Archaea is the second group of prokaryotes (the other group is bacteria). They are many other differences and similarities between Archaea and Bacteria. Both are prokaryotes. Both were regarded as Bacteria until Carl Woese (1953) suggested that they belong to two different domains. 3.5 billion years ago Earth consist of harsh environments conditions; High temperature, UV rays, Low oxygen condition, Shock waves and Toxic gases.
Archaea can tolerate these extreme environments. Most archaea are of conventional morphology, that is, rods, cocci, and helixes, but some are of very unusual morphology. Some are gram-positive, others gram-negative; some may divide by binary fission, others by fragmentation or budding; a few lack cell walls. They can live extreme environments. (Aquatic & terrestrial) These included habitats at the extreme limits that allow life on earth, in terms of temperature, pH, salinity, and anaerobiosis. Typical environments from which pure cultures of archaeal species have been isolated include;
- Hot springs
- Hydrothermal vents
- Salt lakes
- Soda lakes
- Sewage digesters
- The rumen
- Inside glaciers & frozen lakes
Cultivated members of the archaea can be placed into five physiological or nutritional groups. Most described species fall within the phyla Crenarchaeota and Euryarchaeota, while only a handful of species have been described for the Nanoarchaeota, Korarchaeota and Thaumarchaeota. Physiologically, archaea are found under extreme environmental conditions . According to the environment they can be group to ;
According to the 16s ribosomal proteins from sequenced genomes Archaea can be divided to 5 main phylums.
Their cell walls have differences when compared to the cell walls of bacteria. Instead of peptidoglycan they have a similar compound called pseudopeptidoglycan (pseudomurein). It is a polymer of N-Acetyl glucosamine and N-Acetyl glucosaminuronic acid. They are linked by β – (1,3) linkages. So, they are safe from lysozyme activities. Some Archaea have polysaccharide cell walls and some have proteins. Some Archaea has crystalline proteins which are hexagonal.
Extremophilic adaptations of Archaea
- Specific characteristics in archaeal Cell wall
A. Pseudomurein and other Polysaccharide Walls
The cell walls of certain methanogenic Archaea contain a molecule that is remarkably similar to peptidoglycan, a polysaccharide called pseudomurein. Pseudomurein also differs from peptidoglycan in that the glycosidic bonds between the sugar derivatives are β-1,3 instead of β-1,4, and the amino acids are all of the l stereoisomer. Bacterial eptidoglycan, is a polysaccharide composed of two sugar derivatives N-acetylglucosamine and N-acetylmuramic acid and a few amino acids, including l-alanine, d-alanine, d-glutamic acid, and either l-lysine or a structurally similar molecule, DAP. Therefore, Archaea are naturally resistant to the antibiotic penicillin and lysozyme due to lack of peptidoglycan.
B. S- Layers (Surface Layers)
The most common type of cell wall in Archaea is the para crystalline surface layer. S-layers consist of interlocking molecules of protein or glycoprotein.
Biological functions of S- layer
- S-layers are sufficiently strong to withstand osmotic pressures without any other wall components.It prevent the osmotic lysis.
Protecting cell against different ion and pH fluctuations, degrading enzymes.
This layers helps in maintaining shape and envelope rigidity of the cell.
- S- layers act as a receptor for attachment of bacteriophage.
Cell walls of some Archaea lack pseudomurein and instead contain other polysaccharides. For example, Methanosarcina species have thick polysaccharide walls composed of polymers of galactosamine uronic acid, glucuronic acid, glucose, and acetate. This acid complex knows as Methanochondrotin.
Major types of Archaea Cell Wall
A – The most common type of archeal cell wall is an S layer composed of either protein or glycoprotein.
B – In this type, additional layers of material are present outside the S-layer.
C – In here, S-layer is covered by mathanochondroitin.
D – S-layer is the outermost layer and is separated from the plasma membrane by a psedomurien.
E – In this type, S-layer is absent and Instead archaeal cell wall is single thick, homogenous layer resembling Gram positive bacteria.
Ex. Methanobacterium, Halococcus
F -The outermost membrane contains protein complexes that form pores, much like bacterial porin proteins create pores in the outer membrane of typical Gram negative bacteria.
Ex. is Ignicoccus hospitalis
2. Specific characteristics in archaeal Cellular membrane
Their are few fundamental differences between the archaeal membrane and other prokaryotic cells.
(1) Chirality of glycerol
(2) Ether linkage
(3) Isoprenoid chains
(4) Branching of side chains
(5) Formation of tetra ether
3. Specific characteristics in archaeal flagella
Few Archaeal members are capable of swimming, including methanogens, extreme halophiles, Thermosacidophiles, and Hyperthermophiles. Archaeal flagella are roughly half the diameter of bacterial flagella. But, impart movement to the cell by rotating, as do flagella in Bacteria. One of most important feature of the flagellar proteins is that they are similar to the proteins in bacterial type IV pili. Archaeal flagella are directly powered by ATP rather than by the proton motive force.
4. Molecular biological characteristics
Molecular biological characteristics of Archaebacteria is more closely related with Eukarya than Bacteria. Transcription factor proteins are similar in both Archaea and Eukarya. During the initiation of Transcription process, TATA-binding protein has bound to the TATA box and transcription factor (TF) B has bound to the B recognition element, then archaeal RNA polymerase can bind and initiate transcription. Archaeal RNA polymerase is more closely related to eukaryotic RNA polymerase than bacterial RNA polymerase. However, Archaeal RNA Polymerase is complex than bacterial one and it is more similar than Eukaryotic RNA Polymerases. Stability of RNA polymerase is high in Archaea. No Rho-like proteins are involved for the transcription process in Archaea Both Archaea and Eukarya have RNA polymerase which have 11 or 12 subunits. But Bacterial RNA Polymerase have only 5 subunits (including sigma factor).
Translation starting amino acid (which is encoded by the Start codon) in Archaea is Methionine. It is same as in Eukarya. But in bacteria, Formyl-methionine is the starting amino acid. Archaea is highly resistant for Chloramphenicol and Streptomycin which are translational inhibitors of bacteria. And also, they are resistant to Penicillines because their cell walls lack of peptidoglycan. Extreme halophilic archaea are inhibited by Novobiocin (a DNA gyrase inhibitor) and Mevinolin (an inhibitor of isoprenoid biosynthesis). Methanogenic archaea are inhibited by Puromycin and Neomycin (both protein synthesis inhibitors).
Michael T. Madigan, John M. Martinko, Kelly S. Bender, Daniel H. Buckley, D. A. S. (2015). BROCK BIOLOGY OF MICROORGANISMS (14th ed.). Pearson Education,Inc
Pasindu Chamikara – Microbiologist