Photosynthesis requires that a substance be split to release hydrogen cations and electrons. Plants use water and release oxygen as a byproduct, but other substances can be used as well. There is a type of bacteria, known as purple sulfur bacteria, which uses hydrogen sulfide rather than water. The hydrogen sulfide is split into hydrogen cation and elemental sulfur, which may further be reacted with water to make sulfuric acid. There are other bacteria that can reduce elemental sulfur or sulfuric acid to hydrogen sulfide and close the cycle in a type of respiration. The animals of a sulfur-breathing world would work in the same way.
Sulfur is a solid at mild temperatures, so it is not as convenient to breathe as oxygen. As the heat increases, it melts, becoming easy to ingest. An animal swimming in a molten sulfur bath could breathe the scalding liquid and exhale hydrogen sulfide. The plants would oxidize the hydrogen sulfide back to elemental sulfur. A hot world with little water might be able to support such a sulfur cycle if the native life can stand the high temperatures needed to keep the sulfur liquid. A wider temperature range is allowed by sulfuric acid, as in the acidic aerosol clouds over Venus, as the acid is a liquid from room temperature up to heat that melts lead. A mixture of hydrogen sulfide and carbon dioxide can be made into sulfur, water, and sulfuric acid through photosynthesis.
Game and Story Use
- A blood-red ocean of molten sulfur? A boiling cloud of sulfuric acid? There's a lot of room for hellish imagery when it comes to sulfur, so make the most of it. The foul, toxic breath of the animals will only add to the demonic undertones, while purple vegetation that drips a hot, bloody liquid or seethes with sulfuric acid would be creepy in its own right.