Archaebacteria are single-celled organisms that are commonly found in highly inhospitable locations with extreme temperature fluctuations or in water with high alkaline or acid levels. The term archaebacteria has been widely abandoned for the use of archaea, as they are their own class of organism and are not bacteria.
Archaebacteria were discovered in the 1970's by Dr. Carl Woese, who determined that they could not be classified as other single-celled organisms such as eukaryotes or eubacteria due to their genetic and biochemical differences. Archaebacteria have highly resistant cellular membranes with sugars and amino acids that are different than those typically found in the cellular membranes of bacteria. Archaebacteria are able to withstand external influences that would be fatal to bacteria, due to the chemical structure of their cell walls. They are highly adept at surviving in areas where other organisms cannot, such as near underwater geothermal vents, or in water of high salinity that would otherwise kill or repel fish and other microorganisms.
Archaebacteria known as methanogens are found in the digestive systems of several types of mammals, insects and fish, and produce methane as part of their energy production process. Thermophiles are able to survive in habitats with high temperatures, while psychrophiles dwell in extremely cold environments. Halophiles are found in environments that have a high salt content. Archaebacteria are also found in more hospitable environments such as marshlands, soil and ocean water.