polyester protocells were likely more widespread than previously thought, potentially forming in confined spaces like rock pores or even in high-salt environments such as briny pools or oceanic environments.

In 2019, the research team discovered that polyester microdroplets could form through a simple dehydration process. When gently heated to 80°C, phenyllactic acid (PA), a type of αHA, transitioned into a gel-like substance that subsequently formed membraneless droplets when rehydrated.

In their latest study, the researchers investigated whether these microdroplets could form under more dilute or lower volume conditions, similar to those expected on prebiotic Earth.

"Earlier laboratory tests often used high initial concentrations and volumes of αHAs in the hundreds-of-millimolar or microliter range, respectively, which may not reflect the conditions on prebiotic Earth, where such conditions were unlikely; this is why we needed to push the limits of the polymerization droplet assembly processes to see whether assembly of such protocells would have actually been viable on early Earth," explains Jia.

To simulate these more realistic conditions, the researchers reduced the concentration and volume of PA in synthesis and subsequent droplet formation studies. They found that polyesters could be synthesized and droplets could form with as little as 500 µL of 1 mM PA or 5 µL of 500 mM PA.

This suggests that polyester microdroplets could have naturally emerged both in confined spaces, such as rock pores, or dilute environments, such as those following flooding or precipitation.

To further test real-world conditions, the team simulated reactions in salinities resembling those in the ancient ocean. They introduced 1M NaCl, KCl, and MgCl₂ to the PA reactants, finding that polyester synthesis and microdroplet assembly could proceed in NaCl and KCl but not in MgCl₂.

This suggests that polyester microdroplets would have been more likely to form in water bodies with specific salt compositions, such as those high in NaCl and KCl but low in MgCl₂, favorable to αHA polymerization and subsequent polyester microdroplet assembly.

"The conclusions of this study clearly show that polyester protocells were likely more common on early Earth than previously thought and also informs the next generation of laboratory studies of the system," says Chandru.

"Thus, a wide range of primitive environments—including oceanic, freshwater, briny, and confined spaces like rock pores—could have ultimately supported the formation of these protocells both on Earth or elsewhere."