General Microbiology - Schlegel, H. 1987

Prokaryotes. Systematic Review
Methanogenic Bacteria and Other Archaebacteria (Group 13)

In Bergey's Manual, methane-producing Bacteria are assigned to Group 13. For many reasons, they have recently been grouped with two other groups under the general name "archaebacteria". They differ from all other bacteria—the eubacteria—in A number of important traits and are found in extreme habitats. These conditions are similar to those that likely existed at the dawn of life on Earth. Among archaebacteria, there are lithoautotrophs and heterotrophs, anaerobes and aerobes. All of them share many unique features characteristic exclusively of this group. However, individual archaebacteria apparently exhibit the same variations in Cell Morphology, cellular components, and METABOLISM as those found among eubacteria. The Current state of research allows archaebacteria to be divided into three main groups: methanogens, halobacteria, and thermoacidophilic bacteria.

General characteristics. Archaebacteria share a whole range of common features. This applies to the COMPOSITION OF THE Cell wall, Lipids, Transcription and Translation machinery, prosthetic groups and Coenzymes, The Mechanism of autotrophic CO₂ fixation, as well as the mode of energy generation. Although research findings are still quite fragmentary, we will attempt to provide a General Overview of this group.

The Cell wall lacks a peptidoglycan backbone; at best, it contains pseudomurein or solely Proteins and Polysaccharides. This explains why archaebacteria are insensitive to Antibiotics that disrupt eubacterial cell wall synthesis, such as penicillin, cephalosporin, and D-cycloserine.

Instead of fatty acid glycerol esters, The Plasma Membrane contains glycerol ethers with C₂₀ (phytanyl) and C₄₀ (biphytanyl) isoprenoid alkyl groups. Free C₁₅ and C₃₀ isoprenoid Hydrocarbons also occur as neutral lipids:

Class="center">

Archaebacteria differ from eubacteria in the composition of their DNA-dependent RNA polymerases; in archaebacteria, these Enzymes consist of more than four subunits and are insensitive to the antibiotics rifampicin and streptolydigin. Ribosomal Nucleic Acids (16S and 5S) differ significantly in nucleotide sequence. Translation is insensitive to chloramphenicol, yet it is inhibited by diphtheria toxin, which does not affect eubacteria but suppresses Protein Synthesis in eukaryotes.

Among coenzymes and prosthetic groups, there are components that, although similar to those in eubacteria and eukaryotes, are not identical to them: the 5-deazariboflavin derivative F₄₂₀, the nickel-tetrapyrrole factor F₄₃₀, methanopterin, coenzyme M, and others. These factors were discovered in methanogenic bacteria (Section 9.4). However, it is still too early to draw overarching Conclusions.

Autotrophic CO₂ fixation is not carried out via the ribulose bisphosphate cycle. Methanogenic bacteria utilize the acetyl-CoA pathway for this purpose (Section 9.4). However, this pathway also operates in a number of eubacteria.

The energy-converting process (ATP regeneration) is likely based on the establishment of a proton gradient and the functioning of ATP synthase, and can be viewed as a primitive form of "Anaerobic Respiration". The electron acceptor is CO₂, sulfur, or (in a few archaebacteria) molecular oxygen.

Methane-producing bacteria. Among methane-producing bacteria, virtually all morphological forms known in eubacteria are represented: cocci (Methanococcus vannielii), rods (Methanobacterium formicicum), short rods (Methanobrevibacter ruminantium, M. arboriphilicus), spirilla (Methanospirillum hungatei), coccal packets (Methanosarcina barkeri) or filaments (Methanothrix soehngenii), and even forms that produce cell plates (Methanoplanus limicola). Both mesophilic and thermophilic species exist (Methanobacterium thermoautotrophicum, Methanothermus fervidus). Currently, six families are already distinguished, and the number of described genera and species is growing rapidly. The GC content ranges from 27 to 61 mol %. The ecology and metabolism of these bacteria will be discussed in Section 9.4.

Halobacteria. The genera Halobacterium and Halococcus¹ comprise extremely halophilic forms. These are aerobic heterotrophs found in salt evaporation ponds where marine salt is harvested. During mass proliferation of halobacteria containing carotenoids, the Water appears bright red. They grow best in a 3.5–5 M NaCl solution. Another characteristic feature is their ability to utilize light energy in their metabolism (Section 12.3).

Thermoacidophilic bacteria. This group still encompasses non-methanogenic thermophilic archaebacteria that have very little in common with one another. Among them are autotrophs and heterotrophs, extremely acidophilic and neutrophilic, as well as aerobic and anaerobic representatives.

Sulfolobus acidocaldarius inhabits acidic hot springs, where this bacterium oxidizes sulfur to sulfate (Section 11.2). Thermoplasma acidophilum stands apart; much like Mycoplasmas, it lacks a cell wall. This bacterium grows best at 59°C and pH 1–2. Its habitat includes self-heating coal mine refuse heaps, but it has also been found in hot springs. Its genome is the smallest among all studied genomes of non-parasitic bacteria (1∙10⁹). It is capable of growing in Yeast extract as an aerobic heterotroph. Opposing these aerobic species is a group of anaerobic species collectively termed Thermoproteales. They were isolated from hot springs on volcanic slopes and on the ocean floor. These are extreme thermophiles (with optima ranging from 85 to 105°C) and exhibit a type of metabolism known as "sulfur respiration" (Section 9.3); they oxidize H₂ and reduce elemental sulfur to hydrogen sulfide. Among them are facultative autotrophs (Thermoproteus tenax), obligate autotrophs (Thermoproteus neutrophilus, Pyrodictium occultum), and heterotrophic bacteria (Desulfurococcus, Thermococcus, Thermodiscus).

¹ Recently, three new genera of halobacteria have been described: Haloarcula, Natrobacterium, and Natrococcus. — Ed. note.

As this group of organisms is studied further, it becomes increasingly clear that archaebacteria, judging by the characteristics of their cellular components, diverged from eubacteria very early on; furthermore, most of them are likely direct descendants of primeval bacteria that learned to utilize inorganic H Donors (H₂) and H acceptors (CO₂, sulfur) available at the dawn of evolution, thereby contributing to The formation of reduced carbon deposits in sedimentary rocks more than 3 billion years ago.