Members of the Archaea show considerable diversity of both morphology and physiology. In view of the fact that the Archaea remained unidentified as a separate group for so many years, it should come as no surprise that they do not display any obvious morphological differences from true bacteria, and all the main cell shapes (see Chapter 2) are represented. More unusual shapes are also encountered in archaea; members of the genus Haloarcula have flattened square or triangular cells! Both Gram-positive and Gramnegative forms of archaea are found, but neither possesses true peptidoglycan. Some types have a so-called pseudomurein, composed of different substituted polysaccharides and l-amino acids (Figure 7.2). Most archaea, however, have cell walls composed of a layer of proteinaceous subunits known as an S-layer, directly associated with the cell membrane. This difference in cell wall chemistry means that members of the Archaea are not susceptible to antibacterial agents such as lysozyme and penicillin, whose action is directed specifically towards peptidoglycan. Differences are also found in the make-up
W-acetyltalosaminuronic acid OH,
Figure 7.2 Pseudomurein, found in the cell walls of certain members of the Archaea, comprises subunits of N-acetylglucosamine and N-acetyltalosaminuronic acid. The latter replaces the N-acetylmuramic acid in true peptidoglycan (c.f. Figure 3.6). (Ac represents the acetyl group). As with peptidoglycan, the amino acids in the peptide chain may vary, however in pseudomurein they are always of the L-form. The components in parentheses are not always present.
of archaean membranes, where the lipid component of membranes contains branched isoprenes instead of fatty acids, and these are joined to glycerol by ether-linkages, rather than the ester-linkages found in true bacteria (Figure 7.3). The diversity of archaea extends into their adopted means of nutrition and metabolism: aerobic/anaerobic and autotrophic/heterotrophic forms are known. Many members of the Archaea are found in extreme environments such as deep-sea thermal vents and salt ponds. Some extreme thermophiles are able to grow at temperatures well over 100 °C, while psychrophilic forms constitute a substantial proportion of the microbial population of Antarctica. Similarly, examples are to be found of archaea that are active at extremes of acidity, alkalinity or salinity. Initially it was felt that archaea were limited to such environments because there they faced little competition from true bacteria or eucaryotes. Recent studies have shown however that archaea are more widespread in their distribution, making up a significant proportion of the bacterial biomass found in the world's oceans, and also being found in terrestrial and semiterrestrial niches. The reason that this lay undetected for so long is that these organisms cannot as yet be cultured in the laboratory, and their presence can only be inferred by the use of modern DNA-based analysis.
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