At first glance, members of domain Archaea look very much like Bacteria in morphology, but biochemical and evolutionary studies have shown that they are a unique branch of life, separate from Bacteria (Eubacteria) and Eukaryotes. This was first recognized by comparing the sequences of their ribosomal deoxyribonucleic acid (DNA) and their type of cell wall to those of other organisms. Although Archaea also have a prokaryotic cell organization, other differences set them apart from Bacteria. While most Archaea have cell walls, they do not contain murein as in Bacteria, but are made of a number of different molecules, including proteins. The lipids found in their cell membranes are also different from those found in Bacteria and eukaryotes. Archaea can be motile by rotating flagella, but the proteins that make up the flagella are different from those found in Bacteria. Archaea have a number of traits that make them more similar to eukaryotes than to Bacteria. For example, in Archaea ribonucleic acid (RNA) polymerases and other proteins involved in making RNA from DNA are more similar to those in eukaryotes than those in Bacteria. Because of these and other similarities to eukaryotes, Archaea are thought to be the ancestors of the nuclear and cytoplasmic portions of eukaryotes.
Archaea include many organisms that live in extreme environments or that have unique metabolisms. These include methanogenic (methane-making) Archaea, halophilic (salt-loving) Archaea, extremely thermophilic (heat-loving) sulfur metabolizers, and thermoacidophiles, which live in acidic high-temperature environments.
Methanogens are killed in the presence of oxygen and live in anoxic places, such as the muds of rice fields and the guts of animals, particularly insects and cows. They produce methane, or natural gas, which is used by humans as a source of energy.
Halophiles can only live in places with very high salt concentrations, much saltier than the open oceans. They contain a pigment similar to one found in human eyes, bacteriorhodopsin, which allows them to use light energy to make adenosine triphosphate (ATP). Carotenoid pigments, which help shield the cells from damaging ultraviolet (UV) light, make the cells appear orange-red. High-salt aquatic areas containing many halophilic Archaea can be seen from a distance because of this red color.
Thermoacidophiles are a group of Archaea that can live in very acidic environments at elevated temperatures. They are found in hot springs suchas those in Yellowstone National Park, volcanoes, burning coal piles, or at undersea hydrothermal vents. Many of them use sulfur compounds for their metabolism. Some are hyperthermophiles, organisms that live at the highest temperatures known (between 80° and 113°C). They are even found living in boiling water.
Colored transmission electron micrograph of the archaea Methanospirillum hungatii undergoing cell division.
Because Archaea inhabit extreme environments that were probably prevalent on the early Earth, some believe that they are an old group of organisms (hence their name) that may hold clues to the origin of life. However, extreme thermophiles have also been found among the Bacteria, and Archaea have been shown to be abundant in more moderate environments as well. Environmental studies using DNA survey techniques (PCR) show that low-temperature Archaea make up a significant portion of the prokaryotic biomass in terrestrial and planktonic marine environments. From these types of environmental PCR studies, which can tell us what kind of organisms are present in the environment without relying on traditional methods of culturing, we know that both Archaea and Bacteria are abundant in the biosphere, and that the majority of these organisms and their ecological role have yet to be described and understood.
