On the surface of the world the sun is the source of energy for plants and the foo chain goes up from that. How could a similar base for a food chain be established in the large world below?
I am looking first for "realistic" options, so anything like methane eating bacteria would work. I am also open to magical options, like luminescent crystals of which mosses/lichens feed.
Hmm... What about a thermal mushroom?
I don’t mean an mushroom that makes heat, I mean one that eats heat. Perhaps it is a super-basic but large organism that manufactures the compounds of life by reaching its roots deep into the ground and using the difference in temperature between the dark caverns where it is found and the deeper crust/upper mantle to extract energy from the heat of the planet’s core. This way, it can apply that energy to turn basic chemicals, like carbon dioxide or limestone or what have ye, into the stuff it needs to grow. Then, when it has excess, it grows mushrooms that are edible to some species of subterranean creature, and the food chain starts there.
Seems plausible enough, if that’s what you’re going for.
In real world subterranean ecosystems most of the ecology is based on nutrients from the surface transported into the system either by water or by animals that shelter in the caves but feed outside e.g bats (in the form of their bodies and waste products).
So you I suppose posit two 'parallel' ecosystems existing side by side in your subterranean world. The first is reliant on surface nutrients for survival which are transferred to the cavern from the surface via the two mechanisms described above. The second source as you hinted are methagenic microorgansims.
Combined the two sources of nutrients are sufficient to support a diverse network of fungi and animals filling niches equivalent to their surface dwelling counterparts. Only in this case none of the animals have eyes and instead use heat, sound/sonar sonar and smell/taste to navigate their world.
