[gjc8]

The basic model of acetogenesis, as I understand it, takes in carbon dioxide and hydrogen and spits out acetic acid and water, not oxygen. The energy source here is really the hydrogen, not the carbon dioxide, though I think both are actually generally intermediates in actual organisms.

If you have another carbon source, like food, I can't imagine that using carbon dioxide as a feedstock makes much sense. Perhaps it would help to actually write out the basic chemical equation for what you imagine is happening, which would allow you to work out an energy balance for it.

Regular respiration, for example:

C6H12O6 [glucose] (-1271 kj/mol) + 6 O2 (0 kj/mol) -> 6 CO2 (-393 kj/mol x 6) + 6 H20 (-242 kj/mol x 6) + 2539 kj/mol

Now, if I were designing an anaerobic heterotroph, I might say that, as long they're well fed, they don't need to breath at all, based on fermentation:

C6H12O6 (-1271 kj/mol) -> 2 C2H6O [ethanol] (-277 kj/mol x2) + 2 CO2 (-393 kj/mol x2) + 69 kj/mol

Note the drastically reduced energy yield per mole of glucose. So, in GURPS terms, this could be Doesn't Breath (Temporary Disadvantage: Increased Consumption 5 -50%), which implies that you need to eat a full meal for every 15 minutes you go without air.

Alternatively, I might say you breath hydrogen.

C6H12O6 (-1271 kj/mol) + 12 H2 (0 kj/mol) -> 6 CH4 (-75 kj/mol x 6) + 6 H20 (-242 kj/mol x 6) + 631 kj/mol

If you also had CO2, you could use this energy to run respiration in reverse. But there's no reason to unless you need molecular oxygen for some reason; you'd burn more glucose than you'd get back.