We all know by now that plants need light to absorb carbon dioxide--whether it be sunlight, moonlight or even artificial cave light.
But what about total darkness, like a cave or a forest floor beneath an understory thick enough to be considered a "false bottom", similar to the deep scattering layer of our oceans? Could any plant species, either in a future or alternate evolution, evolve to absorb carbon dioxide into their leaves in total darkness?
Yes. This is called crassulacean acid metabolism. https://en.wikipedia.org/wiki/Crassulacean_acid_metabolism
Crassulacean acid metabolism, also known as CAM photosynthesis, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions[1] that allows a plant to photosynthesize during the day, but only exchange gases at night. In a plant using full CAM, the stomata in the leaves remain shut during the day to reduce evapotranspiration, but they open at night to collect carbon dioxide (CO2) and allow it to diffuse into the mesophyll cells. The CO2 is stored as the four-carbon acid malic acid in vacuoles at night, and then in the daytime, the malate is transported to chloroplasts where it is converted back to CO2, which is then used during >photosynthesis.
So these plants store up CO2 in the cool dark night, then use it in the hot bright day.
Crassulacean acid metabolism is a riff on the C4 photosynthetic pathway which includes other adaptations to hot climates.
They can absorb CO₂, but what are they going to do with it?
Rubisco will happily grab CO₂ and bind it provided ribose 1-5 biphosphate is available, and provided the plant has energy, (from ATP, from sugar) it can make that.
But where will this plant get that energy. It has to come from somewhere. There are alternative sources of energy. Animals eat stuff, and get energy from their food (and so don't need to fix CO₂). There are some plants to do the same, they get energy by stealing it from other plants, and so don't need chlorophyll in their leaves.
There are chemoautotrophs, who get the energy to fix CO₂ from their environment. But they normally live in "extreme" environments, such as near undersea volcanic vents, and they are bacteria, not plants.
Perhaps your best option is something like Pterospora. This is a fungi parasite. It acquires energy from a fungus. It is possible to imagine a plant that fixes CO₂ in its leaves, but this is powered by energy stolen from a fungal partner. The fungus in turn gets energy by breaking down detritis that is trapped in the cave. One could even imagine a symbiotic relationship in which the plant provides something back to the fungus, perhaps an effective way of distributing spores. Or perhaps the fungus takes the polypeptide in the death material, decomposes it to provide ATP that would normally be produced by chlorophyll, and takes some of the sugars that the plant makes from CO₂
Some microbes in the deep ocean will metabolise sulfur coming from thermal vents and use the energy to fix carbon. And inside the broken nuclear plant in Chernobyl there is a fungus that uses melanin in a way similar to how plants use chlorophyl, but the fungus gets energy from radiation instead (which is invisible to us).
The fact that those organisms do it in real life means that under the right circumstances, plants could have evolved to do so as well. Notice that these methods of carbon fixing only work in environments that are very unfriendly to most life on Earth though.
Transforming CO2 and water into carbohydrates costs energy, so the plants in the cave need to get it from somewhere.
Surface plants take that energy in the form of sunlight, but in a cave, that would require other energy sources:
This is even more important. Otherwise, you have a heat death scenario.
If your energy source is heat, you need the sink for another reason: You can't extract energy out of heat, you need a temperature difference.
You also need to access the hot and the cold point, which means the plants would fight over who gets to the hottest and the coldest place of the cave. It would be a very weird ecosystem, and probably an unstable one because it's essentially a winner-takes-all scenario, and as soon as that single winning plant dies, the cave is lifeless again, until another ecosystem evolves. (There's also be parasites on that plant, and they'd quickly decompose the carcass and then die or hibernate because they're running out of an energy source.)
