[Build Advice] Model Anaerobic/Aerboic Breathing?

[Anthony]

Originally Posted by Ashtagon

I'm confused on this point too. Afaik, the only natural organic reaction that produces O2 and consumes CO2 is photosynthesis, found in plants (which incidentally also do the O2->CO2 reaction at the same time, but in different areas). I'm not seeing anything that implies these creatures are photsynthesising.

The original post mentioned acetogenesis, which consumes H2 and CO2, though based on this I think both are intermediate steps in the process, not feedstock. In any case, such a metabolism would be (a) very slow, and (b) almost certainly poisoned by oxygen.

[Sunrunners_Fire]

Originally Posted by Ashtagon

I'm confused on this point too. Afaik, the only natural organic reaction that produces O2 and consumes CO2 is photosynthesis, found in plants (which incidentally also do the O2->CO2 reaction at the same time, but in different areas). I'm not seeing anything that implies these creatures are photsynthesising.

I compartmentalize my questions, usually.

I have about six pages of typed notes sitting in local documents and another dozen/ish pages of handwritten notes. It isn't practical to post all of that every time I have a question about how to model something in game terms. It would only be confusing (especially as its' rather subject to change). In this case, I asked about how to model (the current version of) their respiration. Photosynthesis isn't respiration. Though, yes, the Mnemi do possess an assortment of autotrophic features in a variety of flavors. Its' not relevant to the question though; or at least I didn't think it was. :grins:

Of course, if this is intended to be a "ignores science" racial feature, carry right on :)

Such is not the intent, no. Bend it some, sure. I know that I'm engaging in a number of data errors; some intentionally, but mostly by accident. Its' a learning exercise.

Originally Posted by Anthony

The original post mentioned acetogenesis, which consumes H2 and CO2, though based on this I think both are intermediate steps in the process, not feedstock. In any case, such a metabolism would be (a) very slow, and (b) almost certainly poisoned by oxygen.

Google notes facultative acetogenic bacteria as existing in the here and now. If its' poisoned by oxygen, its' an obligate anaerobe not facultative. Various things note that there are only bacterial examples of acetogens known on Earth, but I don't believe its' physically impossible for them to become larger/more-complex than that.

Also, methanogenesis is different than acetogensis. They're both carbonate reduction but the products are different. (Methanogenesis is used by (some? all?) archaea and results in methane/CH4; acetogenesis is used by some bacteria and results in acetate.)

[David Johnston2]

Originally Posted by Anthony

The breathing CO2 part to exhale oxygen is almost certainly energy-negative even counting in the food.

Plants do it, although of course they do it by not being very active.

[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.

[RyanW]

Originally Posted by David Johnston2

Plants do it, although of course they do it by not being very active.

And they also have to absorb energy from the environment (i.e. energy negative). CO2 can be thought of as a dead battery of the chemical world.

[gjc8]

Originally Posted by David Johnston2

Plants do it, although of course they do it by not being very active.

And also having the sun to use as an energy source.

[gjc8]

Originally Posted by RyanW

CO2 can be thought of as a dead battery of the chemical world.

Well, the dead battery of the organic chemistry world, anyway.

[Bruno]

Originally Posted by gjc8

And also having the sun to use as an energy source.

The AND part is important. Photosynthesis is SO energy consuming the plant doesn't have enough energy left over to support an energetic lifestyle at all. Even "active" plants like venus flytraps aren't very active, and perform most or all of their motion through a really clever use of elasticity. Basically venus flytraps "wind up" the "spring" mechanism for their leaves through slow growth, taking days to reset.

[Anthony]

Originally Posted by Sunrunners_Fire

Google notes facultative acetogenic bacteria as existing in the here and now. If its' poisoned by oxygen, its' an obligate anaerobe not facultative. Various things note that there are only bacterial examples of acetogens known on Earth, but I don't believe its' physically impossible for them to become larger/more-complex than that.

It's quite low energy compared to the oxygen reactions.

[Sunrunners_Fire]

Originally Posted by Anthony

It's quite low energy compared to the oxygen reactions.

Yep.

The stuff I'm reading tells me that most/all of the anaerobic methods generate 2 ATP per, while the aerobic method generates 30/ish ATP per (theoretically up to 38 ATP though nothing on Earth does that level of efficiency). Aerobic is up to 15 times more efficient than Anaerobic.