Microbial single-celled organisms are prokaryotes. They include bacteria and archaea.
One of several differences between archaea and bacteria is archaeal tolerance for living in the most extreme environments, at temperatures and in chemical conditions that no other life can withstand. Bacteria are robust, but archaea are tremendously tough. Despite distinctions between the two, their cell structure and functioning are largely selfsame.
Prokaryotes are everywhere: in the soil, water, and air. Their reach far exceeds all other life. Microbes are by far the most abundant and diverse life on the planet; some 25 times the total biomass of all animal life. There are well over a million different types of prokaryote.