Random thought on plant-based aliens

[Dînadan]

Archive binging Irregular Webcomic! and reading #3558 something occurred to me; in a sci-for setting with plant-based aliens, would having said aliens on your crew help save resources with regards to atmosphic scrubbing? Animals inhale oxygen and exhale carbon dioxide while plants (well Earth plants at least, so for the sake of arguement let's assume this applies to alien plants/plant-people) take in carbon dioxide and produce oxygen, so would plant-aliens do some of the work scrubbing CO2 for animal-based crew (and conversely the animal-aliens recycling O2 into CO2 for the plants).

Now obviously some work would still need to be done by artificial atmospheric processors to remove other gases and remove pathogens and the like and would still be needed to maintain the right O2/CO2 balance, but would a mix of plant and animal crew alleviate some of the work?

[LittleRaven]

Oh boy. How hard do you really want to think about this?

Because on the surface, the answer is...yes! Well, kinda, anyway. As you point out, you'd still have to do a lot to manage the atmosphere, but it could definitely help with the oxygen cycle. It'll be important to remember that you need way, WAY more plant crew than humans, though. Like, WAY more. It takes a lot of plant mass to balance out one of us mouth breathers.

But if you want to dive into the actual chemistry....well, no. Remember this bad boy?



Plants inhale carbon dioxide because they need to combine it with water to create sugar. (useful energy for cells, essentially) Photosynthesis is very, very cool...but it has some pretty obvious energy limits. You need a lot of surface area and a lot of radiant energy before you start to get a substantial amount of energy. A plant alien that actually runs on photosynthesis simply doesn't have much energy to work with, and thus is going to act very much like...well, a plant. You could maybe get around this limitation by having them get their energy from food instead, but however their digestive tracts work, they're almost certainly not converting carbon dioxide into sugar. Quite the opposite, actually. Or maybe they could be really, really big (so lots of surface area)....but also really, really thin (so not much mass to support)..and somehow the concentrate all the energy for cognitive functions? That might work, as long as you rule out mobility. But you have to remember that while you are getting some oxygen for free, you're now having to make sure that Mr. Greene is properly illuminated most of the time. And given how big Mr Greene probably has to be to make a substantial dent in the oxygen cycle, that's no small task. You could probably get some additional gains in efficiency if your human crew also harvested Mr Greene for food, though that might lead to some awkward conversations.

[Dînadan]

And these issues are probably why we'll never encounter plant-aliens (as opposed to alien plants which are like our plants, but In Space); so I suppose it'll depend on where on the scale of sci-fi softness a work falls (although that raises the question of at which point on that scale does it become so soft as to render the issue moot as techtech solves the issues so well that there's no need to consider it).


As for eating them and awkward conversations;
1) there might be no need to eat Mr. Greene, just mutliate him for his reproductive organs...which is an even more awkward conversation...
2) technically it's not cannibalism (ditto if he eats you instead)
3) depending on his society's mores, he might have no issue with you eating another member of his species as it'd be like him composting him first for his own needs (of course in that case he'd probably expect you to return the favour by composting your dead for him and would probably be surprised you don't partake in some choice cuts for yourself)

[TGLS]

I'd bet a plant alien would be consuming it's sugars nearly constantly (and probably need to supplement it with other nutrients), so it may very well be stoichiometrically balanced (or simply lighter oxygen consumers) in terms of life support.

[SabreMau]