Path 5: Life Without Phosphorus – Alternate Backbone Linkers and Energy Carriers

Rationale: All known life relies on phosphorus (P) in critical ways: the phosphate (PO₄³⁻) group forms the backbone links of DNA/RNA and the high-energy bonds of ATP, and phospholipids build cell membranes. This path asks: Is P truly indispensable, or could life use an alternative element for these roles? The question gained fame with the controversial 2010 NASA study claiming a bacterium (GFAJ-1) could use arsenic instead of phosphorus in DNA – an assertion later refuted. Nonetheless, exploring “phosphorus-free biochemistry” remains important to understanding life’s flexibility. Perhaps on worlds where P is scarce, life might evolve to use, say, arsenate (AsO₄³⁻) as a substitute (arsenic sits below phosphorus in the periodic table). Or life could avoid that class of molecule entirely – using sulfonate, carbonate, or peptide linkages in place of phosphodiesters, and storing energy in compounds like thioesters (sulfur-based) instead of ATP’s phosphoanhydride. This path also relates to origin-of-life theories: some models suggest early life used thioester compounds as an energy currency before ATP arose. By researching life without P, we test how unique Earth’s solution is – could “alien DNA” have a non-phosphate backbone? Could cells form membranes without phospholipids (perhaps using sulfolipids or other amphiphiles)? Understanding this has practical implications: if we find environments (e.g. certain ocean worlds) that are leaching phosphorus and might be P-poor, can life there adapt? Additionally, this path touches on biohazard mitigation: if we ever create artificial life without P (using different backbones and energy molecules), it might be orthogonal (unable to exchange genes with Earth life, a biosafety feature).

Prerequisites: First, a clear chemical understanding of why phosphate is used by Earth life. Research by Benner (2002) and others indicates the phosphate diester backbone has special properties: it’s negatively charged (polyanionic), preventing DNA strands from folding on themselves and enabling predictable base-pairing. Any alternative must replicate these functions. So a prerequisite is identifying candidate linker chemistries that could form a stable information polymer. Candidates include: arsenate esters (chemically similar but known to be much less stable in water – arsenate diesters hydrolyze ~100× faster than phosphate, causing chain breakdown); sulfate or sulfonate links (stable but doubly charged and bulkier); peptide-like links (neutral amide bonds as in PNA, but then polymer becomes uncharged and tends to misfold). The infamous GFAJ-1 experiment showed how tricky this is: even if a cell is forced into high arsenate conditions, it struggled – later analysis showed the bacterium did not actually build DNA with arsenate reliably, it was just surviving toxicity by scavenging trace phosphate. So prerequisites include in vitro experiments: e.g., synthesize DNA analogs with arsenate or other linkers and test their stability in aqueous solution. Another prerequisite: exploring non-phosphorus membranes – some Earth organisms already hint at this (certain marine bacteria substitute phosphate lipids with sulfur-containing lipids when starved of P). Understanding if stable bilayers can form from wholly P-free amphiphiles (like SQDG – sulfoquinovosyldiacylglycerol – which has sulfur instead of phosphate) is important. Tools needed range from organic synthesis (to make analog nucleotides or lipids) to evolution experiments (e.g., evolving a polymerase enzyme to accept a non-phosphate backbone). Also, knowledge of geochemistry: Are there environments with abundant arsenic but low phosphorus (like certain alkaline lakes)? Life there would be a testing ground.

Dependencies: Path 5 connects with Path 6 (XNA and alternate genetics) because many XNA polymers also explore different backbones (e.g., PNA has no phosphate). Insights from XNA studies show what happens when backbone charge is removed or altered. It’s dependent on Path 1 as well: in some solvents, phosphate might not be the best choice (phosphate works well in water, but maybe in formamide another linker is preferable). For instance, Benner et al. noted that repeating charge backbones might be universal in water, but if our life formula changes solvent, perhaps not. So interplay with solvent research is relevant. There’s also a dependency on metabolism studies – Path 5 isn’t only about genetic material, but also about energy molecules (ATP alternatives). Here it links to origin-of-life (did life start with thioesters? de Duve’s “Thioester World” concept). Understanding that can suggest how a modern life form might run without ATP. Dependencies on analytical chemistry exist too: detecting life that doesn’t use phosphate might require different techniques (our standard nucleotide-detecting methods might miss it – similar to how Cleland & Copley point out life not using DNA/RNA would evade many detection methods reliant on nucleic acid amplification). Finally, Path 5 depends on biochemistry: we may need engineered enzymes that bind alternative backbones or transfer alternative high-energy groups, which ties into synthetic biology capabilities (Path 6).

Signs of Progress: A concrete sign would be an enzyme or organism that functions with an alternate backbone or energy currency. For example, if a bacterium can be engineered to replace ATP with another molecule (say, polyphosphate or a thioester-based metabolism) and still grow, that demonstrates flexibility. Already, some organisms can survive with reduced phosphorus by using polyphosphate storage or replacing membrane phospholipids with non-P lipids – these are partial successes. A landmark achievement would be the in vitro evolution of a DNA/RNA analog polymer that lacks phosphate but still undergoes replication and evolution. Pinheiro et al. (2012) partially achieved this with XNAs (some XNAs like TNA, glycol-DNA, etc., still have phosphate backbones, but others like PNA or sulfone DNA have modifications) – showing heredity without natural DNA. If an XNA with a non-phosphate linkage (e.g., a sulfone or peptide linkage) is shown to faithfully transmit information, that’s a breakthrough. Another sign: stable “arseno-DNA” in lab (even if life doesn’t use it naturally) – if someone could create a long arsenate DNA strand and show it can hold genetic info for a while, that informs this path (though arsenate’s instability makes this unlikely with current chemistry). On the metabolism side, detecting evidence of thioester-utilizing life would be huge: for instance, if in a strange habitat we found life forms that completely lack ATP/ADP and instead use acetyl-CoA analogs as their energy intermediate (we can detect those via metabolomics). In the lab, a proto-cell that uses acetyl-thioesters to drive polymerization (instead of ATP) could be constructed as a proof of concept. Finally, an unambiguous biosignature on another world of life that doesn’t produce any phosphates (e.g., no phospholipid remnants in cell envelopes, but some other lipid) would signal success – though such detection is very challenging without sample return.

Base Camp 5A: Arsenic for Phosphorus Substitution

Scope: Directly address the most famous phosphorus alternative: arsenic (same group as P). This base-camp covers chemical viability of arsenate (AsO₄³⁻) in roles normally played by phosphate, such as DNA backbone linkages and ATP-like molecules. It also analyzes the outcome of the GFAJ-1 experiment and subsequent refutations, extracting lessons about stability of arsenate esters in water (very low – they hydrolyze in seconds to minutes at neutral pH, versus DNA’s millions of years) and how life might circumvent that (maybe by continually regenerating them or living in environments where hydrolysis is slower, like extremely cold or non-aqueous conditions).

Stepping Stones: (i) Measure baseline stability: perform a comparative hydrolysis experiment of a simple diester: e.g., methyl-phosphate vs methyl-arsenate in water at pH ~7, 25 °C. Literature says arsenate diesters hydrolyze ~10^5 times faster – confirm such magnitude if possible (though they are so fast you might need low temperature or immediate monitoring). (ii) Attempt enzymatic polymerization of an arsenate DNA analog: use a template DNA polymerase but feed it arsenate nucleotides (if chemically possible to prepare). See if any chain extension happens (likely not efficiently, but even a little incorporation could be informative). (iii) Study extremophile response: take a phosphate-dependent microbe and slowly acclimate it to high arsenate/low phosphate (like what was done with GFAJ-1). Use modern sensitive techniques (ICP-MS for element content, etc.) to see if any arsenate is being incorporated into nucleic acids or just adsorbed. Maybe use radiolabeled arsenate to track it. This replicates the 2010 experiment with better controls. (iv) Consider environment: arsenic-rich environments (e.g., Mono Lake, or the arsenic groundwater in some regions) – is there any unique biochemistry? Survey microbes in those for unusual metabolites (like arsenolipids, which do exist in some algae). If arsenate-based life were to exist, perhaps it needs an environment with limited water or where chemical turnover is extremely rapid to outrun hydrolysis.

Base Camp 5B: Thioesters and Alternative Energy Currency

Scope: Explore whether life could replace ATP (adenosine triphosphate) and other phosphate-based energy carriers with different high-energy compounds, notably thioesters (R–C(=O)–S–R’). Christian de Duve’s “Thioester World” hypothesis proposes thioesters might have been early energy currency before ATP. This base-camp looks at modern analogs (e.g., acetyl-CoA, a thioester, is central in metabolism) and asks if a life form could use a network of thioester reactions instead of ATP/ADP. Also consider polyhydroxybutyrate or other non-phosphate polymers as energy storage. If life had no phosphate at all, thioesters are attractive because the sulfur bond can carry energy and many enzymes already use thioesters (Coenzyme-A, etc.). The scope extends to other phosphate replacements in metabolism like using pyrophosphate (still P) vs. maybe arsenate or simple proton gradients directly.

Stepping Stones: (i) Assess energy yield: Compare the free energy of hydrolysis of a typical thioester (e.g., acetyl-CoA hydrolysis) to ATP hydrolysis. It’s on the order of ATP’s (acetyl-CoA ~ -31 kJ/mol, similar to ATP ~ -30 kJ/mol). So quantitatively, thioesters can do similar work – confirm those numbers and conditions. (ii) Investigate if any organisms can live with minimal ATP by using thioester-driven cycles. Some bacteria run substrate-level phosphorylation to generate ATP from thioesters (via succinyl-CoA to succinate in Krebs cycle generating GTP). Think backwards: could a cell run mostly on a cycle that generates thioesters and uses them directly to drive work (like flagellar rotation or polymer synthesis)? Possibly design a cell-free system where a thioester (like acetyl-CoA) drives a reaction normally driven by ATP (maybe polymerizing actin or something in vitro, substituting chemically). (iii) Synthetic biology: attempt to evolve a strain of bacteria to rely more on thioester and less on ATP. For example, knock out some ATP-generating pathway and supply an alternative sulfur compound to see if cells can reroute. Another approach: provide an alternative energy source such as acetyl-CoA in high amounts plus a way to regenerate it (like a cycle with pyruvate + CoA + redox to acetyl-CoA) to see if cells survive when ATP synthesis is inhibited. (iv) Investigate any known phosphate-limited ecosystems: Are there organisms that survive with very low phosphate by altering their energy metabolism? It’s known some marine microbes substitute phospholipids with non-P lipids when P-starved. Do they also accumulate alternative energy stores (maybe sulfur compounds)? Check literature on cells under extreme P starvation – do they accumulate polyhydroxyalkanoates or DMSP (dimethylsulfoniopropionate) or others as energy reserves instead of polyphosphate.

Bibliography (Path 5)

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