# Could an organism as complex as a human breathe sulphur dioxide?

On my world sulphur dioxide makes up most of the atmosphere (42%). It comes from constantly active land-based and Hydrothermal vents. I know there are basic, or basic compared to a human, organisms that can 'breathe' sulphur dioxide. Would it be possible for a sentient and relatively large creature to breathe sulphur dioxide?

So my question is:

• Is it possible for my organism to breathe sulphur dioxide?

Sorry if this has been asked before, I searched this site and other sites but I found little that could help. Thank you in advance.

• Welcome to Worldbuilding! It's an interesting idea, but all the sub-questions make it appear quite broad - if you want to know something in such detail, it might(?) be more useful if you spread the above over several questions and link them together Oct 14, 2017 at 15:28
• You might get better answers on the chemistry site, but I think the answer is probably not. The problem is that SO2, like CO2 on Earth, is the end product of an exothermic reaction. So you might have the equivalent of plants taking up SO2 and using energy to split that into S and O2, with "animals" eating the sulfur-based compounds they produce. Oct 14, 2017 at 16:52

Probably.

Aerobic respiration of glucose (the primary source of energy for animal cells) proceeds according to the following formula:

6O2 + C6H12O6 -> 6H2O + 6CO2

We can approximate the energy gained in this reaction by looking at the heat of formation of each of the participating molecules; the energy gained is the difference between the total energies on each side of the reaction.

Heats of formation are as follows (all values taken from NIST Web Book):

C6H12O6  174 kJ/mol
H2O  -285.83 kJ/mol
CO2  -393.52 kJ/mol


Which results in the following totals:

6*0 + 174 = 174 -> 6*(-285.83) + 6*(-393.52) = 4076.1

for a net energy gain of 4250.1 kJ/mol of glucose.

If we directly replace O2 with SO2 in the above reaction, we get

6SO2 + C6H12O6 -> 6H2O + 6CO2 + 6S

Elemental sulfur has a heat of formation of 0, just like diatomic oxygen, so the only change in energetics comes from the non-zero heat of formation of SO2, which is -296.84 kJ/mol. Since there are six of those molecules involved, that takes 1781.04 kJ out of our net energy gain, leaving us with 2469.06 kJ/mol of glucose- a little over 60% of what we get by using oxygen.

Other reaction products are possible, including H2S, CS2, and COS, with the following heats of formation:

H2S  -20.50 kJ/mol
CS2  89.41 kJ/mol
COS  -139.0 kJ/mol


so you can try fiddling around with different ratios of reaction products, but I don't think you'll get better results than just producing elemental sulfur as a byproduct.

So, if nothing else changes about these creature's metabolism, the kinds of energy storage molecules they use, and so forth, an SO2-breathing creature would need to eat about 66% more food and breathe 66% more air to maintain the same power levels as an equivalent Earth creature. Given the enormous variability in food intake and metabolic rates among existing large animals on Earth, it does not seem at all implausible to me that you could have a large, energetic creature breathing SO2 under those constraints. I could certainly survive just fine having to eat twice as much food and breathe twice as quickly as I do now--and birds, for example, have much more efficient respiration than humans, so were I properly adapted for this environment, with more efficient lungs, I could probably get by just fine merely by altering my diet to increase the ratio of starch and sugars and only eating a smidge more than I have to as a normal, O2-breathing human.

• neat, although you might have an issue with the sulfur forming sulfuric acid with the water as that stuff tends to rip apart most organic carbon compounds on contact (but I suppose such a creature would have pretty non-standard chemistry to begin with) Oct 14, 2017 at 20:29
• @Samwise I don't think that would be a major concern. Sulfur doesn't react with water very easily, and at high temperatures you end up with an equilibrium between hydrogen sulfide and sulfur dioxide; sulfur dioxide in solution gives you a weak sulfurous acid (H2SO3), but the scenario posits large amounts of sulfur dioxide, and thus sulfurous acid, present already. Native organisms would presumably be adapted to that, and could probably buffer the pH if necessary. You don't get sulfuric acid unless there is also excess oxygen around to form sulfur trioxide. Oct 14, 2017 at 21:12