Last Chance Lake in September 2022. At the end of the summer, the water has almost all evaporated, leaving a salty crust on the surface.
Credit: Zachary R. Cohen/Washington University
According to a new study by Zachary R Cohen of the Chemistry Department of Washington University, Seatle, WA, USA and colleagues, soda lakes which are rich in sodium and carbonates could have provided the right conditions for the simplest, RNA-based cells to have arisen.
It is widely assumed that the earliest simple cells were based on RNA enclosed in a lipid membrane but there is a problem in that RNA requires divalent ions such as Magnesium (Mg2+ to function, but Mg2+ damages lipid membranes, so it isn't easy to explain how they could have co-existed within the same cell structure.
So the question is whether the low levels of (Mg2+ found in soda lakes, such as the Last Chance Lake in British Columbia, Canada, could have provided the right conditions of enough (Mg2+ for RNA to function but not so much as to prevent the formation of an enclosing membrane.
To explore this possibility, Zachary Cohen and his colleagues collected water from Last Chance Lake and the similar but subtly different Goodenough Lake after seasonal evaporation. According to information from Washington University published in phys.org:
These soda lakes each contained ~1 M Na+ and ~1 mM Mg2+ at pH 10. The authors found that spontaneous extension of short RNA primers occurred in lake water at a rate comparable to the rates in standard laboratory conditions.The scientists give more technical details in their open access paper in PNAS Nexus:
The authors added fatty acids, which could have been available on the early Earth, to the lake water to see if the molecules would assemble into membranes. The membranes formed in dilute water that simulates a rainfall event, and the membranes persisted even when surrounded by concentrated lake water from the dry season.
According to the authors, soda lakes on the early Earth could have supported key features of protocell development, with RNA copying and ribozyme activity taking place in the dry season and vesicle formation occurring during the wet season.
AbstractSo, little doubt there then that the simple beginnings of RNA-based cellular organisms could have arisen in similar conditions on an early Earth. What creationists frauds will now need to do is prime their dupes to parrot the phrase - 'So intelligence was needed, then!" because scientists did what creationists have been demanding they do and shown that living organisms could have arisen naturally from inorganic precursors by replicating it in a lab. It matters not to creationists frauds that this exposes them as disingenuous because they can rely on their dupes not having the logical ability to work out how.
The origin of life likely occurred within environments that concentrated cellular precursors and enabled their co-assembly into cells. Soda lakes (those dominated by Na+ ions and carbonate species) can concentrate precursors of RNA and membranes, such as phosphate, cyanide, and fatty acids. Subsequent assembly of RNA and membranes into cells is a long-standing problem because RNA function requires divalent cations, e.g. Mg2+, but Mg2+ disrupts fatty acid membranes. The low solubility of Mg-containing carbonates limits soda lakes to moderate Mg2+ concentrations (∼1 mM), so we investigated whether both RNAs and membranes function within these lakes. We collected water from Last Chance Lake and Goodenough Lake in Canada. Because we sampled after seasonal evaporation, the lake water contained ∼1 M Na+ and ∼1 mM Mg2+ near pH 10. In the laboratory, nonenzymatic, RNA-templated polymerization of 2-aminoimidazole-activated ribonucleotides occurred at comparable rates in lake water and standard laboratory conditions (50 mM MgCl2, pH 8). Additionally, we found that a ligase ribozyme that uses oligonucleotide substrates activated with 2-aminoimidazole was active in lake water after adjusting pH from ∼10 to 9. We also observed that decanoic acid and decanol assembled into vesicles in a dilute solution that resembled lake water after seasonal rains, and that those vesicles retained encapsulated solutes despite salt-induced flocculation when the external solution was replaced with dry-season lake water. By identifying compatible conditions for nonenzymatic and ribozyme-catalyzed RNA assembly, and for encapsulation by membranes, our results suggest that soda lakes could have enabled cellular life to emerge on Earth, and perhaps elsewhere.
Significance Statement
Where did cells originate on the early Earth? The first cells (protocells) are thought to have consisted of informational and catalytic RNAs inside membrane vesicles. However, RNA function requires divalent cations, such as Mg2+, whereas membranes of environmentally available amphiphiles (e.g. fatty acids) are disrupted by divalent cations. Here, we show that natural soda lake water, which contains ∼1 mM divalent cations, could provide a suitable environment for three processes likely important for the origin of cellular life: nonenzymatic RNA polymerization, ribozyme activity, and encapsulation by prebiotic membranes. Our results suggest that soda lakes deserve further study as potential environments for the formation of the first cells.
Introduction
An early stage of life on Earth may have consisted of protocells that had genetic molecules encapsulated by lipid membranes (1). RNA may have served an important role in protocells because it can store and transmit genetic information and catalyze reactions (2). Protocell membranes may have been composed of fatty acids, which enable the exchange of small molecules like RNA building blocks and metabolites with the external environment while encapsulating large polymers (3). However, typical concentrations of divalent cations like Mg2+ that promote nonenzymatic RNA copying (4) and ribozyme activity (5) disrupt fatty acid membranes, leading to the release of encapsulated solutes (6). Chelation of divalent cations by excess citrate preserves fatty acid vesicles and enables internal RNA reactions (7), but high concentrations of citrate on the early Earth are unlikely. Serine and glycine maintain the integrity of fatty acid vesicles in 10 mM Mg2+ by increasing lamellarity (8), but the assembly of RNA has not been investigated inside these vesicles. Interestingly, nonenzymatic synthesis of a genetic polymer similar to RNA (NP-DNA (3)) and ribozyme activity (9, 10) have both been observed within fatty acid vesicles when the concentration of divalent cations is below 4 mM.
Soda lakes, in which Na+ and (bi)carbonate are the dominant dissolved species (Fig. 1), are promising sites for the co-assembly of RNA and fatty acids into protocells. These lakes contain 0.1–1 M of bicarbonate and carbonate anions (11). Because salts of divalent cations and carbonate anions, i.e. (Ca, Mg, Fe)CO3, have poor solubility, the concentration of divalent cations in soda lakes is on the order of ∼1 mM (12). Additionally, soda lakes on the early Earth may have concentrated phosphate (13), ferrocyanide (14), and sulfur species (15), potentially enabling the synthesis of ribonucleotides during photochemically driven reductive homologation of hydrogen cyanide (16, 17). Fatty acids could have been delivered to Earth by meteorites (18), synthesized endogenously on metal surfaces (19), or synthesized during electrochemical sparking (20), and those fatty acids could have been subsequently concentrated in soda lakes via evaporation to enable membrane formation (21). Although there is uncertainty about how much subaerial continental crust was present on the early Earth (22), zircon geochemistry shows that continental crust was present from ∼4.4 Gya and plausibly consistent with surface exposure (23). Also, volcanic hotspot islands and extensive plateaus are expected (24), where the former are sites of soda lakes today (25). Indeed, modern soda lakes are common on volcanic terrain (26).
Because soda lakes provide unique opportunities for the prebiotic formation of RNA and fatty acid vesicles, and because relatively low concentrations of divalent cations have been shown to enable RNA assembly within fatty acid vesicles, we investigated key components of protocell assembly within natural soda lake water. We collected water samples from Last Chance Lake and Goodenough Lake on the Cariboo Plateau in British Columbia, Canada, in November 2021. These soda lakes have some of the highest soluble phosphate concentrations of lakes on Earth (27), with Last Chance Lake reaching the highest of all. We collected after summertime evaporation when these lakes each contained ∼1 M Na+ and ∼1 mM Mg2+ at pH 10 (Tables S1–S3). We tested for nonenzymatic, RNA-templated synthesis of RNA in water from both soda lakes. Additionally, we established the activity of a ligase ribozyme in natural lake water, and we tested whether membranes composed of fatty acids and fatty alcohols can encapsulate solutes in natural lake water. Our findings directly inform the search for natural environments that could have supported the origin of cellular life on Earth.Fig. 1.Composition of soda lake waters used in experiments, illustrating their dominant sodium–carbonate–chloride composition (Eq/L = molar concentration × charge). Positive charge on the left of each pie chart is balanced by negative charge on the right. The data are for single measurements of water samples collected in November 2021. All concentrations, including smaller components <10 mM, such as Mg2+, Ca2+, and PO43− (measured by Haas et al. (27)), are shown in Tables S1–S3.
Nonenzymatic, RNA-templated synthesis of RNA
Before enzymes emerged on the early Earth, short RNAs that formed during wet-dry cycling (28), freeze-thaw cycling (29), or on mineral surfaces (30) may have been replicated by successive additions of chemically activated ribonucleotides. For example, after pairing between a short RNA primer and the template, 2-aminoimidazole-activated nucleotides can base pair with the template and react nonenzymatically with the primer to form a 3′-5′ phosphodiester linkage (Fig. 2A). Unlike polymerization of triphosphate nucleotides, 2-aminoimidazole-activated nucleotides spontaneously form 5′-5′ bridged dinucleotide intermediates that enable rapid, nonenzymatic extension of RNA primers (31). Separation of the strands may enable a new cycle of polymerization to generate a copy of the original template. Nonenzymatic primer extension may have been a necessary step in the formation of the first ribozymes, so we investigated whether this process can proceed in soda lake water.Fig 2.
RNA primers on RNA templates are extended at comparable rates in soda lake water and in standard laboratory conditions. A) Schematic of RNA primer extension by 2-aminoimidazole-activated ribonucleotides. Our data suggest that primer extension in lake water proceeds through a bridged dinucleotide intermediate (Fig. S3), which is consistent with previous results (31). The primer is fluorescently labeled at the 5′ end. B) Gel image showing RNA-templated extension of an RNA primer in Last Chance Lake water within 10 min. RNAs smaller than the primer were not detected, suggesting that the RNA primer is not hydrolyzed. C) First-order rate constants for extension of an RNA primer by one nucleotide. Standard laboratory conditions are 50 mM MgCl2 and 0.2 M bis-tris-propane at pH 8, and artificial lake water is 0.5 M Na2CO3, 1 mM MgCl2, and 0.2 M bis-tris-propane at pH 10. Error bars correspond to the SD from three independent experiments in lake waters, and six independent experiments in standard laboratory conditions. LC is Last Chance Lake and GE is Goodenough Lake.
[…]
Conclusion
Our results suggest that soda lakes on the early Earth could have supported key features of protocell development. The nonenzymatic synthesis of RNA proceeds on RNA templates at a comparable rate in soda lake water as under standard laboratory conditions. We find that the yield of nonenzymatic RNA polymerization increases with the concentration of activated nucleotides. Activated nucleotides could have reached high concentrations during evaporation from natural lakes, although continued nucleotide synthesis may become limited by the availability of phosphate [∼40 mM in Last Chance Lake, but potentially higher in prebiotic lakes (13, 27)]. 2-Aminoimidazole could be produced from nucleotide precursors (47), and nucleotide activation with 2-aminoimidazole could occur in the eutectic brines of partially frozen lakes (34). While nonenzymatic copying of sequence-general RNA templates remains challenging (48), our results show that soda lakes are capable of supporting nonenzymatic RNA synthesis in a comparable manner to optimized laboratory conditions.
[…]
Additionally, we show that prebiotic membranes can encapsulate solutes despite flocculation in soda lake water. Although the internal volume of vesicles may decrease as the external salt concentration increases, our results show that material can remain encapsulated after the vesicles swell again following dilution of the salt. It is likely that nonenzymatic RNA synthesis (50) and ribozyme-catalyzed ligation (10) can proceed inside flocculated vesicles whose exterior solution is soda lake water…Fig 6.
A hypothetical development cycle for protocells based on seasonal fluctuations in the water content of soda lakes.
Zachary R Cohen, Dian Ding, Lijun Zhou, Saurja DasGupta, Sebastian Haas, Kimberly P Sinclair, Zoe R Todd, Roy A Black, Jack W Szostak, David C Catling,
Natural soda lakes provide compatible conditions for RNA and membrane function that could have enabled the origin of life PNAS Nexus, Volume 3, Issue 3, March 2024, pgae084, https://doi.org/10.1093/pnasnexus/pgae084
Copyright: © 2024 The authors.
Published by Oxford University Press. Open access.
Reprinted under a Creative Commons Attribution 4.0 International license (CC BY 4.0)
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