GENERAL MICROBIOLOGY - T.P. Pyrog - 2004

7. PROKARYOTE SYSTEMATICS

7.5. CHARACTERISTICS OF TAXA ACCORDING TO THE NINTH EDITION OF BERGEY'S MANUAL OF SYSTEMATIC BACTERIOLOGY

7.5.4. Division Mendosicutes

Archaea (archaebacteria) are primarily soil or aquatic microorganisms found in anaerobic conditions within hypersaline, hydrothermal, and geothermal environments, as well as symbionts in the animal digestive tract. This group includes aerobes, anaerobes, and facultative anaerobes capable of growing as chemolithoautotrophs, heterotrophs, or facultative heterotrophs. Archaea can be mesophiles or thermophiles, with some species able to grow at temperatures above 100 C.

A unique biochemical feature of archaea is that their Lipids contain isoprenyl glycerol ethers. The absence of murein (peptidoglycan containing muramic acid) in their Cell walls makes archaea resistant to β-lactam Antibiotics (Penicillins). The nucleotide sequences of 5S, 16S, and 23S rRNAs differ significantly from those of eubacteria and eukaryotes.

Carbon assimilation in autotrophic archaea does not occur via The Calvin Cycle. CO₂ is fixed through the acetyl-CoA pathway or the reductive Tricarboxylic Acid Cycle. Some archaea are capable of fixing molecular nitrogen. Their Coenzymes and prosthetic groups include components that, while similar to those in eubacteria and eukaryotes, are not identical: a 5-deazariboflavin derivative F₄₂₀, the nickel-tetrapyrrole factor "MISO" methanopterin, and coenzyme M.

Gram-staining results can be either positive or negative within the same order due to wide variations in cell envelope types. In Gram-positive species, cell walls consist of pseudomurein, methanochondroitin, and Heteropolysaccharides. Gram-negative Cells possess surface layers composed of Glycoproteins.

Cell Morphology is diverse: spherical, spiral, plate-like, or rod-shaped. Both unicellular and multicellular forms occur as filaments or aggregates. Individual cell diameters range from 0.1 to 15 µm, and filament lengths can reach 200 µm. Reproduction occurs via binary fission, budding, constriction, fragmentation, or unknown mechanisms. The color of cell masses can be red, purple, pink, orange-brown, yellow, green, dark green, gray, or white.

According to Bergey's Manual of Determinative Bacteriology (1997), archaea are placed in groups 31–35 and divided into five main groups.

Methanogens (Group 31). These include representatives of 18 genera (Methanobacterium, Methanococcus, Methanosarcina, Methanospirillum, Methanobrevibacterium, Methanogenium, Methanothrix, etc.). Notably, only 13 genera of these Bacteria were described in 1984.

Strict anaerobes capable of producing methane as the primary end product of METABOLISM. Substrates can include H₂ + CO₂, formate, acetate, methanol, methylamines, or H₂ + methanol. Sulfur (S⁰) can be reduced to H₂S without energy generation. They are meso- to thermophiles, neutrophiles (optimal pH 7.0), and include halophilic species. They may exhibit blue-green fluorescence when irradiated with light at a wavelength of 420 nm. They contain coenzyme M, factor 420, factor 430, and methanopterin.

Sulfate-reducing archaea (Group 32). This group includes the genus Archaeoglobus, with two described species: Archaeoglobus fulgidus and Archaeoglobus profundus. Data on Archaeoglobus species are absent in Bergey's Manual of Systematic Bacteriology, as they were described in 1988–1990.

Cells are irregularly coccoid, often triangular, occurring singly or in pairs. Gram-negative. They form greenish-black colonies 1–2 mm in diameter. Strict anaerobes capable of producing H₂S from sulfate via dissimilatory sulfate reduction. Additionally, they produce small amounts of methane. Extreme thermophiles (growth up to 92 C), pH range 4.5–7.5 (optimum around 6), salinity range 0.9–3.6% sodium chloride. Capable of chemolithotrophic, chemoorganotrophic, or chemomixotrophic growth. Thiosulfate and H₂ are required for autotrophic growth. Under heterotrophic conditions, they utilize formate, lactate, glucose, starch, and Proteins as electron Donors. Under ultraviolet light (wavelength 420 nm), they exhibit blue-green fluorescence. They contain factor 420 and methanopterin, but lack coenzyme M and factor 430.

Extremely halophilic aerobic archaea (halobacteria) (Group 33). These include representatives of six genera (Halococcus, Halobacterium, Haloarcula, Haloferax, Natronobacterium, Natronococcus). In 1984, only three genera of halobacteria were described: Halococcus, Haloarcula, and Halobacterium.

Gram-negative or Gram-positive, aerobic or facultatively anaerobic chemoorganotrophs. Cells are rod-shaped, ranging from regular to highly irregular in morphology. They require high concentrations of sodium chloride (1.5 M and above). Neutrophiles or alkaliphiles. Mesophiles or somewhat thermophilic (up to 55 C). Some species contain the photoactive red-purple pigment Bacteriorhodopsin and are capable of utilizing light for ATP synthesis.

Cell wall-less archaea (Group 34). Thermoacidophilic aerobes (comprising a single genus, Thermoplasma). Optimal Temperature is around 60 C, optimal pH 1–2. Cells are coccoid, lacking a cell wall. They can be referred to as thermoacidophilic "Mycoplasmas." The cytoplasmic membrane contains a mannose-rich glycoprotein and lipoglycan.

S⁰-metabolizing extreme thermophiles and hyperthermophiles (Group 35). Comprising 14 genera (Acidianus, Desulfurolobus, Sulfolobus, Pyrobaculum, Pyrococcus, Desulfurococcus, etc.). Only seven genera were described in 1984.

Obligate thermophiles, aerobes, facultative anaerobes, or strict anaerobes. Gram-negative rods, filaments, or cocci. The optimal growth temperature ranges from 70 to 105 C. Acidophiles and neutrophiles. Autotrophs or heterotrophs. Most species metabolize sulfur.

Some key features distinguishing eubacteria from archaea are presented in Table 7.3.

Class="center">Table 7.3

Selected characteristics distinguishing eubacteria from archaea

The discovery of archaea (1977) sparked a debate regarding their place in The system of living organisms and revived Structure/149.html">The problem of cellular evolution. Currently, two hypotheses exist concerning the pathways of cellular evolution (Fig. 7.1).

Fig. 7.1. Possible pathways of cellular evolution: a — monophyletic; b — polyphyletic

According to one hypothesis (Fig. 7.1, a), a population of ancestral Prokaryotic Cells emerged from a population of primary cells, subsequently giving rise to various groups of prokaryotes (a monophyletic group). The Introduction/5.html">Eukaryotic Cell evolved through endosymbiosis between a host Prokaryotic Cell and an endosymbiont. Such host cells could have been archaebacteria that branched off from ancestral prokaryotes at an early stage of their existence and engulfed free-living prokaryotes, which later evolved into eukaryotic Chloroplasts and Mitochondria. According to another concept (Fig. 7.1, b), eubacteria, archaebacteria, and the nuclear-cytoplasmic component of the eukaryotic cell (mitochondria, chloroplasts) diverged independently from a common ancestor, eventually reaching the organizational level of modern prokaryotic cells (polyphyletic origin).