Bread, alcoholic spirits. Note lots of acid, lower pH than many other fermentations. Certain bacteria can do this. Hydrogen gas can be liberated directly. Can only happen if H 2 is removed, so its concentration remains low.
Fermentations play major role.
Many sugars can be fermented. Respiration Use an external electron acceptor. Oxygen as prototype. Alternative solution is to use some non-organic molecule that has a low redox potential, can accept electrons and become some reduced molecule. In mitochondria, this is a cytochrome, cyt a.