[Build Advice] Model Anaerobic/Aerboic Breathing?

[Sunrunners_Fire]

Okay, so ... I have this alien species. (hehe) The question at hand is "How do I model their respiration?" A tag search and forum search came up blank, so ...

[What is being modeled]

"Mnemi are facultative anaerobic organisms which primarily utilize acetogenesis for their respiration -- they can utilize aerobic respiration as circumstances warrant it; the presence (or lack thereof) of oxygen doesn't harm them." In practice, this means they breathe both carbon dioxide and oxygen, though they continue to breathe if there is no carbon dioxide and they continue to breathe if there is no oxygen; that breathing purely carbon dioxide results in them exhaling oxygen; that breathing purely oxygen results in them exhaling carbon dioxide; and all of these things mean that they'll breathe just fine as long as they have either or both available. Assuming they meet the other requirements for breathing (food and water are important parts of breathing!), of course.

[How do I model it]

Breath-Holding won't work for this. Doesn't Breathe also doesn't work, even when modified, as none of the existing modifications cover this. I could just describe it as a Perk, but it gives more benefit than I'd expect a Perk to give. You can seal a human and a handful of mnemi in a small air-tight room; they'll keep breathing just fine until they start dropping from dehydration and starvation. Yes, even the human will be fine (though the mnemi will start dropping before the human does).

I'm either missing something or I need to get creative. Suggestions? Did I miss a relevant thread or post when searching?

(EDIT) ... and I misspelled the thread title. Eww.