Archaebacteria , diverse group of bacteria (prokaryotes), sometimes called the archaea and considered a major group unto themselves. Archaebacteria are contrasted with the Eubacteria, from which they differ biochemically in the arrangement of the bases in their ribosomal RNA and in the composition of their plasma membranes and cell walls. There are three major known groups of Archaebacteria: methanogens, halophiles, and thermophiles. The methanogens are anaerobic bacteria that produce methane. They are found in sewage treatment plants, bogs, and the intestinal tracts of ruminants. Ancient methanogens are the source of natural gas. Halophiles are bacteria that thrive in high salt concentrations such as those found in salt lakes or pools of sea water. Thermophiles are the heat-loving bacteria found near hydrothermal vents and hot springs. Many thermophiles are chemosynthetic (see chemosynthesis), using dissolved sulfur or other elements as their energy source and iron as a means of respiration. Archaebacteria emerged at least 3.5 billion years ago and live in environments that resemble conditions existing when the earth was young.
Archaebacteria are a type of prokaryote, that is, a unicellular organism without a cell nucleus. They make up the kingdom Archae, one of the main kingdoms of life. These organisms are difficult to classify because they have similarities to both normal bacteria and the larger eukaryotes. In structure, they are like unicellular prokaryotes, but the genetic transcription and translation underlying their creation is similar to that of the more complex eukaryotes.
Able to live in a variety of environments, archaebacteria are known as extremophiles. Certain species are able to live in temperatures above boiling point at 100° Celsius or 212° Fahrenheit. They can also thrive in very saline, acidic, or alkaline aquatic environments. They employ a variety of chemical tricks to accomplish this, with one species, halobacteria, able to convert light into adenosine triphosphate (ATP) or cell energy, using a non-photosynthetic process. Halobacteria live in waters almost completely saturated with salt, and unlike photosynthetic plants, are incapable of extracting carbon from atmospheric carbon dioxide.