Im thinking about starting a speculative evolution project about this becauze i really like the idea of exotic biochemistry, ive looked at a few other posts and i have the gases that are breathed and used for photosynthesis as well as sulfuric acid for the solvent, but i wanted to go a little bit further, like what other biological processes could function in this system? i saw one post where some guy went crazy and listed all the reactions for silicon fixation and posted a silicon analog to glucose, i hope he replies to this lol

anyway, the planet has an atmosphere of fluorine, silicon tetrafluoride, and hydrogen fluoride, fluorine is breathed by animal analogues and the other 2 are used for photosynthesis and breathed out by animal analogues, they use sulfuric acid as their solvent in an environment around 400°F, their planet is a carbon planet dominated by carbon based geology with very little oxygen, and their star emits tons of UV to help photosynthesizers break up the gases they need

so, how could things like chemosynthesis work in this environment? what are some molecules/compounds that could function here? i think itd be cool to find a few molecules that would work in the place of stuff like glucose. What could that energy storage molecule be?

  • $\begingroup$ I narrowed the question so as to fit WBstack requirements better: one question per post. Glucose analog is question enough! $\endgroup$
    – Willk
    Commented Jun 13, 2022 at 17:43

2 Answers 2


This comes pretty close to being a duplicate of Biochemistry of a sulfuric acid world?, but specifying that the creatures here breathe fluorine is a large enough departure that I suppose it warrants answering separately.

their planet is a carbon planet dominated by carbon based geology with very little oxygen

In that case, there is almost certainly a lot of carbon tetrafluoride around as well, and plenty of carbon as a heteroatom in the siliceous biochemistry.

If you have oxygen in the biochemistry (which you do, as part of H2SO4 at least), you will also probably have some free oxygen in the atmosphere, because fluorine will displace it from pretty much everything. Oxygen may thus end up serving a similar role in biogeochemical cycles as nitrogen does in Earthling biology--it has to be fixed out of the air into silicorganic forms for use as a heteroatom.

I am also very skeptical of the plausibility of maintaining significant quantities of atmospheric F2 at 400F... but we'll do the best we can!

As a carbon planet, you'll have a lot of silicon carbide minerals, in place of the usual silicates (silicon oxides). One usually expects that to result in a much more rigid geology... except in this case, it can be attacked by hydrofluoric acid! Some polymorphs of silicon carbide (e.g., α-SiC) are essentially immune to acid attack, but others break down rapidly, producing SiF4, CF4, and hydrogen gas. The surface minerals that survive will be fluorine-terminated, much like the surfaces of silicate rocks are hydroxyl-terminated on Earth.

so, how could things like chemosynthesis work in this environment?

Well, there's the "chemo-" part, and then there's the "-synthesis" part. The "-synthesis" part is reusable, so let's focus on the "chemo-" first.

Plenty of chemical energy sources that are used on Earth should still be accessible to life forms on this planet--oxidation of metals, reduction of oxidized substrates (e.g., methanogenesis), etc. But there are a couple of unique processes that your world's organisms could catalyze, and in the process help transform a primitive world with an inert atmosphere into the modern world where things breathe fluorine.

  1. Sulfate ion exchange. Organisms immersed in sulfuric acid solvent and with access to halogen salt-containing minerals can gain energy by extracting halogens to produce hydrofluoric and hydrochloric acid and depositing sulfate minerals in their place.

  2. Silicon Carbide decomposition. Where sufficient quantities of HF are available in the environment, organisms can catalyze the decomposition of SiC into SiF4 and CF4, producing waste hydrogen. Symbiotic methanogens could then convert additional SiC, or partially-oxidized silicorganic substrates, into methane and elemental silicon (and probably some extra regenerated HF as well), and additional layers of organisms (or perhaps, sometimes, the same organisms--horizontal gene transfer is a wonderful thing!) can then convert elemental silicon and HF into more SiF4 and hydrogen, and so on. (Methane will of course burn in F2, just as it does in O2, but it's inert against reaction with H2SO4 and HF, and primitive anaerobic methanogens can be expected to survive into the fluoro-aerobic age just as they have survived in anaerobic environments on Earth.)

Carbide-decomposers have a nice leg up on sulfate-exchangers, in that their energy production process will also produce useful biosynthesis inputs (unsaturated fluorosilicons and fluorocarbons) as intermediate steps. Sulfate-exchangers would need environmental access to structural-food molecules as well as energy-food molecules--which is not unusual for extremophilic Earth bacteria, and it's perfectly suitable to an abiogenic environment where proto-lifeforms are assumed to be assembling out of a soup of non-biologically-produced components anyway.

Now, what would primary producers actually be synthesizing, whether they get their energy from chemical reactions or capturing photons?

Given the environmental prevalence of silicon carbides, and the relatively low accessibility of oxygen (yes, you can rip it out of H2SO4 with a lot of energy, and eventually it will be accessible from the atmosphere, but none of that will be true in the early days when basic universal biochemical pathways are being established), I would expect some kind of partially (or even fully) halogenated polycarbosilane as the rough equivalent of glucose (as opposed to polysiloxanes, which would be expected from an oxygen-rich silicate world). E.g., some derivative of 1,3,5-trisilacyclohexane, or Si3C3H6, where some or all of the hydrogens are substituted with other functional groups that make it more stable against fluorination and provide active sites for polymerization and manipulation by enzymes--e.g., most hydrogens could be substituted by fluorines, chlorines could stand in for polarizing hydroxyl groups, and difluoromethyl groups (-CHF2) could stand in for reactive hydroxyl groups permitting polymerization by defluorination rather than dehydration. In general, you can expect to see partially-saturated carbons in this environment standing in for reactive oxygens in our lower-temperature, water-based biology. When oxygens do end up incorporated into biochemistry, they will either be part of siloxane polymers & rings (providing a stronger bridging bond between silicon centers than carbon can), or as part of polyatomic ions, like sulfate and phopshate.

Fluorogenic synthesis would most generally proceed by cracking SiF4 and CF4 to produce SiF2 and CF2 radical units, with splitting of HCl and HF for hydrogen to use to attack the relatively inert terminal fluorides as an internal process which is the actual source of free fluorine gas, much like water cracking for hydrogen is an internal process in Earthling photosynthesis, even though the big-picture photosynthesis equation makes it look like oxygen is being removed from carbon. Exactly how much fluorine is released in the photosynthesis cycle would depend on just how much hydrogen actually ends up incorporated into the structure, vs. how much the carbosilanes remain perfluorinated and how much hydrogen just ends up being used to regenerate HF.

  • $\begingroup$ so you think the glucose analog could be something like Si3H3C3F3 or Si3C3F6? $\endgroup$ Commented Feb 3, 2023 at 16:01
  • 1
    $\begingroup$ @sesese2368420 More likely the former than the latter, but probably something slightly more complex with some reactive functional groups hanging off, like glucose's hydroxyls. $\endgroup$ Commented Feb 3, 2023 at 16:48
  • $\begingroup$ would something like this work? imgur.com/a/rR5SwPG $\endgroup$ Commented Feb 4, 2023 at 0:24
  • $\begingroup$ @sesese2368420 Your silicons appear to be oversaturated and carbons undersaturated. $\endgroup$ Commented Feb 5, 2023 at 3:06

Silicon disulfide

silicon disulfide


Backing into this - your animals breathe fluorine and exhale fluorinated silicon. That means they are oxidizing (fluorinating) reduced silicon for energy. You want something like glucose - reduced carbon in polymeric form to store photosynthetic energy.

I present silicon disulfide. Reduced silicon ready for the fluorinating and in handy polymeric form. The sulfur is released back to the working fluid where it equilibrates with the sulfuric acid.

I am pondering the cellulose equivalent of silicon disulfide where it is wrapped in such a way so as to resist attack by the ambient atmospheric acids...

  • $\begingroup$ thats really cool, how did u find out what molecule would be good for this? im thinking about researching some other silicon molecules but idk how to know if they would be functional for this kind of stuff $\endgroup$ Commented Jun 13, 2022 at 20:54
  • 1
    $\begingroup$ Well fluorinating pretty much anything will be exothermic. You say what your animals exhale which is terminally fluorinated silicon,. I wanted a silicon molecule using one of the other molecules in your environment. I wanted a polymer because that is part of what makes glucose a good energy storage medium: it can polymerize for storage as starch., I knew silicon likes oxygen because that combo is all over earth and so I wondered about sulfur, oxygen's edgy older brother., I googled for silicon sulfide and the polymer image popped up - done! $\endgroup$
    – Willk
    Commented Jun 13, 2022 at 21:05

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