Bacteria and Archaea are two domains of prokaryotic microorganisms. Bacteria usually reach several micrometres in length, while their cells can have a variety of shapes: from spherical to rod-shaped and spiral. Bacteria are one of the first forms of life on the Earth and are found in almost all terrestrial habitats. They inhabit soil, fresh and marine bodies of water, acidic hot springs, radioactive waste and the deep layers of the earth's crust. Bacteria are often symbionts and parasites of plants and animals. Bacteria, unlike plants and other eukaryotes, do not have a nucleus and membrane organelles.
Similar to bacteria, archaea are unicellular microorganisms that do not have a nucleus, as well as any membrane organelles. Previously, archaea were combined with bacteria in a common group called prokaryotes, and they were called archaebacteria. However, it was found that archaea have their own independent evolutionary history and are characterized by many biochemical features that distinguish them from other life forms. Despite the resemblance to bacteria, some genes and metabolic pathways of archaea bring them closer to eukaryotes. None of the known representatives of archaea is neither a parasite nor a pathogenic organism. However, they are often mutualists and commensals. Archaea are especially numerous in the oceans. Like bacteria, archaea play an important role in the carbon and nitrogen cycles.