The name "archaebacteria," with its prefix meaning "ancient," suggests that this is an extremely old group. The fact that most of these Monerans live in extremely hostile environments similar to those found on primitive Earth leads many to believe that archaebacteria may have been the earliest forms of life on the planet. However, as a separate phylogenetic group, the Archeabacteria are actually younger than the Eubacteria, sharing a much more recent common ancestor with eukaryotes than eubacteria do.
While some archaebacteria are heterotrophic, the vast majority are chemoautotrophs, meaning they produce their own food from chemicals found in their environments. Based on the method by which they do this and the type of environment in which they are found, archaebacteria can be classified into four groups: methanogens, halophiles, sulfur reducers, and thermoacidophiles.
Methanogens are anaerobic, feeding on decaying plant and other organic material, producing water and methane gas. They can be found in bogs and marshes, deep in the oceans, and in the gastrointestinal tracks of cellulose- fermenting herbivores where they aid in the digestion of cellulose. Some methanogens thrive near volcanic vents. The ability of these archaebacteria to survive near such vents greatly interests scientistst, since the water in these areas reaches temperatures of up to 110 degrees Celsius. Most organisms are not able to endure these conditions: their proteins lose shape and cease to function at around 45 degrees Celsius. How methanogens have adapted to this extreme heat is not known.
Halophiles are phototrophs (producing their energy from light) that use a purple version of chlorophyll called bacteriorhodosin. They live in extremely salty conditions such as those found in the Great Salt Lake and the Dead Sea. Such environments present two challenges. First, the difference in salt concentration inside and outside the cell is tremendous, creating huge osmotic pressure. While other organisms would rapidly lose all of their water and die, halophiles have adapted to survive within such a difference in water gradient. Second, the salty environments are very alkaline, some having a pH of up to 11.5. Beyond simply surviving within these inhospitable environments, halophiles have incorporated the conditions into their unique photosynthetic pathway. Most halophiles are aerobes.
Like methanogens, sulfur reducers live near volcanic vents and pools. As their name suggests, they use the abundant inorganic sulfur found near these vents, along with hydrogen, as food. They also have very high heat tolerances, living in temperatures up to 85 degrees Celsius.
Thermacidophiles also live off of sulfur, but they do so by oxidizing it, combining the sulfur with oxygen molecules rather than hydrogen. Like the methanogens and sulfur reducers, these archaebacteria live near volcanic vents and pools and thus are adapted to high temperatures (65 to 80 degrees Celsius). Unlike the other two classes, though, thermoacidophiles also prefer extremely acidic conditions, living in environments with a pH as low as 1.0. Almost all thermoacidophiles are obligate anaerobes.
The structure of Archaebacteria varies greatly due to the extremely dissimilar environments among which these organisms range. While most have cell walls similar to those of the eubacteria, their composition differs greatly both from that of the eubacteria and between the different types of archaebacteria. Some methanogens have cell walls made of pseudopeptidoglycan, a molecule similar to the peptidoglycan that makes up eubacterial walls. The cell walls of other archaebacteria lack pepitoglycan-like molecules and are made of polysaccarides, glycoproteins, or proteins.
Compared to the wealth of information we have about eubacteria, little is known about the archaebacteria. The phylum's general simple structure and life processes are similar enough to those of the eubacteria that the two groups are classified together as the kingdom Monera; to date, however, the differences that enable archaebacteria to live in the extreme circumstances that would kill eubacteria have not been discovered. Perhaps when those difference come to light, the classification will change.