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.
Diversity of Archaebacteria
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
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
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.
Sulfur Reducers
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.
Thermoacidophiles
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.
Structure
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.
