The Dark Warlock Susie has heard of a cave system with a lethal density of argon that is protecting a burgeoning monster population. In the interest of beefing up her security, Susie wants to recreate the underground environment near her magical spire. However some of the monsters are non-carnivores and she cannot figure out what they consume without leaving the cave nor killing off outside plant life by digging into their roots. Also, even warlocks fear creating new life and would prefer using something already in existence.
Assuming a hunter-gatherer mentality, where the food organisms are encouraged to grow but are not cultivated. What would be the primary anaerobic food stuff that actively grows for non-carnivores in a light deprived environment?
The environment can be altered in any way except it must contain 1% or less oxygen in the air, the argon is not a concern here. Susie is only a Dark Warlock after all!