Archaebacteria live in some of the most harsh habitats. They are present in the gut of several ruminant animals.
They are characterized by the presence of a cell wall made of pseudomuerin and not rigid cellulosic cell wall. Mycoplasmas are not included in archaebacteria. They only include methanogens, halophiles and thermoacidophiles.
Cyanobacteria are referred to as blue green algae.