Introduction
|
Chapter 17: Microbial Diversity: Archaea Introduction |
|
Chapter 17 (p 744-788) focuses on the members of the domain Archaea. While the Archaea are prokaryotes, they are as distinctive from the Bacteria as they are from the eukaryotes. The Archaea were once known as archaebacteria and live in many extreme environments. The Archaea can be divided into four groups: the methanogens, the halophiles, the hyperthermophiles and the genus Thermoplasma. The distinctive features of this group were also briefly discussed in Section 18.7 and this section should be reviewed
The Archaeal membranes differ from Bacterial membranes in that they contain ether-linked lipids bonded to glycerol. The ether-linked lipids are common to all Archaea. Glycerol diethers and diglycerol tetraethers are the major types of lipids present in the cell membrane. The Archaea also contain large amounts of non-polar lipids. The overall arrangement of the cell membrane is similar to that found in Bacteria and Eukarya. The Archaea can alter the thickness of their membrane by including or removing pentacyclic rings in the structure.
Archaeal cell walls do not contain muramic acid and D-ammino acids, the building blocks of peptidoglycan; particular species may contain pseudopeptidoglycan, polysaccharide, glycoprotein, or protein in their cell walls.
Metabolism in the Archaea is varied, ranging from chemoorganotrophic reactions to autotrophic utilization of CO2. In general, the types of metabolism within the group are similar to what is found in the Bacteria. The major exception are the reactions leading to methanogensis.
The six genera of extremely halophilic Archaea inhabit hypersaline environments and will not grow at NaCl concentrations less than 1.5 molar. All extreme halophiles can grow at salt concentrations near the salt's saturation point. Two genera are not only halophilic, but also alkalinophilic. That is, they grow best at pH values above 9. These organotrophic bacteria require Na+ ions to stabilize their glycoprotein cell wall. The high external salt concentration is balanced by the intracellular accumulation of K+ ions.
Certain species of Halobacterium can synthesize ATP using light energy. However, the process does not involve chlorophyll pigments as in photosynthesis, but rather a membrane protein called bacteriorhodopsin. The absorption of light by retinal associated with this protein is used to pump protons across the cell membrane. The resulting proton motive force can drive ATP synthesis via a membrane-bound ATPase.
Methanogens are strict anaerobes which convert one of three classes of substrate: CO2, methyl compounds, or acetate to methane gas. The formation of methane can be viewed as a type of anaerobic respiration. These organisms contain a unique set of coenzymes which are necessary for the reduction of C-1 intermediates to methane; for example, coenzyme M is involved in the final step of methane formation. When CO2 is the carbon source, the acetyl CoA pathway is used to produce organic carbon, rather than the Calvin cycle. Although their physiological diversity is limited, methanogens comprise seven morphological groups.
Hyperthermophilic Archaea includes the most thermophilic of all known prokaryotes. All representatives require reduced sulfur compounds for their metabolism; in most cases, reduced sulfur is used as an electron acceptor to carry out anaerobic respiration. However, Sulfolobus can grow autotrophically using elemental sulfur as an energy source.
Thermoplasma is a cell-wall-less prokaryote similar to the mycoplasmas. However, it is an acidophilic, aerobic chemoorganotroph that is also thermophilic. It is generally found in self-heating coal refuse piles. The cell membranes of the organism are chemically unique, containing lipopolysaccharide that consists of tetraether lipid with mannose and glucose subunits.
A Pearson Company Distance Learning at Prentice Hall Legal Notice |