Single-celled, microscopic organisms that reproduce by dividing in half, they lack nuclei, and live in extreme places like hot springs are

Archae bacteria

Explanation:

Archae bacteria are single celled prokaryotes. They are different from Eubacteria which are considered as true bacteria. Archebacteria have few properties which make them different from both bacteria and eukaryotes. They are similar in size and structure to bacteria but their cell membrane is made of ether lipids which makes them resistant to hostile environment. Archae bacteria are found in extreme places and are also classified according to their location. For ex: halophiles are found in extreme marine environment, thermophiles are found in extremely hot places like hot springs.