I made an organism called the Skarlix for a art/xenobiology project called Sagan 4.
The Skarlix is a eukaryotic, somewhat amoeba-like organism that "eats" various metal compounds and "breathes" (oxidizes) hydrogen (e.g., in the form of hydrogen gas, unless other compounds are more plausible). Its metabolism is based partly on the brief Wikipedia description for knallgas bacteria. However, I think I messed up at some point in its description, probably because I didn't understand the difference between "eating" and "breathing" at the molecular level.
I think "eating", on the cellular level, means gaining energy for growth. Is there a way to make it gain mass by eating metal compounds of some sort, but use hydrogen in roughly the same ways human cells use oxygen? If not, is there a way to make it need very little to no oxygen, "breathe" an alternative element or chemical, and rely on metal in sediment?
More information: This is a free-living, somewhat amoeba-like (in shape) simple eukaryote which lives in sediment in the ocean. While I don't need to go into detail on electron donors and acceptors in its description, I do need to know whether this metabolic combination is even plausible. If not, I should alter it. Ideally, any alteration wouldn't prohibit its descendants from plausibly living in the guts of a creature that consumes very iron-rich foodstuffs, or within the blood of a creature with manganese-based blood.
The Skarlix was created in an era equivalent to the Precambrian/Early Cambrian on its planet, after a big extinction event. Since it was created, many microbes have developed, probably including those that make hydrogen compounds. While pairing it with a microbe that consumes hydrogen makes sense, that's going much too far into its timeline to be plausible. As for its diet of "metals", that was left unspecified in the description. Early lithotrophs in the Sagan 4 project didn't have their exact diets specified, presumably so even people unfamiliar with lithotrophic biochemistry could stock the planet with lithotrophs. Any metal which makes sense in this situation is appropriate, except copper, which I specified is too poisonous to it. (not sure about the plausibility of that, but it's stuck there. Another organism was later developed which ate copper, an under-utilized resource because the Skarlix wouldn't eat it.)