A carbonate-rich lake solution to the phosphate problem of the origin of life

High Phosphate Concentrations in Present-Day, Carbonate-Rich Lakes

Phosphate concentrations are remarkably high in closed-basin, carbonate-rich lakes and generally increase with increasing carbonate alkalinity (Fig. 1) (see SI Appendix, Appendix A for other dissolved species). Searles Lake, located within the Mojave Desert in California, is an economically important carbonate-, boron-, and sulfate-rich groundwater brine within a dry lake basin that contains 5 to 17 mM phosphate (19). Even higher total phosphorus concentrations are found in Goodenough and Last Chance Lakes in British Columbia, which can exceed 50 mM during periods of desiccation in the summer and autumn (20, 21). Other carbonate-rich lakes, such as Lake Magadi in Africa (22, 23) and Mono Lake in California (24), contain ∼1 mM phosphate.

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Fig. 1.

Phosphorus concentrations in carbonate-rich lakes, including Searles Lake (19, 69), Lake Magadi (22, 23), Goodenough and Last Chance Lakes (20, 36), various other North American lakes (36, 69⇓–71), and various other African lakes (72⇓⇓–75). Inorganic phosphate was determined in most analyses; however, the Goodenough and Last Chance Lake analyses are for total phosphorus (inorganic + organic).

Closed-basin lakes accumulate high salt concentrations, including phosphates, from chemical weathering within their hydrologic basins, followed by the inflow of weathering products via streams to evaporating lake brines (25). Carbonate-rich lakes, in particular, develop when atmospheric or volcanogenic CO₂ gas dissolves in water to form carbonic acid (H2CO30), which chemically weathers rocks to form dissolved cations (Na⁺, K⁺, Ca²⁺, and Mg²⁺) and carbonate alkalinity (CO32− and HCO3−). Upon evaporation of these fluids, Ca²⁺ and Mg²⁺ ions precipitate as carbonates and silicates, and the residual carbonate alkalinity concentrates as Na–HCO₃–CO₃-rich brine (26).

Phosphate is thought to accumulate in carbonate-rich lakes because the carbonate alkalinity strongly precipitates Ca²⁺ from solution as low-solubility Ca²⁺ carbonates, which allows phosphate to reach much higher concentrations before saturating with respect to apatite (27, 28). However, the specific minerals that control Ca²⁺ and phosphate concentrations, and the maximum phosphate concentrations possible, are unknown because apatites and carbonates form poorly defined and/or metastable phases in carbonate-rich brines. Na⁺, Mg²⁺, CO32−, SO42−, and F⁻ ions typically found at high concentrations in carbonate-rich lakes extensively substitute in apatites (26, 29, 30), which greatly influences their solubility, nucleation, and crystallization rate (30). Furthermore, even low phosphate concentrations inhibit the nucleation and growth of both calcite (CaCO₃) and aragonite (a low-temperature CaCO₃ polymorph), so other carbonate phases likely control Ca²⁺ concentrations in carbonate-rich lakes (31). A final issue is that biology cycles organic and inorganic phosphorus in carbonate-rich lakes, which makes it difficult to separate biotic vs. abiotic influences on phosphate concentrations.

Experiments on Phosphate Accumulation in Carbonate-Rich Brines

The preceding considerations motivated a series of experiments to determine how much phosphate can accumulate by abiotic processes in carbonate-rich lakes. We start by noting that carbonate-rich lakes can only accumulate phosphate if Ca²⁺ precipitates in carbonates instead of apatites; otherwise, phosphate will be removed from solution by much higher Ca²⁺ influxes to lake basins. Consequently, how much phosphate can accumulate in carbonate-rich lakes is determined by what phases Ca²⁺ preferentially precipitates in. If Ca²⁺ precipitates as apatite, it will limit phosphate concentrations in the brine. On the other hand, if Ca²⁺ carbonates form, then phosphate can accumulate at higher concentrations.

To investigate Ca²⁺ precipitation in phosphate- and carbonate-rich brines, we slowly added Ca²⁺ using gypsum (CaSO₄·2H₂O) and phosphate using Na₂HPO₄ to various saturated Na₂CO₃/NaHCO₃ brines in the laboratory at room temperature (∼23 °C). We then analyzed the salts that precipitated for mineralogy and chemistry (Methods). Results indicate that the formation of apatite and Ca²⁺ carbonate salts depends primarily on the relative concentrations of carbonate (CO32−) vs. bicarbonate (HCO3−) ions in solution (Fig. 2A). High CO32− ion concentrations strongly precipitate Ca²⁺ from solution as gaylussite (Na₂CO₃·CaCO₃·5H₂O) and/or calcite (open circles in Fig. 2A), which allows phosphate to accumulate up to ∼100 mmol⋅kg⁻¹. Gaylussite is commonly found in carbonate-rich lakes, such as in Searles Lake and nearby Mono Lake (32); however, the formation of calcite is surprising because even trace phosphate levels inhibit calcite nucleation and growth (31). We infer that calcite probably formed indirectly through incongruent dissolution of primary gaylussite based on the slow conversion of gaylussite to calcite we observed over 4 wk (SI Appendix, Appendix B, Fig. S4), which is supported by observations of gaylussite converting into CaCO₃ in Mono Lake (33, 34).

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Fig. 2.

Results from experiments on phosphate- and carbonate-rich brines. (A) The concentration of phosphate in saturated Na–HCO₃–CO₃ brines after 2 wk of equilibration with 50 to 100 mmol⋅kg⁻¹ Ca²⁺ in the form of gypsum (CaSO₄·2H₂O) and initial phosphate concentrations from 10 to 100 mmol⋅kg⁻¹. Solid circles indicate solutions saturated with respect to both apatite and calcite (determined via XRD; see Methods). Open circles indicate solutions that are saturated with respect to calcite and/or gaylussite (Na₂CO₃·CaCO₃·5H₂O); hence, phosphate must be more concentrated than these points for apatite to precipitate (indicated by the upwards arrows). The x axis gives the equivalent percentage of CO32− ions relative to the total carbonate alkalinity, that is, 100×2CO32−/(2CO32−+HCO3−). (B) The modeled pCO₂ and pH of the experimental saturated Na–HCO₃–CO₃ brines at 25 °C, which correspond to the solutions in A.

At low CO32− ion concentrations, Na-rich carbonated apatites and calcite phases occur together, and phosphate concentrations in equilibrium with these minerals decrease with decreasing CO32− (black circles near the left of the vertical axis in Fig. 2A). These results are consistent with Ca²⁺ from carbonate salts controlling the solubility of apatite. A decrease in CO32− ion concentration increases the solubility of calcite/gaylussite, which increases the concentration of Ca²⁺ in solution and causes phosphate to precipitate as apatite.

The relative proportion of CO32− vs. HCO3− ions in solution also controls the pCO₂ and pH, which we model for the experimental saturated Na–HCO₃–CO₃ brines in Fig. 2B (SI Appendix, Appendix B, Table S1 and Methods). By comparing modeled pCO₂ values to recent estimates ranging from 0.01 to 1 bar CO₂ on the early Earth (35), we find that corresponding phosphate concentrations in our experiments are between 20 and 100 mmol⋅kg⁻¹, respectively. Furthermore, the pH ranges from circumneutral at 1 bar pCO₂ to pH 9 at 0.01 bar pCO₂ in saturated Na–HCO₃–CO₃ brines because the relatively high pCO₂ acidifies the solutions. This suggests that elevated phosphate concentrations could have occurred in CO₂-rich atmospheres on the early Earth.

Evaporative Concentration of Phosphate-Rich Brines

Our experiments indicate that when Ca²⁺ is added to phosphate- and carbonate-rich brines, apatite minerals only precipitate above 10 to 100 mmol⋅kg⁻¹ phosphate. Applied to a closed-basin lake setting, this means that phosphate delivered via streams can accumulate up to ∼100 mmol⋅kg⁻¹; however, at higher phosphate concentrations, Ca²⁺ delivered by stream inflows will precipitate phosphate from solution as apatite. Given that streams have much more Ca²⁺ than phosphate (36), closed-basin lakes in hydrologic balance (i.e., stream inflows balance water loss via evaporation) are constrained to <100 mmol⋅kg⁻¹ phosphate. However, once a phosphate-rich lake forms, the brine contains much more phosphate than Ca²⁺ because Ca²⁺ is sequestered into carbonate minerals (see SI Appendix, Appendix A for lake data). Consequently, in the absence of stream inflows, evaporating lake brines can reach much higher phosphate concentrations. Such evaporation may occur seasonally because of wet and dry cycles. Alternatively, lake brines may desiccate in shoreline environments by evaporation on rocks wetted by wave action or evaporation in local pools.

During lake evaporation (in the absence of Ca-rich stream inflows), phosphate will accumulate in the residual brine until it precipitates with a more concentrated cation as a salt. This phosphate-limiting salt cannot be a Ca²⁺ or Mg²⁺ phase because these ions are minor in carbonate-rich brines. We can also exclude K⁺ because it is minor relative to Na⁺ in carbonate-rich lakes and only precipitates phosphate salts at extremely high concentrations (37). The remaining major cation is Na⁺, which implies that phosphate concentrations will increase in evaporating lake brines until the brine reaches saturation with respect to sodium phosphate salts. In addition, high phosphate concentrations would only form after sodium carbonate/bicarbonate and chloride salts had precipitated because phosphate is minor relative to both carbonate alkalinity and chloride.

To determine the maximum phosphate concentrations possible in such brines, we model solutions saturated with respect to sodium phosphate, carbonate, and chloride salts at variable temperature (0 to 50 °C) and CO₂ gas pressure (log pCO₂ −3.5 to 0 bars). We find that up to 3 molal phosphate occurs in equilibrium with either Na₂HPO₄·7H₂O or Na₂HPO₄·2H₂O salts (Fig. 3A). Relative to pure sodium phosphate solutions (Fig. 3A, red lines), phosphate concentrations in saturated mixed brines (Fig. 3A, black lines) decrease by about one-third because the additional Na⁺ associated with chlorides and carbonates forces Na₂HPO₄ to precipitate from solution. The carbonate alkalinity precipitates as either nahcolite (NaHCO₃), natron (Na₂CO₃·10H₂O), or trona (Na₂CO₃·NaHCO₃·2H₂O) depending on the pCO₂, and chloride ions precipitate as halite (NaCl).

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Fig. 3.

Phosphate concentrations and pH modeled in saturated, carbonate-rich lake brines. (A) The concentration of phosphate in solutions saturated with respect to sodium phosphate. The red lines indicate the solubility of pure Na₂HPO₄·XH₂O, where XH₂O refers to the hydration state (X = 2, 7, and 12). The black lines indicate the solubility of sodium phosphate in solutions saturated with respect to sodium phosphate, chloride, and carbonate salts. Solid lines are for 1 bar atmospheric pCO₂, and dashed lines are for log pCO₂ = −3.5 bar (the pCO₂ of present-day Earth). (B) The pH of solutions saturated with respect to sodium phosphate, chloride, and carbonate salts at variable temperature and pCO₂. The range of pH and temperature of these solutions encompasses that of the black lines in A.

The modeled pH of saturated phosphate brines depends on the temperature and atmospheric pCO₂ (Fig. 3B). At present-day pCO₂ levels, solutions are highly alkaline (pH ∼ 10), consistent with high pHs measured in modern soda lakes (38); however, in CO₂-rich atmospheres on the early Earth (log pCO₂ = −2 to 0) brines range from moderately alkaline (pH 9) to slightly acidic (pH 6.5) because of acidification by CO₂. These are maximum pH values because the solutions are saturated with respect to carbonate alkalinity; hence, for undersaturated solutions the pH will be lower. Temperature also affects the pH because CO₂ is more soluble in solutions at lower temperatures. These results provide a plausible environmental scenario for molar phosphate concentrations at moderate pH levels used in prebiotic syntheses (8).

Carbonate-Rich Lakes on the Early Earth

Carbonate-rich lakes may have been relatively common on early Earth due to strong chemical weathering of abundant, fresh volcanic rocks (39) under early Earth’s CO₂-rich atmosphere (40). Weathering would release phosphate from apatites and carbonate alkalinity from other minerals, which would accumulate in closed basins (41, 42). A potential analog for such carbonate-rich lakes on the early Earth is Lonar Lake in central India, a NaHCO₃-rich lake formed from an impact crater in basalt host rock (43). Lonar Lake also precipitates gaylussite and calcite salts (44), but phosphate concentrations are relatively low (∼5 μM) because algal blooms consume the phosphate (45).

A consequence of early Earth’s CO₂-rich atmosphere is that it would have enhanced the weathering of hydroxyl- and fluorapatites in mafic rocks by lowering the pH of surface waters (46, 47). Apatites are more soluble at lower pH and weather more rapidly in CO₂-acidified stream and rainwater, resulting in potentially high phosphate fluxes to carbonate-rich lakes on the early Earth. By modeling the equilibrium solubility of fluorapatite and hydroxyapatite in the presence of calcite buffer as a function of temperature and CO₂ pressure (Fig. 4 and Methods), we find that relatively low CO₂ pressures on present-day Earth (log pCO₂ = −3.5) limit phosphate to ≤1 μM, which is consistent with phosphate concentrations found in most present-day rivers and surface waters. However, in CO₂-rich atmospheres relevant to the early Earth (pCO₂ = 0.01 to 1 bar) (35, 40), modeled phosphate concentrations are 10 to 100 times higher, which implies a much higher phosphate weathering flux on the early Earth from streams into lakes.

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Fig. 4.

Modeled phosphate concentrations (log PO₄) in equilibrium with fluorapatite, hydroxylapatite, and calcite at variable pCO₂ and temperature relevant to the release of phosphate into streams that supply lakes. The pH is given by dashed contours at 0.5-pH intervals.

Phosphate concentrations were likely higher in prebiotic, carbonate-rich lakes because microbial phosphate sinks were absent. Modern carbonate-rich lake environments are some of the most productive ecosystems on Earth in terms of biomass turnover because of high nutrient levels (38). As a result, phosphate measured in modern carbonate-rich lakes is probably a minimum because microbes consume the phosphate and eventually deposit it in organic matter. This may explain why Lake Magadi in Africa, a surface lake with high biomass turnover, has over 10 times lower phosphate concentrations than Searles Lake (which cannot support photosynthetic microbes because it is a groundwater brine), despite the much higher carbonate alkalinity found in Lake Magadi (Fig. 1). Consequently, in the absence of microbial consumption, carbonate-rich lakes on the prebiotic Earth may have hosted higher phosphate concentrations than typically occur today.

A potential sink for soluble phosphate on the early Earth is reduced soluble iron (Fe²⁺), which precipitates with phosphate as the low-solubility mineral vivianite [Fe₃(PO₄)₂·8H₂O]. However, Fe²⁺ also precipitates as siderite (FeCO₃) in carbonate-rich brines, which limits Fe²⁺ to low levels and increases the solubility of phosphate from vivianite, similar to solubility relations in the CaCO₃–apatite system shown in Fig. 2. Furthermore, siderite is less soluble than calcite, and unlike calcite, the nucleation and growth of siderite is not inhibited by phosphate. Although siderite crystallizes relatively slowly from solution at low temperatures (48), near-equilibrium conditions occur on a timescale of hours to days, which is fast relative to inflow and evaporation processes in closed-basin lakes. This implies that Fe²⁺ concentrations in anoxic, phosphate- and carbonate-rich brines will be lower than Ca²⁺, which suggests that Ca²⁺, not Fe²⁺, would have controlled phosphate concentrations in carbonate-rich lakes on the early Earth.

Implications for Prebiotic Chemistry

Our experiments and model results show that carbonate-rich lakes on the early Earth would have accumulated phosphate at concentration and pH levels relevant to laboratory prebiotic syntheses. Molar phosphate concentrations can occur during evaporation in lake marginal pools, on rocks wetted by wave action, and in shore sediments. Drying promotes condensation reactions such as phosphorylation by removing water (49), as well as more generally facilitating polymerization reactions (50). Condensation reactions may also have been promoted by deliquescence of highly soluble evaporites (51). Furthermore, carbonate-rich lakes are rich in clay, zeolite, and silica mineral assemblages that catalyze phosphorylation reactions because of their high reactive surface areas (52). Once phosphate reacts, changing lake levels or wave action could reintroduce the reaction products to the lake, or reaction products could undergo successive wet–dry cycles.

More broadly, closed-basin lakes are ideal sites for “one-pot” organic syntheses because they accumulate and concentrate reagents via evaporation across hydrologic basins (41, 49, 53). As a result, closed-basin lakes may have accumulated organic precursors for phosphorylation, as well as condensation agents that promote phosphorylation by forming reactive intermediate PO3− species (54) and polyphosphates (14). Boron also accumulates at high concentrations in closed-basin lakes (e.g., Searles Lake contains ∼0.5 molal boron; SI Appendix, Appendix A), which promotes the regioselective phosphorylation of ribose and stabilizes the furanose form needed to synthesize nucleic acids (55).

A difficulty for an origin of life in saline lakes is that high salt concentrations tend to destabilize lipid vesicles (56) and inhibit RNA oligimerization (57), but these issues are potentially resolvable. For example, lipid vesicles can stabilize in high-salt environments by binding to nucleotides and/or amino acids (58, 59). Given that concentrated prebiotic reagents and wet/dry cycles often used in the laboratory imply saline environments, more research is needed on prebiotic chemistry in salty solutions.

Recent attempts at “one-pot” syntheses have proposed closed-basin lakes as sites for the formation of amino acids, nucleotides, and lipid precursors in reaction networks involving ultraviolet light, phosphate, sulfur, and cyanide (53, 60). Our results indicate that high phosphate concentrations used in these reaction networks (8) plausibly occurred in carbonate-rich lake environments. Furthermore, the key cyanide component needed for this chemistry also preferentially accumulates in carbonate-rich lakes to make sodium ferrocyanide evaporites, which thermally decompose to sodium cyanide above 700 °C (possibly via magmatic activity or flash heating from asteroid impacts) (42, 53, 61). Concentrated sulfur needed for prebiotic syntheses may also have accumulated as sulfite ions (62). Consequently, carbonate-rich lakes accumulate a number of key reagents at high concentrations relevant to experimental prebiotic syntheses.