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Archaea are prokaryotic but are phylogenetically distinct from eubacteria. They are a diverse group that has not been widely studied due to difficulties in culturing them from various environments. They are considered briefly, as many survive in severe environments that may be biocidal to other microorganisms and offer some interesting, if not rare, examples of microbial resistance mechanisms (see sections 8.3.9 and 8.3.10). It should be noted that bacteria and other microorganisms can survive and even multiply over a quite wide range of conditions, including temperature, pH, and the presence or absence of oxygen (aerobic or anaerobic). Those that grow under extremes of these conditions are referred to in combining form as “-philes”; for example, thermophiles (or thermophilic microorganisms) can survive at high temperatures, psychrophiles grow in cold environments, halophiles survive extreme salt conditions, and acidophiles or alkaliphiles are found in low- or high-pH environments (for further discussion, see section 8.3.10). In general, the archaea are found under extreme conditions within these ranges. For this reason, they are often referred to as extremophiles and can be considered to form four general groups: thermophiles (which survive in extreme high or low temperatures), halophiles (which survive in extreme high-salt concentrations), methanogens (which can survive under unique anaerobic conditions), and barophiles (which can survive high hydrostatic pressure). Examples are given in Table 1.12.

It should be noted that in many cases these extreme conditions are actually required for the growth of archaea. As an example, Pyrococcus cells have an optimum temperature of 100°C but require at least 70°C for growth. Further, halobacteria, such as Halobacterium, require a minimum salt level of 1.5 M for growth.

TABLE 1.12 Examples of extremophile archaea

Type	Description	Habitat example(s)	Typical conditions	Example(s)
Halophiles	Grow under high-saline conditions	Salt or soda lakes	9–32% NaCl	Halobacterium, Natronobacterium
Thermophiles	Grow at high temperatures, some under extreme acidic or basic conditions	Hydrothermal vents, hot springs	50–110°C	Sulfolobus, Thermococcus, Pyrococcus
Methanogens	Strict anerobes that produce methane (CH4) gas from CO2 and other substrates	Sediments, bovine rumens (anaerobic digesters)	Strictly anaerobic; H2 and CO2 used for CH4 production	Methanobacterium, Methanospirillium
Barophiles (or piezophiles)	Grow optimally at high hydrostatic pressure	Deep sea	Low temperature (2–3°C) and high pressure (> 100 kPa, e.g., 20–100 MPa)	Methanococcus

Structurally, the archaea are similar to eubacteria (Table 1.3), but they present diverse cellular mechanisms that allow survival under extreme conditions. Overall, they have unique lipids (generally short-chain fatty acids) in their cell membranes, but also polysaccharides and/or proteins in their cell walls that differ from those of eubacteria. It is interesting that, similar to the mycoplasmas (see section 1.3.4.1), some archaea have no associated cell wall. Examples are Thermoplasma species, which contain a thick, unique cell membrane, which allows the growth and metabolism of the genus (see section 8.3.10). The cell membrane contains a unique LPS consisting of mannose-glucose polysaccharide attached to lipid molecules and glycoproteins that gives the membrane greater rigidity and temperature resistance. Some archaea have a surface structure similar to that of eubacteria, with a cell membrane bounded by a cell wall. The cell wall may contain a polysaccharide similar to peptidoglycan called pseudopeptidoglycan, with alternating N-acetylglucosamine and N-acetylalosaminuronic acid. Others do not have a peptidoglycan but a cell wall made up of proteins and polysaccharides. An example is the halophilic Halobacterium, which contains a salt-stabilized glycoprotein cell wall. Others species produce an external proteinaceous layer, similar to bacterial capsules (see section 8.3.7), which is known as an S-layer. In many methanogens, S-layers consisting of a crystalline structure of proteins may be found.