Distinct Characters of ArchaeaEdit
Although sharing similar size and shape with eubacteria, archaea constitute an independent new domain in the current classification system due to its fundamental differences both to eubacteria and eukaryotes. Archaea cells are most similar to Gram-positive cells because they both have a single lipid bilayer cell membrane and a thick cell wall. However, archaea cells lack peptidoglycan (although some archaea do have cell walls containing pseudopeptidoglycan) in the cell wall composition. Moreover, the lipid bilayer of archaea cells is significantly different from its counterparts in both eubacteria and eukaryotic cells. Archaea cell membranes are composed of glycerol-ether lipids instead of glycerol-ester lipids shared by eubacteria and eukaryotic cells. The stereochemistry of the glycerol moiety in archaea cell membrane is the mirror image of that found in other organisms. The lipid tails in the lipid bilayer of archaea cells are chemically different and sometimes the lipid bilayer is substituted by a single layer of lipid.
Genetics, metabolism and significance of archaeaEdit
Archaeas can obtain energy from inorganic compounds, such as sulfur, ammonia and carbon dioxide. Other archaea can use methane or sunlight as a source of energy. Similar to eubacteria, archaea usually have a single circular chromosome, the sizes of which range from 490kb to 5751kb   . Plasmids are also found in archaea. Archaea reproduce asexually by binary fission, fragmentation or budding. Archaea are often seen as extremophiles because they are usually found in high temperature, high salinity, very acidic, or very alkaline habitats. However, archaea are also found in normal temperature environments and they are often found to live a mutual or commensal life together with other protozoa or animals. Those archaea resistant to heat are of great significance in technology and industry. For example, the Pfu DNA polymerase used in PCR reaction is from Pyrococcus furiousus. Heat resistant amylase and galactosidases from other species of Pyrococcus genus are also commonly used in food industry.
 Waters E, et al. (2003). "The genome of Nanoarchaeum equitans: insights into early archaeal evolution and derived parasitism ". Proc. Natl. Acad. Sci. U.S.A. 100 (22): 12984–8.
 Galagan JE, Nusbaum C, Roy A, et al. (2002). "The genome of M. acetivorans reveals extensive metabolic and physiological diversity ". Genome Res. 12 (4): 532–42. doi:10.1101/gr.223902