The phosphate problem of the origin of life describes the unresolved source of phosphorus in prebiotic chemistry. Phosphorus is an essential component of universal biomolecules including DNA, RNA, phospholipids, and ATP, and therefore a requisite of biological information storage, compartmentalization, and metabolism. However, the present understanding of the early Earth during the Hadean eon, when biological life is estimated to have emerged, suggests that phosphorus would have been locked into insoluble apatite minerals and unavailable to undergo reactions leading to the synthesis of prebiotic molecules. The phosphate problem represents a critical gap in knowledge regarding the origin of life. Researchers have attempted to solve the phosphate problem with hypotheses of prebiotic chemistry and geological context that could explain the chemical species of phosphorus or source of phosphate that could have led to the early synthesis of biomolecules. Either phosphorus was available in another form such as phosphite, or there was an environmental mechanism to concentrate bio-available forms of phosphate. Leading hypotheses supported by both environmental observations and laboratory experiments have provided evidence suggesting large closed-basin soda lakes could have concentrated phosphate for theorized prebiotic synthesis mechanisms.
Progress Solving the Phosphate Problem in the Origin of Life There is no consensus within the scientific community on how early life accessed soluble phosphate or any other phosphorus-containing compound. Research into this subject faces the challenge of lacking substantial environmental evidence from the earliest periods of Earth's history due to plate tectonics, weathering, and other natural phenomena changing and recycling Earth's surface. However, some geological clues, laboratory experiments, and observations of phosphate in our current environment have generated a limited number of hypotheses. Knowing the geological context is helpful in understanding the predominant phosphate source hypotheses. During the mid- to late-Hadean, Earth's surface, ocean, and atmosphere were significantly different from the environment we observe today. The surface of the Earth would have been covered by a shallow, acidic ocean, with land only beginning to form as volcanic islands with some continental crust. Meteorites bombarded the Earth's surface creating craters and ocean basins. The atmosphere was composed of carbon dioxide, methane, and nitrogen, but lacked oxygen. Without oxygen, the Earth had a reducing atmosphere, not conducive for the persistence of soluble and oxidized phosphate. Even today, phosphorus is rarely found in high concentrations in nature because, in water, it binds to calcium to form insoluble apatite minerals ( Ca 5 ( PO 4 ) 3 ( OH , F , Cl ) {\textstyle {\ce {Ca5(PO4)3(OH,F,Cl)}}} ).
Soda Lakes Concentrated Phosphate to Levels Relevant for Prebiotic Chemistry Evidence suggests that carbonate-rich lakes (also referred to as soda lakes) could provide answers to the phosphate problem. In most bodies of water, calcium binds to phosphate to form apatite minerals. This renders the phosphorus inaccessible for prebiotic synthesis. Soda lakes differ in that carbonates bind to calcium in the form of dolomite ( CaMg ( CO 3 ) 2 {\textstyle {\ce {CaMg(CO3)2}}} ), removing it from the environment and allowing dissolved phosphate to remain. The highest known naturally occurring dissolved phosphate concentration reaches up to 37 millimolar (mM) in Last Chance Lake. Researchers have used this and other soda lakes as an analog for early Earth environments that could have led to the origin of life. Extant soda lakes generally form in closed basins (i.e., that lack outflows) and on volcanic bedrock, such as basalt, the weathering of which provides phosphate and carbonates. Streams or springs carry phosphate into lakes and evaporation can concentrate phosphate in the carbonate-rich lake water. Loss of phosphate into relatively insoluble calcium phosphate (or apatite) is small because calcium is precipitated into dolomite instead. A geochemical imbalance of more sources of phosphate than losses allows dissolved phosphate to build up to higher concentrations than in other bodies of water. It is hypothesized that soda lakes were common on early Earth. A CO2-rich atmosphere would have increased weathering rates, volcanic rocks would have been abundant, and meteorite impacts would have made closed crater basins. Without biology as a phosphate sink before the origin of life, calculations and laboratory experiments suggest sustainable concentrations between 1-100 mM of dissolved phosphate, depending on how efficient phosphorus was recycled back into the system. At such levels, cyanosulfidic prebiotic synthesis of RNA building blocks is plausible.
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