# Carbon cycle in a sulphur-based biochemistry

I'm designing a world with advanced life forms that use a sulphur-based biochemistry (see this link for context). I would like to create a carbon cycle analogous to Earth, where:

• Photosynthesis transforms carbon dioxide (CO2) and water (H2O) into glucose (C6H12O6) and oxygen (O2).
• Cellular respiration consumes O2 and releases CO2 + H2O, maintaining equilibrium.

In my world, I would like:

• Photosynthesis to take methane (CH4) and sulphuric acid (H2SO4) to produce sulphur dioxide (SO2) and a hydrocarbon that performs the same role as glucose (let's call it substance X).
• Cellular respiration to consume SO2 and release CH4 + H2SO4, again maintaining equilibrium.
• Substance X would therefore need to have a molecular formula with 1×C, 6×H, 2×O (or a multiple in proportion, e.g., 2×C, 12×H, 4×O).
• No oxygen (O2) in the atmosphere.
• No production of water in the photosynthetic reaction.

The ideal solution would be a hydrocarbon that actually exists, with a molecular formula using the atoms mentioned above. If that's not possible, what if:

• Sulphur trioxide (SO3) is used instead of SO2?
• A third, nitrogen-based gas is added (such as ammonia or nitric acid) such that photosynthesis takes CH4, H2SO4, third gas; and respiration takes SO2 to release CH4, H2SO4 and the third gas.
• What wrong with $6\mathrm{CH}_4 + 6\mathrm{H}_2\mathrm{SO}_4 \leftrightarrow \mathrm{C}_6\mathrm{H}_{12}\mathrm{O}_6 \cdot 6\mathrm{H}_2\mathrm{O} + 6\mathrm{SO}_2$? – AlexP Feb 13 at 23:06
• If people disagree with me, change it back, but I added [chemistry] to the tag mix. Because this is about both chemistry in life forms (for which [biochemistry] is perfect) and chemistry of the atmosphere and other non-living things. Oh and [atmosphere]. – Cyn Feb 14 at 0:53
• Thanks @ArkensteinXII. I'm happy to rename the post to make it clearer - but my question is actually not about photosynthesis. I'm interested in the entire cycle - autotroph organisms converting A to B, where heterotrophs then convert B back to A, keeping a balance. I hope that makes sense? – Allie Enfield Feb 14 at 9:14
• @AllyEnfield Water does indeed react with sulphuric acid in an endothermic reaction. It is possible that your organisms could utilise that energy perhaps? It is going to be very difficult to exclude water from a planetary environment in that it periodically falls from space, and is produced by a variety of geological processes. – Arkenstein XII Feb 14 at 19:23
• @AllyEnfield The first thing that comes to mind is that, much like cold-blooded organisms of Earth, they could use it simple to maintain body temperature? There are undoubtedly mechanisms by which a lifeform could capture that energy, even if only in part. – Arkenstein XII Feb 17 at 0:18

You have invented sulfate reducing microorganisms

Sulfate-reducing microorganisms (SRM) or sulfate-reducing prokaryotes (SRP) are a group composed of sulfate-reducing bacteria (SRB) and sulfate-reducing archaea (SRA), both of which can perform anaerobic respiration utilizing sulfate (SO42–) as terminal electron acceptor, reducing it to hydrogen sulfide (H2S). Therefore, these sulfidogenic microrganisms "breathe" sulfate rather than molecular oxygen (O2), which is the terminal electron acceptor reduced to water (H2O) in aerobic respiration....In terms of electron donor, this group contains both organotrophs and lithotrophs. The organotrophs oxidize organic compounds, such as carbohydrates, organic acids (e.g., formate, lactate, acetate, propionate, and butyrate), alcohols (methanol and ethanol), aliphatic hydrocarbons (including methane), and aromatic hydrocarbons (benzene, toluene, ethylbenzene, and xylene)

You do not need photosynthesis because there is enough power in sulfate to reduce methane. These creatures are the heterotrophs in your world (and ours), the equivalent of animals up in the oxic world.

You can model your plants on purple sulfur bacteria. They do photosynthesis. Like plants they kick out oxidized stuff as waste; oxygen for plants, sulfuric acid for the PSBs.

The purple sulfur bacteria (PSB) are part of a group of Proteobacteria capable of photosynthesis, collectively referred to as purple bacteria... Unlike plants, algae, and cyanobacteria, purple sulfur bacteria do not use water as their reducing agent, and therefore do not produce oxygen. Instead, they can use sulfur in the form of sulfide, or thiosulfate.. The sulfur is oxidized to produce granules of elemental sulfur. This, in turn, may be oxidized to form sulfuric acid.

If you think that kind of thing is cool, you are right. Read up. It is amazing and it is real.

• This is a great answer, +1; just note for the querent that this answer reverses the use of the sulphate group, using it for respiration (playing the role of oxygen). – AlexP Feb 14 at 8:59
• Brilliant, thanks @Willk. This allows me to play with H2S and SO4 as alternatives I hadn't considered before. Having read the article I noticed SR* and PSB reactions produce water as a final byproduct, which wouldn't work in the equation that I'm looking for. – Allie Enfield Feb 14 at 9:31