The answer to that question is found in another question: “What looks like a bacteria, but isn’t?”
All life on Earth can be subdivided into just three Domains. One of these is bacteria. Another is called Eukaryota, which are all plants and animals. The third Domain is something most people have never heard of.
It is called Archaea. They look like bacteria, but have more in common with plants and animals than they do with bacteria.
https://en.wikipedia.org/wiki/Archaea
And they eat things that bacteria do not eat, in places so hostile that bacteria cannot live there. They can live in boiling or freezing water, very acidic or basic places, yet one variety is so ordinary that it lives in the human gut, and actually improves our digestion, indirectly.
And the answer to your question, what do they eat?, is many things that nothing else eats. They can eat methane gas, hydrogen gas, sulfur, carbon, and nitrogen.
Very curious. I have heard of, and even used, “probiotic” supplements, but never heard of a formulation “containing Archaea.”
Think of the sudden and catastrophic consequences of a sudden bloom of a bacteria which aggressively eats plastics. Talk about the ultimate weapon?!
And of course their DNA is similar to what?