The corpses of fungi, you uneducated droolpan. In some cases they just parasitize the fungi (Myco-Heterotrophy). In some cases it's "symbiotic" in that the fungi sends its dead cells and receives sugar in exchange. Also what do you think fertilizer is? It's dead plant matter processed by animals and insects. Without fungus feeding on soil enriched by microbes, plants wouldn't exist on land.

And in the ocean, especially where there is little light, how do you think plants survive, you example of your education system? By eating microbes and the poop of other creatures eating microbes and plants.

And where do those microbes, when we go far enough down, get their energy? By eating amino acids that would otherwise form into life. I guess you could say this is the bottom of the chain as long as fetuses aren't people so amino acids aren't life, but that seems like a cop out at that point.