Taxonomy of Bacteria and Archaea
Bacteria ( /bækˈtɪəriə/ ( listen ) ; singular : bacterium ) constitute a large domain of prokaryotic microorganisms . Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals. Bacteria were among the first life forms to appear on Earth , and are present in most habitats on the planet, growing in soil, water, acidic hot springs , radioactive waste , [ 2 ] and deep in the Earth's crust , as well as in organic matter and the live bodies of plants and animals, providing outstanding examples of mutualism in the digestive tracts of humans, termites and cockroaches.
The Archaea ( /ɑrˈkiːə/ ( listen ) or /ɑrˈkeɪːə/ ar- KEE -ə or ar- KAY -ə ; singular: archaeon or archeon ) constitute a domain of single-celled microorganisms . These microbes have no cell nucleus or any other membrane-bound organelles within their cells. In the past Archaea had been classed with bacteria as prokaryotes (or Kingdom Monera ) and named archaebacteria , but this classification is regarded as outdated. [ 1 ] In fact, the Archaea have an independent evolutionary history and show many differences in their biochemistry from other forms of life, and so they are now classified as a separate domain in the three-domain system .