Archaebacteria
single: archaebacterium

Archaebacteria have a wide range of shapes: spheres, rods, spirals, lobed, flat rectangular or irregular. Some exist as single cells, other form filaments or clusters. Some are motile.
They are often called extremophiles because they are found in extreme conditions in the environment, such as in hot springs, or the depths of the ocean. Included in this group are anaerobic methanogens, which produce methane, halophiles living in salty conditions and thermophiles which can live at high temperatures.
Archaebacteria reproduce by either splitting in two (binary fission), budding or fragmentation.