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Phosphorus is central to all life on Earth, being a critical component of DNA, cell membranes, and metabolic systems (via ADP/ATP). However, it's not really all that common, especially compared to the classic CHON (carbon, hydrogen, oxygen, nitrogen), which make it a limiting nutrient for the biosphere. So if some alien biosphere could reduce it's dependence on phosphorus, that would be useful!

Some time ago, there was a report of bacteria suspected of using arsenate instead of phosphate in their DNA. As far as I understand the literature, that has been pretty thoroughly debunked, but the question still remains: could other polyatomic ions substitute for phosphorus in different roles?

The article Why Nature Chose Phosphateslays out several reasons that phosphates are useful, and makes the claim that no other option can fill all of the roles that phosphate does. But that doesn't seem to rule out the option of a variety of other ions filling in in different places, to reduce or possibly even eliminate the need for phosphorus after all.

The obvious option to me for several uses is sulfite, which can also form bridges and chains via sulfite ester bonds; the reduced charge on sulfites (-2 rather than -3) means that, e.g., a sulfite-backboned DNA equivalent would no longer by an acid, but I'm not sure how horrendous a problem that would be--it is also true of PNA, after all. Maybe it just needs to work on a non-polar solvent, or maybe there is some other modification that can make it acidic again?

Additionally, sulfolipids, which retain their negatively-charged hydrophillic heads just like phospholipids, so perhaps phosphorus could be replaced in cell membranes as well?

To summarize: it seems that sulfur and nitrogen-based structures could plausibly stand in for phosphorus in a few places; sulfur is less abundant than phosphorus in Earth's crust, but more abundant overall, so maybe that would be useful elsewhere in the universe (like, say, Io).

So, the actual question: Are sulfur and nitrogen indeed plausible stand-ins for phosphorus? If not, are there other options? And just how much phosphorus can actually be replaced? (Or, equivalently, for what functions can phosphorus not be replaced?)

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    $\begingroup$ Fair warning. I had fun answering this other question, but it points out the basic difficulty with asking "can't we replace X with Y?" questions: if there isn't an actual example of that having been done (e.g., earth-life based on sulfates/nitrates or anything else other than phosphates) then every answer is fundamentally a guess, even when someone a whole lot smarter than I answers the question. The usual result is, "just do it and continue with your story" because sometimes even the pros can't completely say you're wrong. Cheers. $\endgroup$ – JBH May 31 at 20:53
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    $\begingroup$ @JBH True, but there are close approximations to draw on. E.g., PNA doesn't exist in nature, but it has been synthesized and had its properties studied. And sulfolipids aren't used in cell membranes, but they do serve other functions, and have been studied by biochemists, so it's not unreasonable that someone might know if they can form membranes. Etc. $\endgroup$ – Logan R. Kearsley May 31 at 21:07
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As you point out, phosphorus is not that common when compared to the other element you cite: sulfur and nitrogen.

Based on the only sample we have, I would say that if life could have evolved using them in place of phosphorus, it would.

But since life on our planet chose the phosphorus way, it can mean two things:

  1. replacing phosphorus with sulfur/nitrogen is not possible in living organisms
  2. life based on sulfur/nitrogen chemistry instead is less efficient than life based phosphorus chemistry.

Option 1 would mean that it never existed, option 2 means that if it existed it was quickly outperformed and went extinct.

By the way, my chemistry is too rusty to assess it properly, but I am not sure that the sulphurate equivalent of ATP can exist. If my doubt is true, this can be the reason why the above options are in place.

ATP molecule

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