Supplemental Lecture (97/04/20 update) by Stephen T. Abedon (abedon.1@osu.edu)

  1. Chapter title: Archaeobacteria
    1. A list of vocabulary words is found toward the end of this document
    2. Archaeobacteria constitute the third domain of living organisms, one distinct from that represented by the eubacteria and the eucaryotes. Archaeobacteria are procaryotes, like eubacteria, however, and therefore are most facilely compared to eubacteria (i.e., archaeobacteria represent a monophyletic taxon of bacteria-like things). Nevertheless, some aspects of archaeobacteria are more eucaryote-like than eubacteria. Most fascinating about archaeobacteria are the often bizarre environments in which they inhabit including water whose temperature exceeds that of boiling water at sea level, as well as the saltiest of salty habitats.
  2. Overview (of archaeobacteria)
    1. The following is quoted from Prescott et al., 1996 (p. 478):
      1. As a group the archaeobacteria [Greek archaios, ancient, and bakterion, a small rod] are quite diverse, both in morphology and physiologically.
      2. They can stain either gram positive or gram negative and may be spherical, rod-shaped, spiral, lobed, plate-shaped, irregularly shaped, or pleomorphic.
      3. Some are single cells, whereas others form filaments or aggregates.
      4. They range in diameter from 0.1 to over 15 µm, and some filaments can grow up to 200 µm in length.
      5. Multiplication may be by binary fission, budding, fragmentation, or other mechanisms.
      6. Archaeobacteria are just as diverse physiologically. They can be aerobic, facultatively anaerobic, or strictly anaerobic.
      7. Nutritionally they range from chemolithoautotrophs to organotrophs.
      8. Some are mesophiles; others are hyperthermophiles that can grow above 100°C.
      9. Archaeobacteria usually prefer restricted or extreme aquatic and terrestrial habitats.
        1. They are often present in anaerobic, hypersaline, or high-temperature environments.
        2. Recently archaeobacteria have been discovered in cold environments. It appears that they constitute up to 34% of the procaryotic biomass in coastal Antarctic surface waters.
        3. A few are symbionts in animal digestive systems.
    2. Following is quoted from Prescott et al., 1996 (p. 476):
      1. Archaeobacteria differ in many ways from both eubacteria and eucaryotes. These include differences in cell wall structure and chemistry, membrane lipid structure, molecular biology, and metabolism.
      2. Archaeobacteria grow in a few restricted or specialized habitats: anaerobic, hypersaline, and high temperature.
      3. Bergey's Manual divides the archaeobacteria into five major groups: methanogenic archaeobacteria, sulfate reducers, extreme halophiles, cell wall-less archaeobacteria, and extremely thermophilic S0-metabolizers.
      4. Methanogenic and sulfate-reducing archaeobacteria have unique cofactors that participate in methanogenesis.
      5. Archaeobacteria have special structural, chemical, and metabolic adaptations that enable them to grow in extreme environments.
  3. Cell wall
    1. Lack of peptidoglycan:
      1. The archaeobacteria cell wall differs chemically from that of the eubacteria cell wall.
      2. Specifically, they lack in peptidoglycan.
    2. Gram staining:
      1. Archaeobacteria nevertheless often may be differentiated in terms of Gram staining.
      2. This is because the Gram stain is a measure of physical aspects of cell walls that are shared between the eubacteria and the archaeobacteria (though gram-negative archaeobacteria lack outer membranes).
    3. There exist cell-wall less archaeobacteria which live in the high temperature (55 to 59°C) and acidic piles of coal tailings.
  4. Membranes
    1. Branched chain hydrocarbons:
      1. Archaeobacteria lipid bilayers consist of branched chain hydrocarbons linked by ether (as opposed to ester) linkages to glycerol.
      2. Typical structure of eubacteria monoglyceride:
        3. H                      
H-C-OH  O                
  |     ||H H H H H H H  
H-C -O- C-C-C-C-C-C-C-C-H
  |     H H H H H H H H  
H-C-OH                   
  H
      3. Typical structure of archaeobacteria monoglyceride:
      H         H       H    
H-C-OH    H-C-H   H-C-H  
  |     H H | H H H | H  
H-C -O- C-C-C-C-C-C-C-C-H
  |     H H H H H H H H  
H-C-OH                   
  H
    2. Membrane-spanning lipids:
      1. Archaeobacteria lipid bilayers also contain lipids consisting of ether-linked hydrocarbons stretched between glycerol moieties, linked at both ends (think of two fats joined at the end of their fatty acid chains and you'll get an idea: |==| where | is glycerol, = are two parallel fatty acids, and |= is a eubacterium diglyceride).
      2. For these linked lipids each glycerol is found in the opposite membrane leaflet, at the hydrophilic-hydrophobic interface.
      3. An archaeobacteria membrane spanning, glycerol-based lipid (only one of expected two spanning hydrocarbon chains shown):
        4. H       H       H       H         H         
H-C-OH  H-C-H   H-C-H   H-C-H     H-C-H       
  |   H H | H H H | H H H | H H H H | H   H   
H-C-O-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-C-O-C-H 
  |   H H H H H H H H H H H H H H H H H   |   
H-C-OH                                  H-C-OH
  H                                       |   
                                        H-C-OH
                                          H
      4. One obvious explanation for the existence of such lipids is that they may make the archaeobacteria membrane sufficiently stable, at least in part, to allow growth and survival in the extreme environments in which many archaeobacteria may be found.
  5. Molecular genetics
    1. Similar but smaller genomes:
      1. Both archaeobacteria and eubacteria have closed circular DNA genomes.
      2. However, the average eubacteria genome may be as much as two or more times larger than the average archaeobacteria genome.
    2. Archaeobacteria, unlike eubacteria, tend to display few plasmids.
    3. Some detailed aspects of the archaeobacterium molecular genetics resemble those of eucaryota rather than eubacteria (the RNA polymerase and, less so, the ribosome).
  6. Methanogens
    1. This diverse group (3 orders, 18 genera) is the largest among the archaeobacteria.
    2. Produce methane anaerobically:
      1. They convert various substrates to methane (or methane and CO2) under strict anaerobic conditions.
      2. Archaeobacteria Methanogens contribute significantly to the release of methane, a potent green house gas, into the atmosphere.
    3. Methane generation and ATP synthesis apparently occur via anaerobic respiration.
    4. Among the methanogens there are examples which:
      1. can live on H2 as their energy source and CO2 as their carbon source (to do this they convert 2 molecules of CO2 to acetyl-CoA).
      2. thrive includes gastrointestinal tracts as well as various anaerobic aquatic environments (such as sediments).
      3. are extreme halophiles which grow in the high pressure, 100°C-plus temperatures of deep sea hydrothermal vents.
  7. Sulfate reducers
    1. Sulfate reducers are extreme thermophilic archaeobacterial lineage which displays apparently low diversity (one known genus), found at deep sea hydrothermal vents.
    2. The sulfate reducers, though capable of reducing various sulfur containing compounds, nevertheless differ from the extreme thermophilic S0-metabolizers in not being capable of reducing molecular sulfur (i.e., using molecular sulfur as a final electron acceptor in anaerobic respiration).
    3. Since sulfate reducers resemble the mathanogens is some ways, a logical phylogenetic placement (based on phenetic considerations) might be as a form intermediate to that of the archaeobacteria methanogens and the extreme thermophilic S0-metabolizers.
  8. Extreme halophiles
    1. Fastidious salt lovers:
      1. There are six genera of archaeobacteria extreme halophiles.
      2. They are aerobic chemoheterotrophs which require complex energy and carbon sources (particularly, proteins and amino acids).
      3. Osmotic requirements exceed 1.5 M NaCl (8% wt/vol) for all extreme halophiles and tolerances range up to 36% wt/vol).
    2. High salt environments:
      1. The environments in which such high osmotic pressures exist include land locked seas (such as the Great Salt Lake and the Dead Sea) as well as any place in which high salt content water tends toward high levels of evaporative loss.
      2. (Normal sea water is only about 0.5 M NaCl; our bodies are about 0.15 M NaCl or equivalent.)
      3. Some extreme halophiles are known to grow on salted foods (such as salted fish) and even cause spoilage.
    3. Example: Halobacterium salinarium:
      1. Halobacterium salinarium is an extreme halophile capable of a unique form of photosynthesis which does not utilize chlorophyll.
      2. These archaeobacteria employ blue and red light photoreceptors to position themselves in the water column as near to the surface as possible (toward the red light) without being too close to the DNA damaging ultraviolet light (away from the blue light.
  9. Extreme thermophilic S0 metabolizers
    1. Molecular sulfur users:
      1. The extreme thermophilic S0-metabolizers are a truly bizarre grouping (3 orders, 9 genera) of extremely thermophilic, mostly strictly anaerobic archaeobacteria.
      2. Some of these archaeobacteria utilize molecular sulfur as a final electron acceptor in anaerobic respiration.
      3. Others of these archaeobacteria employ sulfur as an energy-containing compound (i.e., they are lithotrophs).
    2. Acidophiles, too:
      1. Many extreme thermophilic S0-metabolizers are also acidophiles.
      2. The thermoacidophiles are aerobic, acidophilic, extreme thermophilic S0-metabolizers of which two types are known (Sulfolobus and Thermoproteales).
      3. Habitats in which extreme thermophilic S0-metabolizers are found tend to be geothermic or volcanic, i.e., where molecular sulfur and high temperatures are together found.
  10. Vocabulary
    1. Cell wall
    2. Extreme halophiles
    3. Extreme thermophilic S0-metabolizers
    4. Membranes
    5. Methanogens
    6. Molecular genetics
    7. Sulfate reducers
  11. Practice questions
    1. Describe an environment in which you might expect an archaeobacteria to grow. Describe a single, significant biochemical characteristic of this archaeobacteria. (hint, if an organism lives and grows in a low temperature environment, resistance to low temperatures would not be a sufficiently significant biochemical characteristic, but ability to photosynthesize would---such an organism would be a low temperature photosynthesizer, though that specifically would not a correct answer) [PEEK]
    2. Name or describe an unusual characteristic of archaeobacteria lipid bilayers. [PEEK]
  12. Practice question answers
    1. (i) very hot, derives energy from molecular sulfur, (ii) gastrointestinal tract, methane producer, (iii) high salt environment, photosynthesizer, (iv) highly acidic environment, derives energy from molecular sulfur, etc.
    2. ether rather than ester linkages; hydrocarbons stretched between glycerol moieties found in opposite leaflets.
  13. References
    1. Prescott, L.M., Harley, J.P., Klein, D.A. (1996). Microbiology. Third Edition. Wm. C. Brown Pub., Dubuque, Iowa. pp. 477-490.
    2. Raven, P.H., Johnson, G.B. (1995). Biology (updated version). Third Edition. Wm. C. Brown publishers, Dubuque, Iowa. pp. 601-602.
    3. Tortora, G.J., Funke, B.R., Case, C.L. (1995). Microbiology. An Introduction. Fifth Edition. The Benjamin/Cummings Publishing, Co., Inc., Redwood City, CA, p. 289.