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For weeks now, I have been trying to find a significantly different alternative to normal biochemistry; I've had some good ideas, but I sadly don't know enough about Earth's biochemistry to be sure if any of them are actually viable.

The idea which, for the longest time, kept coming up was a biochemistry which is very low in nitrogen - normally considered both an essential element and a limiting nutrient - while very high in sulphur, an element which, though certainly used by earth life, many scientists would (as far as I know) argue is not essential for a carbon-hydrogen-oxygen biochemistry.

In a sense, this is technically two questions:

  1. Can a CHOS biochemistry (that is, a biochemistry where sulphur is much more heavily utilised than on Earth but where nitrogen is a comparatively rare element) actually exist, based on what is currently known and hypothesised?
  2. If "yes" to the above, what kinds of molecules would this biochemistry contain?

To clarify:

  • A CHOS biochemistry can still have nitrogen in some of its most important molecules, it simply can only have as much of it as Earth's CHON biochemistry has of sulphur
  • Similarly, CHOS biochemistry is still primarily based on carbon, hydrogen, and oxygen, it simply contains sulphur as the next most abundant element after those three
  • A CHOS biochemistry, though still based on the same basic elements as Earth's (except, obviously, for the nitrogen), need not have a one-to-one correspondence to the molecules of Earth's biochemistry, that is, it can lack any direct analogue to a biomolecule found on Earth so long as it can still perform all the basic functions of life

I sincerely apologise if this is the wrong place to ask this question, but I don't know where else I'd ask a question like this.

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    $\begingroup$ Maybe? That is the best you are going to get for an answer, this is high order speculation. $\endgroup$
    – John
    Apr 9, 2023 at 13:41
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    $\begingroup$ I don't see you learning anything from asking this question. It is the sort of question you need a PhD to answer or understand the answer. And you claim you do not know much about real biochemistry to begin with. $\endgroup$
    – Daron
    Apr 9, 2023 at 16:47
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    $\begingroup$ @JBH And this definitely isn't a duplicate--at least not of that question you found. That one is about replacing carbon with sulfur. This one is the much smaller ask of increasing the prevalence of sulfur in an otherwise normal carbon-based biochemistry. $\endgroup$ Apr 9, 2023 at 23:06
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    $\begingroup$ Everything we know about carbon based life, we've learned by example and experimentation. I think I can say with confidence that, if we were a sulphur based life-form, we wouldn't be able to answer whether carbon based life were possible. Organic chemistry is way too complicated to ask "would it all work if we moved the atoms around like this...." Read about glycolysis or the citric acid cycle for a better feel of the topic. $\endgroup$ Apr 10, 2023 at 15:12

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That's a very solid "...maybe?"

Sulfur cannot directly substitute for the functions of nitrogen. It just doesn't form the right kinds of bonds. Nitrogen is the atom you reach for when you need a 3-way branch rather than a 4-way branch or linear chain in a carbon polymer; or a particular bond angle; or a polarized segment. Sulfur only forms either 2 or six bonds, not 3, and likes to form wavy but non-branching chains.

Fortunately, all is not lost, because you didn't ask for sulfur to actually replace nitrogen, just to be more common than nitrogen. And while sulfur can't replace nitrogen, it is a potentially very useful atom. In artificial polymers, it can be a component of a wide variety of functional groups, and participate in a wide variety of reactions. It naturally polymerizes all by itself, especially at high-ish temperatures, and forms bridges between other polymers to produce rigid networks; artificially, that's the basis of vulcanization, but it's also a major usage of sulfur in biology as well, to produce disulfide bridges which stabilize the structures of folded proteins. Hydrogen sulfide and sulfates can also be useful as signalling molecules and as metabolic substrates.

So, if you can find ways to reduce nitrogen usage (potentially doable by, e.g., using sugar-based ribosomes in place of amino-acid-based proteins), and increase the need for sulfur in the functions which it already fills in Earthling biochemistry, maybe you could push the proportion of sulfur above that of nitrogen.

It turns out sulfur is more useful and RNA is more stable at higher temperatures and in more acidic conditions, so that's a very convenient coincidence. If we're not going to completely reinvent biochemistry from scratch, then, your sulfur-heavy life would therefore probably live on a hotter world, under higher pressure so that water remains liquid, with mildly acidic oceans. All three of those conditions are easily attainable with a thick CO2 atmosphere, which both is a greenhouse gas and forms carbonic acid in contact with water.

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  • $\begingroup$ Could it replace oxygen, I wonder? $\endgroup$ Apr 10, 2023 at 17:56
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    $\begingroup$ @მამუკაჯიბლაძე Not one-to-one, but sulfur can substitute for some of oxygen's functions. There are real organisms which breathe sulfur like we breathe oxygen, and organisms which break down hydrogen sulfide to release elemental sulfur in photosynthesis like plants split water to release oxygen. $\endgroup$ Apr 11, 2023 at 0:18
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Do you realise what you are asking?!

It's not possible to answer such a huge question as nobody has sufficient knowledge of the vast range of alternative sulphur compounds that would be needed.

http://biochemical-pathways.com/#/map/1

Have a look around the above diagram showing some key biochemical pathways (use the arrows bottom right) to get a better perspective on the size of the question.

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