| Archaea and Bacteria |
As reviewed in "Evolution and Phylogenetics," living organisms can be grouped into three domains: the Archaea, the Bacteria, and the Eukarya. Members of Bacteria and Archaea are prokaryotes: single-celled organisms lacking true nuclei and other membrane-enclosed organelles. Bacteria and archaea, however, differ in cell wall characteristics and membrane lipid composition. They also differ in RNA polymerase structure and, therefore, protein synthesis.
Many extremophiles (organisms that tolerate high or low temperature, high salinity, or extreme pH) fall within the Archaea. Some archaea, the extreme halophiles (salt lovers), tolerate salt concentrations as high as nearly ten times that of seawater.
They have also been found thriving in the Great Salt Lake and the Dead Sea. Nevertheless, habitat alone does not differentiate the groups. Some bacteria grow at temperatures above 80° C, and some Archaea have been found in environments not considered extreme. For example, methanogenic archaea live in anoxic sediments in marshes and are used in sewage treatment facilities. Another archaean, Methanobrevibacter smithii, lives and generates methane in the human colon.