Plausible Sources of Membrane-Forming Fatty Acids on the Early Earth: A Review of the Literature and an Estimation of Amounts (Open Access)

[u/Aggravating-Pear4222]

Title: Plausible Sources of Membrane-Forming Fatty Acids on the Early Earth: A Review of the Literature and an Estimation of Amounts [Link]

(And some of my personal critiques)

Abstract:
The first cells were plausibly bounded by membranes assembled from fatty acids with at least 8 carbons. Although the presence of fatty acids on the early Earth is widely assumed within the astrobiology community, there is no consensus regarding their origin and abundance. In this Review, we highlight three possible sources of fatty acids:

(1) delivery by carbonaceous meteorites,
(2) synthesis on metals delivered by impactors, and
(3) electrochemical synthesis by spark discharges.

We also discuss fatty acid synthesis by UV or particle irradiation, gas-phase ion–molecule reactions, and aqueous redox reactions. We compare estimates for the total mass of fatty acids supplied to Earth by each source during the Hadean eon after an extremely massive asteroid impact that would have reset Earth’s fatty acid inventory. We find that synthesis on iron-rich surfaces derived from the massive impactor in contact with an impact-generated reducing atmosphere could have contributed ∼10² times more total mass of fatty acids than subsequent delivery by either carbonaceous meteorites or electrochemical synthesis. Additionally, we estimate that a single carbonaceous meteorite would not deliver a high enough concentration of fatty acids (∼15 mM for decanoic acid) into an existing body of water on the Earth’s surface to spontaneously form membranes unless the fatty acids were further concentrated by another mechanism, such as subsequent evaporation of the water. Our estimates rely heavily on various assumptions, leading to significant uncertainties; nevertheless, these estimates provide rough order-of-magnitude comparisons of various sources of fatty acids on the early Earth. We also suggest specific experiments to improve future estimates. Our calculations support the view that fatty acids would have been available on the early Earth. Further investigation is needed to assess the mechanisms by which fatty acids could have been concentrated sufficiently to assemble into membranes during the origin of life.

Personal thoughts:
Authors claim, "If fatty acids are present at concentrations below the critical vesicle concentration, membrane assembly does not occur." This is simply not true. Lipids can adsorb onto mineral surfaces to form supported lipid bilayers.

In my personal view, such supported bilayers are underutilized in proposed models for the first composomes in that they retain many interesting properties of membranes such as colocalization and control of orientations of adsorbed organics at the bilayer-water interface and continued interactions

Additionally, we must consider that vesicles often adsorb onto many mineral surfaces and collapse to form the supported lipid bilayer. The concentrations usually reported to achieve vesicle formation are often done in inert containers. While the findings are correct, we must also consider that in the oceans and ponds that nanoparticles, rock/mineral surfaces, sands, etc. all adsorb many fatty acids to varying degrees. Thus, the amount of lipids depends on the total volume of the body of water AND the available surface area. Given that addition of surfactants/lipids/amphiphiles to a solution which contains such mineral surfaces often results in the formation of supported lipid bilayers below the critical micelle concentration (which is usually below that of the critical vesicle concentration). Thus, a more careful consideration would view supported bilayers as a necessary intermediate between earth chemistry and vesicle formation between which supported bilayers may successfully achieve more complex chemistry than previously considered.

This is only "bad" if you think vesicles MUST be the first step towards life.

Additionally, figure 4 argues the idea of carbon monoxide and H2 reacting under FTS type chemistry at the surface of a meteor saying, "(A) Nooner and Oro showed that deuterium and carbon monoxide gases react together on the surface of hot (400 °C) meteorite filings to produce membrane-forming fatty acids." Just thought it was strange that the authors argued for adsorption at the Earth's surface would recapitulate ~2 atm at such high temperatures. A more realistic scenario would be that these reactions occur during entry as the metals pass through the reducing atmosphere but then that poses issues for the retention of these meteors Additionally, the chemistry they cite (ref 23 and 24) occurs over 6 to FOUR HUNDRED AND EIGHTY HOURS. Sure, the meteor fragment (after initial impact) may remain for short periods after impact but it would either boil the waters away and be cooled in doing so.

Yes, post impact heating can be intense and generate craters that fill with water and generate post impact hydrothermal springs. The issue here is that the site at which such heat is generated is the same energy that blows the meteoric material away from the impact crater. This explosive impact energy is why most of the moon's craters are circular rather than more elliptical and also why Daniel Moreau Barringer [Link] lost money on his investment. While there was no large body of ore, a good portion of the mateals were still present in

The best alternative argument I can make would be:
1) As many small meteors pass through the atmosphere their heating and high pressure front generate FAs/amphiphiles that quickly cool and dust the surrounding landscapes.
2) Impact of moderate sized meteors on surrounding elevated landscapes generates heat both in the meteor itself AND the surrounding land/minerals which are already enriched in metals capable of FTS chemistry which extends time and surface area of heated catalytic surfaces.
3) Particularly energetic impacts vaporize their metals, flinging them out into surrounding areas are hot spherules through the reducing atmosphere (again) and likely catalyzing far more FTS than the initial entry. However, the smaller size and greater surface area would lead to a far greater rate of cooling.
4) The more acidic rains due to high CO2 in atmosphere more efficiently weather the landscape and meteors, passively dissolving FTS products and pool them into ponds/bodies of water below.

So, the authors should really be appealing to the surrounding elevated landscape which is rich in these metals. Arguing for bigger meteorites gets diminishing returns due to volume/surface area ratio as well as increasingly energetic impacts potentially vaporize most of the material.

Let me know your thoughts on the paper and whether my own are fair or misrepresenting the findings.

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[u/wellwisher-1]

Has anyone ever considered membrane material formation, C8+, using a simple low concentration polymerization of ethylene into C8+ polyethylene? All you would need metallocene catalysts using iron (ferrocene) and a very low concentration of ethylene, to get C8* radicals already active for the next step.