What did the first single-cell life on Earth eat?

Eating and eliminating is part of the very definition of life. So the first living things on Earth had to eat something.

Today, Earth’s food chain is one big circle. The tiniest single-cell life is eaten by slightly larger living animals and plants, which are then eaten by larger ones and so on up to the great herbivores which are eaten by the alpha predators. The alpha predators and great herbivores then die, decay, and feed the plants and the single-cell life forms. But before there was any of that life to feed on, what did the first prokaryotes eat?