Domain Archaea
A member of the prokaryotes, Archaea are most notably known for inhabiting extreme environments and their unusual metabolic capabilities. Their structural composition is unique in that their cell walls lack peptidoglycan and their membrane lipids are made up of long, branched hydrocarbons connected to glycerol by ether linkages. There are no known pathogenic species in this domain.

Phylum Euryarchaeota
This group includes gram positive Archaea and other variable species.  It is comprised of halophiles and methanogens.
Halobacterium salinarum
This marine species is a bacillus shaped halophile, found in salted fish, hypersaline lakes, and in salterns.  Their optimum saline levels are ten times the concentration of sea water and thus, many of their proteins are unable to function at lower salt concentrations.  Lacking a cell wall, their single lipid bi-layer forms a lattice with the surrounding cell surface glycoproteins.  This structure maintains a negative charge, and stabilizes the lattice under high saline concentrations.  Additionally, their cell membranes contain pumps to actively transport potassium into the cell, reducing osmotic stress.  While amino acids are their main source of chemical energy, they can also utilize light energy through the protein, bacteriorhodopsin.  This protein is similar to rhodopsin which is found within the human retina.  It works by producing a proton gradient to obtain its energy.  H. salinarum also contains the carotenoid, bacterioruberin, which is not only responsible for their red color, but also absorbs UV light to protect the cell's DNA by acting as an antioxidant.

Phylum Crenarchaeota
This group consists of gram negative thermophiles and acidophiles.

Sulfolobus solfataricus
These thermoacidophiles are found in sulfurous hot springs and mud pools, under high temperatures and varying pH.  While their optimum temperature and pH is 75 degrees Celsius and 2-3 pH, they can survive in temperatures ranging from 55-90 degrees Celsius, with a pH ranging from .9-5.8.  S. solfataricus are coccoid, free-living chemolithotrophs, growing in only aerobic conditions.  Their metabolic processes allow them to oxidize sulfur to sulfuric acid and metabolize various carbohydrates using sulfur as their electron donor.  They are capable of synthesizing all twenty amino acids and have one of the largest chromosomes among the Archaea which is approximately 3.1 Mbp in size.