The word Archaea is derived from the Greek word Archaios which means ancient.
The Archaeabacteria, are quite diverse, both in morphology and physiology. They may be spherical, rod-shaped, spiral, lobed, cuboidal, triangular, 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.
The Archaea are diverse physiologically- can be aerobic, facultatively anaerobic, or strictly anaerobic. They include psychrophiles, mesophiles, and hyperthermophiles that can grow above 100°C. Nutritionally they may be chemolithoautotrophs to organotrophs.
Ecology
Archaea are found in areas with either very high or low temperatures or pH, concentrated salts, or completely anoxic. These are generally referred to as “extreme environments.”
Extreme and hypersaline are situations where humans could not survive. Most of the Earth (the oceans) is an “extreme environment” where it is very cold (about 4°C), dark, and under high pressure. Many Archaea are well adapted to these environments, where they can grow to high numbers. Archaea constitute at least 34% of the prokaryotic biomass in some Antarctic coastal waters. In some hypersaline environments, the brine is red with archaeal pigments. Some archaea are symbionts in the digestive tracts of animals. Archaeal gene sequences have been found in soil and temperate and tropical ocean surface waters.
Thus, the Archaea are highly diverse with respect to morphology, reproduction, physiology, and ecology. Although best known for their growth in anoxic, hypersaline, and high-temperature habitats they also inhabit marine arctic, temperate, and tropical waters. Their RNA, ribosomes, elongation factors, RNA polymerases, and other components distinguish Archaea from Bacteria and eukaryotes. Much of archaeal metabolism appears similar to that of other organisms, but the Archaea differ with respect to glucose catabolism, pathways for CO2 fixation, and the ability of some to synthesize methane.
Archaeal Cell Walls
The Archaeal cell wall, like the bacterial cell wall, is a semi-rigid structure which provide protection to the cell from the environment and from the internal cellular pressure. The cell walls of bacteria typically contain peptidoglycan, but it is absent in Archaea.
The chemistry of Archaeal cell walls is different from that of Eubacteria. Archaea lack the muramic acid and D-amino acids that make up peptidoglycan and thus resist attack by lysozyme and β lactam antibiotics such as penicillin.
Archaeal cell walls stain either Gram positive or Gram negative, depending on the thickness and mass of cell wall.
Gram positive Archaea have a variety of complex polymers in their cell wall. Methanobacterium and some other methanogenic archaea have pseudomurein (a peptidoglycan-like polymer that is cross-linked with L-amino acids), N-acetyl talosaminuronic acid instead of N-acetyl muramic acid and β (1- 3) glycosidic bonds instead of β (1-4) glycosidic bonds. Methanosarcina and Halococcus lack pseudomurein and contain complex polysaccharides similar to the chondroitin sulfate of animal connective tissue. Other heteropolysaccharides are also found in Gram positive cellwalls.
Gram negative Archaea have a layer of protein or glycoprotein (20-40 mm thick) outside their plasma membrane. Some methanogens (Methanolobus), Halobacterium, extreme thermophiles (Sulfolobus, Thermoproteus, Pyrodictium) have glycoproteins in their walls. Other methanogens (Methanococcus, Methanomicrobium, Methanogenium) and extreme thermophile Delsuphurococcus have protein walls.
Structure, function and chemical composition of archaeal cell membranes
One of the most distinctive archaeal features is their membrane lipids.
Archaeal membrane lipids differ from those of other organisms in having glycerol connected to branched chain hydrocarbons by ether links.
Bacterial and eukaryotic lipids have glycerol connected to fatty acids by ester bonds.
Phosphate-, sulfur- and sugar-containing groups can be attached to the third carbons of the diethers and tetraethers, making them polar lipids -phospholipids, sulfolipids, and glycolipids. These predominate in the membrane, making up 70 to 93% of the membrane lipids. The remaining lipids (7-30%) are nonpolar and are usually derivatives of squalene.
These lipids are combined in different ways to yield membranes of various rigidity and thickness. A regular bilayer membrane is formed when C20 diethers are used. A much more rigid monolayer membrane is formed when the membrane is constructed of C40 tetraethers. Archaeal membranes may contain a mix of diethers, tetraethers and other lipids.
The membranes of extreme thermophiles such as Thermoplasma and Sulfolobus contain tetraether monolayers which provide stability. Archaea that live in moderately hot environments have a mixed membrane containing some regions with monolayers and some with bilayers.
Differences between Archaebacteria and Eubacteria
American microbiologist and biophysicist Carl Richard Woese proposed three kingdom classification system in 1990. This classification system divides the life forms into three domains and six kingdoms.
The three domains are archaea, bacteria, eukaryote, and six kingdoms are Archaebacteria (ancient bacteria), Eubacteria (true bacteria), Protista, Fungi, Plantae, Animalia.
Woese classified them based on their differences in the 16S ribosomal RNA (rRNA) structure. 16S rRNA can be used for comparative analysis between prokaryotic and eukaryotic species.
Carl Woese used the rRNA as an “Evolutionary Chronometer” – an evolutionary time clock.
The Archaea (Archaebacteria)
- Archaea are ancient bacteria - believed to have evolved just after the evolution of first life on earth.
- Archaea are prokaryotic cells. The cell walls of Archaea contain no peptidoglycan, hence Archaea are not sensitive to some antibiotics that affect the Bacteria.
- Archaea have membranes composed of branched hydrocarbon chains (many also containing rings within the hydrocarbon chains) attached to glycerol by ether linkages.
- The ether-containing linkages in the Archaea membranes is more stable than the ester-containing linkages in the Eubacteria and are better able to withstand higher temperatures and stronger acid concentrations.
- Archaea often live in extreme environments and include methanogens, extreme halophiles, and hyperthermophiles.
- Archaea contain rRNA that is unique to the Archaea, distinctly different from the rRNA of Bacteria and Eukarya.
Hydrothermal vents on the ocean floor, where the surrounding water can reach over 300° Celsius, are home for some archaeal species.
The Bacteria (Eubacteria)
Bacteria (also known as eubacteria or "true bacteria") are prokaryotic cells that are common in human daily life. Eubacteria can be found almost everywhere and serve as antibiotic producers and food digesters, pathogens etc.
Bacteria are prokaryotic cells. They have membranes composed of unbranched fatty acid chains attached to glycerol by ester linkages.
The cell walls of Bacteria contain peptidoglycan. Bacteria are sensitive to antibacterial antibiotics.
Bacteria contain rRNA that is unique.
Bacteria include mycoplasmas, cyanobacteria, Gram-positive bacteria, and Gram-negative bacteria.
Thus, in short, Archaebacteria are called ancient bacteria whereas the eubacteria are called true bacteria. Eubacteria are usually found in soil, water, living in and on of large organisms. Eubacteria are divided into two groups known as gram positive and gram negative bacteria. Archaebacteria are found in salt brines, ocean depths and hot springs. Three types of archaebacteria are found: methanogens, halophiles and thermoacidophiles.
| ArchaeaBacteria | EuBacteria |
| -Ancient bacteria- | -True bacteria- |
Complexity | Simple in their organization | Complex than archaebacteria
|
Habitat | Can sustain in extremely harsh environment such as oceans, hot springs, marshlands, hot springs and gut of animals | Found everywhere - soil, organic matter, earth’s crust, water, bodies of animals and plants, radioactive wastes, hot springs |
Size | 0.1-15 μm in diameter
| 0.5-5 μm in diameter |
Shape | spheres, rods, plates, spiral, flat or square-shaped | cocci, bacilli, vibrio, rods, filaments or spiral in shape |
Cell wall | Pseudopeptidoglycan | Lipopolysaccharide/ Peptidoglycan with muramic acid |
Membrane lipids | Ether-linked, branched, aliphatic chains, containing D-glycerol phosphate | Ester-linked, straight chains of fatty acids, containing L-glycerol phosphates |
RNA | Consists of single RNA
| Three types of RNA |
RNA polymerase | Complex subunit pattern
| Simple subunit pattern |
Introns (a long stretch of noncoding DNA found between exons (or coding regions) in a gene) | Present in archaebacteria
| Absent in eubacteria |
Metabolism | Methanogenesis- exhibit neither glycolysis nor Kreb’s cycle | Autotrophy, Aerobic and Anaerobic Respiration, Fermentation and Photosynthesis-exhibit both glycolysis and Kreb’s cycle |
Reproduction and Growth | Asexual Reproduction, by fragmentation, budding and binary fission | Other than binary fission, budding and fragmentation, eubacteria can produce spores in order to remain dormant during unfavorable conditions |
Types
| Methanogens, halophiles and thermophiles | Gram positive and Gram negative |
Examples | Halobacterium, Thermoproteus, Pyrobaculum, Thermoplasma and Ferroplasma | Mycobacteria, Bacillus, E. coli, Pseudomonas, Clostridium etc |
Interesting Read
https://www.ck12.org/c/biology/archaea/lesson/Introduction-to-Archaea-Advanced-BIO-ADV/
https://www.science.org.au/curious/earth-environment/what-are-archaea
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