The two domains of prokaryotes, which are archae and bacteria, contain the common organisms of cyanobacteria, halophiles and hyperthermophiles. Prokaryotes occur in many forms, and some species are more common than others. Of the two prokaryotic domains, the bacteria domain is the most populous and diverse.
Organisms in both prokaryotic domains survive in a variety of conditions, including some of the coldest and hottest environments on Earth. Bacteria are among the most diverse and abundant organisms on Earth. They live in oceans, the soil and in digestive tracts of most living organisms, including plants, animals and humans. Bacteria are located in rocks far below the surface of the Earth, and in the aggregate, have a population exceeding five trillion. Bacteria may be benign or malignant, and are responsible for maintaining optimal health as well as spreading disease. In this domain, cyanobacteria are prominent members. These species vary in size and location, and even reproductive habits. These organisms are classified based on specific traits, such as biological differences in their cell walls and outer membranes. The archae domain includes organisms such as extremophiles, which are specially adapted to living in harsh environments. Extremophiles include salt-loving halophiles, which live in salty waters, and hyperthermophiles, which thrive in very hot water.