Archaea

It can be described as, group of (primitive) bacteria living in extreme aquatic and terrestrial environments. They can stain either gram positive or gram negative and may be spherical, rod-shaped, spiral, plate-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.

Cell walls

Archaeal cell wall is made of glycoprotein or protein instead of peptidoglycan. Methanobacterium and some other methanogens have walls containing pseudomurein, a peptidoglycanlike polymer that has L-amino acids in its cross-links, N-acetyltalosaminuronic acid, and β(1→3) 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 walls.

Lipids and membranes

This is the most distinctive nature of archaea is membrane lipids. They are different from bacteria and eukaryotes. Archaeal lipids are derivatives of isopranyl glycerol ethers rather than the usual glycerol fatty acid esters. They also contain phospholipids, sulfolipids and glycolipids. Archaeal nonpolar membranes are the derivative of squalene, 30 carbon compound, presence of diethers, tetraethers are needed for their stability to thrive in extreme environments.

Fig. 4. Archael glycerolipids