There are many unique characteristics that make up these prokaryotes. They are specially resistant to high temperatures and high acid concentrations. They have a plasma membrane which contains high amounts of saturated fats, and its enzymes are able to withstand extreme conditions without denaturation.
The similarities between DNA sequences of thermoacidophiles, and other Archaebacteria, and complex eukaryotes provides support to Archae being the progenitor species for the first cellular life on Earth. They were able to thrive on the early, warmer Earth with an atmosphere that lacked oxygen.
Archaeobacteria constitute the third domain of living organisms, one distinct from that represented by the eubacteria and the eucaryotes.
Archaeobacteria are procaryotes, like eubacteria, however, and therefore are most facilely compared to eubacteria (i.e., archaeobacteria represent a monophyletic taxon of bacteria-like things).
Nevertheless, some aspects of archaeobacteria are more eucaryote-like than eubacteria. Most fascinating about archaeobacteria are the often bizarre environments in which they inhabit including water whose temperature exceeds that of hot springs.
Archaeobacteria nevertheless often may be differentiated in terms of Gram staining.
This is because the Gram stain is a measure of physical aspects of cell walls that are shared between the eubacteria and the archaeobacteria (though gram-negative archaeobacteria lack outer membranes).
There exist cell-wall less archaeobacteria which live in the high temperature (55 to 59°C) and acidic piles of coal tailings.
The following is quoted from Prescott et al., 1996 (p. 478): As a group the archaeobacteria [Greek archaios, ancient, and bakterion, a small rod] are quite diverse, both in morphology and physiologically. They can stain either gram positive or gram negative and may be spherical, rod-shaped, spiral, lobed, plate-shaped, irregularly shaped, or pleomorphic. Some are single cells, whereas others form filaments or aggregates. They range in diameter from 0.1 to over 15 µm, and some filaments can grow up to 200 µm in length. Multiplication may be by binary fission, budding, fragmentation, or other mechanisms. Archaeobacteria are just as diverse physiologically. They can be aerobic, facultatively anaerobic, or strictly anaerobic. Nutritionally they range from chemolithoautotrophs to organotrophs. Some are mesophiles; others are hyperthermophiles that can grow above 100°C. Archaeobacteria usually prefer restricted or extreme aquatic and terrestrial habitats. They are often present in anaerobic, hypersaline, or high-temperature environments. Recently archaeobacteria have been discovered in cold environments. It appears that they constitute up to 34% of the procaryotic biomass in coastal Antarctic surface waters. A few are symbionts in animal digestive systems.
The archaeobacteria cell wall differs chemically from that of the eubacteria cell wall. Specifically, they lack in peptidoglycan.
Typical structure of eubacteria monoglyceride: H H-C-OH O
| ||H H H H H H HH-C -O- C-C-C-C-C-C-C-C-H
| H H H H H H H HH-C-OH
Typical structure of archaeobacteria monoglyceride: H H H H-C-OH H-C-H H-C-H
| H H | H H H | HH-C -O- C-C-C-C-C-C-C-C-H
| H H H H H H H HH-C-OH
Archaeobacteria lipid bilayers also contain lipids consisting of ether-linked hydrocarbons stretched between glycerol moieties, linked at both ends (think of two fats joined at the end of their fatty acid chains and you'll get an idea: |==| where | is glycerol, = are two parallel fatty acids, and |= is a eubacterium diglyceride).
For these linked lipids each glycerol is found in the opposite membrane leaflet, at the hydrophilic-hydrophobic interface.
An archaeobacteria membrane spanning, glycerol-based lipid (only one of expected two spanning hydrocarbon chains shown): H H H H H H-C-OH H-C-H H-C-H H-C-H H-C-H
| H H | H H H | H H H | H H H H | H HH-C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-O-C-H
| H H H H H H H H H H H H H H H H H |H-C-OH H-C-OH
One obvious explanation for the existence of such lipids is that they may make the archaeobacteria membrane sufficiently stable, at least in part, to allow growth and survival in the extreme environments in which many archaeobacteria may be found.
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