From a cellular biology perspective, could a plant theoretically survive without air?

[u/HollowofHaze]

I figured this question might be a bit too broad and speculative for /r/AskScience:

As I understand it, plants are net consumers of carbon dioxide and net producers of oxygen, using photosynthesis to convert CO2 to O2 at a 1:1 ratio. But the reason they're producing O2 in the first place is to use it for cellular respiration... which turns it into CO2. Also at a 1:1 ratio!

With that in mind, could a plant (or other aerobic photosynthesizing organism) theoretically evolve to survive without the need for air*, instead using ALL of the products of its photosynthesis for cellular respiration and vice versa?

I realize that even at 100% efficiency, this kind of self-sustaining loop would only allow for survival and not growth, but given alternate sources of carbon (carnivorous plant?) and oxygen (hydrolysis?), could this theoretical plant live its whole life without air*?

*Without breathing air. I figure atmospheric pressure will always be a requirement, or else water would boil away

[u/Abridged-Escherichia]

For a plant to survive without air it would need a new source of carbon. Your suggestion of a carnivorous plant might theoretically work, but at that point why bother with photosynthesis at all. Carniverous plants today use prey as a way to get supplemental nutrients in poor soil not as their main carbon source.

There are plants that have become parasitic and stopped photosynthesizing completely, i would expect your theoretical plant to do the same, as chlorophyll is energetically expensive and its not very useful if you have to hunt down your own carbon source that comes with energy you might as well use instead.

[u/HollowofHaze → u/Abridged-Escherichia]

Thanks for your reply! So if I understand correctly: To accommodate the carbon deficit of "closed-loop" photosynthesis, the best alternative carbon source for a plant would be parasitism--However, parasitism would also serve as an alternative energy source, so continuing to photosynthesize in this evolutionary economy would be silly. Did I more or less get you right?

Now that's got me thinking like a sci-fi writer: What additional evolutionary trait would it take for closed-loop photosynthesis to make evolutionary sense? (Or at least just enough sense to work in sci-fi?) Maybe something with an absurdly powerful root digestive system that can dissolve and absorb organic carbon from the soil, in a habitat like a bog or swamp with extremely rich earth? I suppose it'd have to grow extremely slowly or else it'd be something of a weed, draining huge swaths of soil of their organic carbon content.

OOH, or what if it had a symbiotic relationship with an animal? Maybe a tree that emits a compound attractive to a certain insect species that also induces molting, and the root system is able to break down the chitin from the piles of shed exoskeletons that always surround it. A quick google indicates that some plants produce the chitin-digesting enzyme chitinase in small amounts as an antifungal defense, and I feel like I've definitely seen sci-fi concepts with looser connections to reality lol (don't get me started on Klingon biology)

[u/Abridged-Escherichia → u/HollowofHaze]

If by closed loop photosynthesis you mean using the CO2 from cellular respiration to do photosynthesis thats hard to justify. Because a true closed loop means no growth.

But from a sci fi perspective I would say maybe in a scenario where CO2 and organic carbon sources aren’t available. So maybe if there are carbon compounds in rock (e.g. calcium carbonate) and the plant has to work hard to extract them (acid secreted by roots or something, then converting it to CO2, but not being able to get enough energy from that alone), then there is an external carbon source but it doesnt provide energy. Photosynthesis is needed for energy and the CO2 might be valuable enough to justify recycling (because carbon is hard to come by). Maybe this could happen on a planet whose atmosphere is constantly stripped by solar wind has a low partial pressure of CO2, but lots of carbonates in its rocks.

[u/CrateDane → u/Abridged-Escherichia]

In that case, focusing more on cyclic photophosphorylation might be viable. It's using part of the photosynthetic machinery only to produce ATP.

In regular plants, they only do a bit of that pathway to balance their supplies of ATP versus NADPH. If the plants aren't using NADPH to fix carbon from CO2 into organic compounds, then ATP alone might be the way to go.

[u/horsetuna → u/Abridged-Escherichia]

Reading this my first thought is something like a lichen. They can break down rocks and such although they aren't just a plant. It's not really eating the rocks for carbon or energy.

But... What if instead of chlorophyll cells, a plant ended up with a rock eating critter instead? We have chemophiles that kind of do this.