I'm working on a planet which has a biosphere that breathes sulfur as an oxidant. What is the most efficient gaseous (By "gaseous" I mean gaseous at the temperature of my planet, which is on average 26°C) sulfur-based compound to use to get sulfur from to use as an oxidant? I've considered disulfur, but that stuff is unstable, and likely wouldn't stay in the atmosphere for very long. It also has to be common enough (or easy enough to produce) that it makes up ~30% of an atmosphere that has ~1.5 times the pressure that earth has at sea level on a planet which has a gravity of ~1.3g. The sulfur compound should take minimal amounts of energy to break the sulfur out of, or be able to be used as an oxidant.
I don't have a lot of knowledge in chemistry, so let me know if I contradicted myself or if there's something that doesn't make sense. Also, I am aware that this seems similar to this post but that was asking about more exotic life, I'm asking about carbon-based life that uses water and similar proteins and nutrients to Earth life.
I'm fine with a "no" as then I can just use oxygen, which would be a bit boring, but I could do some other things to spice it up!
2 Answers
Sulfur dioxide
This isn't a wonderful candidate (it's normally a weak reducing agent), but it's at least possible.
It's stable and can be reduced to S (0) or H2S (i.e. S (-2)) biologically.
I believe people have speculated that this is occurring on Venus.
For anyone wondering, while SO3/H2SO4 can't be an oxidising agent, it's so thermodynamically and kinetically stable that it's just not doing much at all. SO2 is more feasible.
Sadly, there is no compound which matches these criteria. The only sulphur molecule simple and stable enough to be plausible in an atmosphere and gaseous at roughly Earth temperatures is H2S, hydrogen sulfide. It is not an oxidising agent, rather it is a good reducing agent eg. fuel.
It is however quite compatible with carbon based life, playing a crucial part in the metabolism of many bacteria. For some photosynthetic bacteria, it plays a role analogous to CO2 for plants, and others produce it from elemental sulphur when decomposing organic matter. This makes its inclusion in a potential alien biosphere not just possible but a bit uncreative, as it is a molecule which is proven to work well in that role.
Some resources on this: https://sci-hub.se/10.1016/j.tim.2006.09.001
