Archaea Prokaryotic cells

The smallest free-living microorganisms are the prokaryotes, comprising bacteria and archaea (see Chapter 2). Prokaryote is a term used to define cells that lack a true nuclear membrane they contrast with eukaryotic cells (e.g. plants, animals and fungi) that possess a nuclear membrane and internal compartmentalization. Indeed, a major feature of eukaryotic cells, absent from prokaryotic cells, is the presence in the cytoplasm of membrane-enclosed organelles. These and other criteria differ-... 

All living things are composed of either prokaryotic cells or eukaryotic cells. Prokaryotes, which include bacteria and the archaea, lack a membrane-bound cellular organelle called a nucleus. The eukaryotes consist of all the remaining species. These cells contain a nucleus and complex structures that are not observed in prokaryotes. 

One of the most commonly observed surface structures on the prokaryotic cell envelopes is crystalline arrays of proteinaceous subunits, termed surface layers (S-layers). In archaea, S-layers bind directly to the cytoplasmic membrane, in Gram-negative bacteria to lipopolysaccharides of the outer membrane, and in Gram-positive bacteria to the cell wall. 

Figure 1 Schematic illustration of the supramolecular architecture of the three major classes of prokaryotic cell envelopes containing crystalline bacterial cell surtece layers (S-layers). (a) Cell envelope structure of Gram-negative archaea with S-layers as the only cell wall component external to the cytoplasmic membrane, (b) Cell envelope as observed in Gram-positive archaea and bacteria. In bacteria the rigid wall component is primarily composed of peptidoglycan. In archaea other wall polymers (e.g., pseudomurein or methanochondroitin) are found, (c) Cell envelope profile of Gram-negative bacteria, composed of a thin peptidoglycan layer and an outer membrane. If present the S-layer is closely associated with the lipopolysaccharide of the outer membrane. (Modified after U.B. Sleytr, P. Messner, D. Pum, and M. Sdra. Crystalline Bacterial Cell Surface Proteins. Austin, TX R.G. Landes/Academic Press, 1996. With permission.)...

Cell envelopes of prokaryotic organisms (archaea and bacteria) are characterized by the presence of two distinct components the cytoplasmic membrane, which constitutes the inner layer, and an outer supramolecular layered cell wall (for reviews see Ref. 4), which pre-... 

Photosynthetic prokaryotes do not have chloroplasts. Their photosynthetic pigments are embedded in their cell walls. Some use bacteriochlorophyll for light harvesting. In the proteobacteria and archaea, light harvesting is accomplished by the protein rhodopsin, which acts as a photo-driven proton pump that fuels phosphorylation of ADP. 

The hyperthermophilic archaea of the genus Thermococcus, isolated from marine hydrothermal systems, has been found to be a rich source of cyclic methylene-sulfur [135]. The prokaryotic archaea, established as the third domain of life in addition to eukaryotes and bacteria, grow under extreme conditions such as the absence of oxygen, temperatures of 100 °C and saturated salt solutions. Thus, 23 cyclic polysulfides (145-167) could be isolated from the intact cells of Thermococcus tadjuricus (strain Ob9) and T. mococcus acidaminovorans (strain Vc6bk) by using chemical screening methods. 

Non-enzymatic attack In non-enzymatic attack of minerals by microbes, reactive products of microbial metabolism come into play. The microbial enzymes responsible for metabolic product formation are located below the cell envelope, in the cytoplasm of prokaryotes (Bacteria and Archaea) and in cell organelles and/or the cytoplasm of eukaryotes (e.g. fungi, algae, lichens). In these instances of microbial attack, physical contact of the microbial cells with the surface of a mineral being attacked is not essential. The reactive metabolic products are formed intracellularly and are then excreted into the bulk phase where they are able to interact chemically, i.e. non-enzymatically, with a susceptible mineral. Depending on the type of metabolic product and mineral, the interaction with the mineral may result in mineral dissolution or mineral diagenesis by oxidation or reduction or acid or base attack. Mineral dissolution or diagenesis may also be the result of complexation by a microbial metabolic product with that capacity. In some instances mineral attack may involve a combination of some of these reactions. 

Prokaryotic Unicellular, filamentous, colonial or mycelial Little or no differentiation Anaerobic, aerobic, facultatively-anaerobic, microaerophilic or aerotolerant Asexual reproduction Cell walls (with some exceptions) bacteria and archaea Monera... 

Cell envelopes of archaea diifer distinctly from those of bacteria and show remarkable structural and chemical diversity. Murein, the typical sacculus-forming polymer of bacteria, and lipopolysaccharide-containing outer membranes, characteristic of gramnegative bacteria, are not found in archaea. Crystalline surface layers (S-layers) are common in both prokaryotic domains and they consist of protein or glycoprotein subunits (Table 1). However, S-layers in archaea have a form-stabilizing function especially when they are the only envelope layer outside the cytoplasmic membrane, while in bacteria S-layers have no distinct form-stabilizing function. 

The lack of a murein cell-wall sacculus and the discovery of different cell-envelope polymers and structures in some physiologically unusual prokaryotes, was one of the first biochemical and cytological evidences in favour of Carl Woese s archaebacteria concept [46,149,150]. Since then, increasingly more unique cell-envelope polymers and new types of biosynthetic pathways have become known. These findings corroborate the proposal that the archaea represent a third lineage of organisms [150] in addition to bacteria and eucarya, and that the common ancestor or ancestral population of the archaea did not evolve any cell-wall polymer before it radiated into the various sublineages known today [46,151]. 

---