Archaea Bacteria - Structure and chemical composition of Archaeal cell wall and cell membranes

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. 

Archaeal cell walls stain either Gram positive or Gram negative, depending on the thickness and mass of cell wall. 

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. 

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-acetyltalosaminuronic acid instead of N-acetylmuramic 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.

Sometimes, two glycerol groups are linked to form long tetraethers. Usually, the diether hydrocarbon chains are 20 carbons in length, and the tetraether chains are 40 carbons. Cells adjust the  length of the tetraethers by forming pentacyclic rings. Such pentacyclic rings are used by thermophilic archaea to help maintain the delicate balance of the membrane at high temperatures. Biphytanyl chains contain 1 to 4 cyclopentyl rings. 

        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.