Bacteria are the answer. They are ubiquitous and many are autotrophic. Autotrophic is a fancy way to say they make their own food; some literally metabolizing rock. Bacteria are a foundational lifeform and are distinct from animals and plants which have a different cell structure. Even viruses can’t match the work that bacteria do, as viruses require DNA/RNA to live (they require life to live; parasitic bastards…). Bacteria live on their own accord and theoretically could live on Mars, Titan, or any temperate, rocky planetoid. There are bacteria that require no host or life to eat and so are the answer to your question.
I’d suggest you research deep sea vents for more about water-to-lithosphere interfacing bacteria. Those kind use thermal discharges from tectonic activity. They use the hot gases coming from the deep of the Earth to metabolize sulfur and that heat facilitates reactions critical for cellular life in the cold, dense water. I’d next suggest you research bacteria discovered in deep rock from mines. Samples collected from deep mines (3km+) always have some sort of bacteria that is digesting the rock. I don’t know how they do it or what their primary source of fuel is, but bacteria are there, living on rock under high pressure. Makes humans seem weak, eh? In the end, bacteria are the primary lifeform on Earth.
You’ll come to find the organisms of our planet that make their living from nonliving things are bacterial. They likely have viruses in them, but those viruses require their life. The bacterial host requires the Earth’s matter and energy to live. Bacteria are a definition of life moreso than animals.
Puts things into perspective…