Ancient bacteria are single-celled microorganisms that are able to live in extreme environments and have significant biological differences from regular bacteria. In the three-domain classification system for living things, ancient bacteria belong to the archaea domain.
There are three main groups of ancient bacteria. The crenarchaeota group includes microorganisms that live in extremely hot, cold or acidic environments, such as near hydrothermal vents. The euryarchaeota group includes microorganisms that live in extremely salty or oxygen-free environments, such as in salt lakes and mammalian digestive systems. The korarchaeota group contains primitive microorganisms that are largely unstudied as of 2014 and live in hot springs and obsidian pools
Ancient bacteria are similar to true bacteria because they are prokaryotic, which means that they do not have a true nucleus that is separated from the rest of the cell by a membrane. Instead, their DNA is coiled in a portion of the cell called the nucleoid. On the other hand, unlike true bacteria, ancient bacteria get energy from a wide variety of unusual sources, such as hydrogen, metal ions and ammonia, and their cell membranes have a different structure, with ether-linked fats instead of the ester-linked fats found in the membranes of true bacteria.