Another question like this one inspired by Iceworld.

Along with having bodily fluids based on copper chloride, the Sarrians from Hal Clement's Iceworld are described as breathing gaseous sulfur. Even ignoring the copper chloride, the need to have sulfur in gaseous form puts their minimum operating temperature at around 445C--because that's the temperature at which sulfur boils!

Whether complex chemistry capable of supporting life can even exist at that temperature at all is itself a thorny question... but sulfur melts at a mere 113C, which is much more accommodating.

So, assuming that some kind of life can exist at temperatures in which sulfur is fluid, can it ever make sense to actually "breathe" sulfur as a metabolic oxidizing agent in place of oxygen?

The primary problem I see is that carbon disulfide has a positive heat of formation... which means you actually get more energy by just producing elemental carbon and sulfur than by trying to react them together. Forming silicon disulfide, on the other hand, is exothermic, so maybe that would work for some kind of high-temperature siloxane-based life... but silicon disulfide is a polymer with a melting point over a thousand degrees, so either we really stretch the limits of where we think life can exist far beyond what even Clement was willing to do, or we're probably stuck with a microbial world.

But, on the other hand, the thermodynamic information I have access to is only for STP conditions--maybe CS2 is a more favorable product at higher temperatures. Or maybe there's some other favorable reaction path--like, just reacting sulfur with hydrogen, and doing something else to handle the carbon wastes from metabolizing fats and carbohydrates. I dunno! Hence, well... I'm asking.

(Note that this is not a question about Iceworld, merely inspired by. Answers need not address potential Sarrian biology at all, and positive answers need not be compatible with how Sar and Sarrians are depicted. If they are, that's just a bonus.)

  • $\begingroup$ If silicone disulfide is energetically favorable to form, why are we worried about it's melting point? Just solubilize or precipitate it. Use it for something. Do you need to exhale it? $\endgroup$
    – DWKraus
    Feb 10 at 20:58
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    $\begingroup$ Gaseous sulfur, at 445c may be too high for biology to form. How about other forms of sulfur? H2S boils at -60c. there is also HSO4 (which I cant find the boiling point) $\endgroup$
    – Sonvar
    Feb 10 at 22:52
  • $\begingroup$ @DWKraus It is also insoluble in all known solvents. And you wouldn't last long if you couldn't excrete CO2, and just had to let it build up in all of your cells indefinitely. $\endgroup$ Feb 10 at 23:16
  • $\begingroup$ @Sonvar H2S isn't an oxidizer; there are microbes that breathe sulfate (i.e., SO4+2, whether bonded to hydrogen or not), but they just use the oxygen and precipitate elemental sulfur, or use the oxygen to bind carbon and produce hydrogen sulfide. $\endgroup$ Feb 10 at 23:18
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    $\begingroup$ Good, but I can't put together a whole answer. Glad to help. $\endgroup$
    – DWKraus
    Feb 11 at 4:24

2 Answers 2


Not carbon! Metal.

In a superhot environment, sulfur - metal chemistry could be the backbone of a life cycle. Sulfidation of metal is usually exothermic. Imagine a world where the autotrophs used ambient energy to separate metal from sulfides, storing metals in their tissues and releasing the sulfur as a waste gas. Metal here is the equivalent of sugar in our world. Heterotrophs would eat the metal in the autotrophic "plants" and release energy by breathing ambient sulfur and regenerating the sulfide.

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    $\begingroup$ Hmm... a high temperature, small world that can't retain atmospheric nitrogen or oxygen, so metal oxides eventually end up depleted, perhaps? Finding solubility data for metal sulfides in high temperature solvents is gonna be a pain... apparently, copper sulfide is soluble in potassium cyanide, but that's not exactly a likely thallasogen! Well, there's a whole lot more details to work out somehow, but that fits in with what little is known about Sarrians, at least. $\endgroup$ Feb 13 at 2:16
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    $\begingroup$ I wonder if the oxygen would be depleted or would eventually wind up on the metal to which it was most attracted - kind of like iron will give up its oxygen to aluminum if they are in a hot mix. Re solubility your other question had this all in molten copper chloride and it looked to me like most metals dissolve to some extent in molten salts. $\endgroup$
    – Willk
    Feb 13 at 19:29
  • $\begingroup$ Metals, yes. Metal sulfides... I have no idea! $\endgroup$ Feb 14 at 1:04

but silicon disulfide is a polymer with a melting point over a thousand degrees

The fact that we have gas-phase metabolic waste products in the form of CO2 doesn't mean that everything has to have them. If we say the average human exhales a kilo of CO2 a day (which isn't quite right, but it is close enough), that's in the same ballpark as the amount of urine and feces they excrete per-day.

Clearly humans are capable of shifting that amount of solid and liquid-phase waste products in a day, so there's no reason that your silicon-based sulfur-breathers couldn't just poop more instead of exhaling (or maybe the stuff gets excreted as a fine powder through gill-like slits or extruded as spines or hairs or whatever else takes your fancy).

Obviously some of the plumbing gets more complex which might make a lot of body plans rather awkward and maybe that rules out large mobile organisms, but there's plenty of scope for multicellular life, such as coral-like things that could use their own exhaled waste products as a skeleton or scaffolding.

If there's a source of water as well (or, more likely, steam) then the SiS2 can be broken down into silicic acid and gaseous hydrogen sulfide. This might provide a possible decomposition route for all the exhaled crud lying around everywhere, though I wonder if the mere presence of water in any notable quantity might mean there are more favorable metabolic pathways to use instead.


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