630 Microbiology for Teachers

Archaeal Diversity

Euryarchaeota

• Extreme Halophiles
  • Gram-negative - many have yellow-to-red carotenoid pigments that protect them against sunlight
    • rods (0.6-1 x 1-6 µm) with glycoprotein walls (Halobacterium, motile via lophotrichous flagella; Natronobacterium)
    • cocci (0.6-1.5 µm) with sulfated polysaccharide walls (Halococcus, Natronococcus)
    • flattened disks or cup-shapes (Haloferax)
    • irregular disks, triangles, rectangles (Haloarcula)
  • Organotrophic aerobes that require very high salt (1.5-5.5 M, prefer 3-4 M) and require complex nutrients (vitamins, etc.) for growth (fastidious)
    • potassium ions bind to acidic proteins, rather than organic compatible solutes, to prevent dehydration
    • amino acids, organic acids or carbohydrates (modified Entner-Duodoroff pathway) are used for energy (via TCA cycle, respiratory metabolism)
    • carbon dioxide fixed via Calvin cycle
    • anaerobic respiration used by some Halobacterium species with nitrate, sulfur, thiosulfate as terminal electron acceptors
    • ATP synthesized by other Halobacterium species using a light-mediated bacteriorhodopsin/retinal system
  • Hypersaline aquatic environments with alkaline pH such as Great Salt Lake, Dead Sea, solar salt evaporating ponds (salterns) and salted fish, are the normal habitat for these Archaea
• Methanogens
  • Gram-negative or Gram-positive - some have pseudopeptidoglycan, some have protein, some have polysaccharide cell walls; coenzyme F420 fluorescence used for identification of methanogens; 3 orders and 13 genera based on 16S rRNA structure
    • long (Methanobacterium, Methanothrix) or short (Methanobrevibacter) rods (0.5-1 µm wide) that occur as very long filaments
    • cocci (1.5-2.5 µm) that occur in singlets (Methanococcus), aggregates (Methanolobus), packets (Methanosarcina), or chains (Methanopyrus)
    • spirals (Methanospirillum)
    • plate-shaped (Methanoplanus)
  • Strictly anaerobic, fastidious (require vitamins, amino acids, nickel {part of coenzyme F430, hydrogenase, carbon monoxide dehydrogenase}, iron, cobalt); Methanopyrus is an extreme thermophile; no known methanogens have a complete TCA cycle; some can fix nitrogen
    • lithotrophs - generate methane using hydrogen and carbon dioxide that is fixed via a reductive acetyl-CoA pathway (Methanobacterium, Methanothermus, Methanococcus, Methanogenium, Methanoplanus)
    • organotrophs - generate methane from methyl groups of formate, methanol (Methanosphaera, Methanosarcina, Methanococcoides) or acetate (Methanothrix, Methanosaeta)
  • Environments rich in organic matter such as anaerobic sewage digestors, anoxic freshwater sediments and the rumen of cattle are normal habitats for these Archaea; they are very important as sources of methane (natural gas)
• Extreme Thermophiles
  • Gram-negative
    • rods (Methanopyrus, Thermoproteus)
    • disc-shapes (Thermodiscus)
    • filaments (Thermofilum)
    • irregular (lobate) spheres 0.8-1 µm in diameter (Sulfolobus)
    • spheres that are motile via lophotrichous flagella (Thermococcus, Pyrococcus, Staphylothermus)
  • Obligate anaerobes that require temperatures above 50C for growth (sometimes higher than 70C) and use sulfur as electron acceptor (except Sulfolobus, which uses oxygen or ferric iron)
    • organotrophs - use a modified Entner-Duodoroff pathway to oxidize organic compounds for energy (oxygen or sulfur serves as final electron acceptor)
      • organic compound + S H₂S + CO₂ (Thermococcus, Thermoproteus, Desulfurococcus, Thermofilum, Pyrococcus)
      • organic compound + O₂ H₂O + CO₂ (Sulfolobus)
      • organic compound CO₂ + fatty acids (Staphylothermus)
      • organic compound CO₂ + H₂ (Pyrococcus)
    • lithotrophs - can also grow organotrophically using sulfur as an electron acceptor (see above)
      • Methanopyrus - oxidizes hydrogen for energy (4 H₂ + carbon dioxide yields methane + 2 water) and fixes carbon dioxide at the same time (thermophilic methanogen)
      • Pyrodictium - optimum temperature, 105C; uses hydrogen as energy source, sulfur as electron acceptor (H₂ + S H₂S) and carbon dioxide as carbon source (via a reverse (reductive) acetyl-CoA pathway)
      • Sulfolobus - optimum temperature for growth, 70-80C; requires a pH of 2-3; aerobically oxidizes sulfur (2 S + 3 O₂ + 2 H₂O 2 H₂SO₄) and ferrous iron (4 FeS + 15 oxygen + 2 water 2 Fe₂(SO₄)₃ + 2 H₂SO₄) for energy; fixes carbon dioxide via a reverse (reductive) TCA cycle much like that seen in green sulfur bacteria
      • Thermoproteus - grows at 78-96C, pH 1.7-6.5; uses hydrogen as energy source, sulfur as electron acceptor (H₂ + S H₂S) and carbon dioxide as carbon source
  • Geothermally heated, acidic soils or waters which contain sulfur (solfatara), such as those in Yellowstone National Park and near marine hydrothermal vents are the usual habitats for these Archaea
• Sulfate Reducers (Archaeoglobus)
  • Irregular, motile spheres
  • Thermophilic (83C is optimum), methanogenic, anaerobes which extract electrons from hydrogen, lactate, glucose, etc. and reduce sulfate, sulfite, or thiosulfate to sulfide
  • High-temperature, sulfate-containing environments such as marine hydrothermal vents are the usual habitats for these Archaea
• Wall-Less Archaea (Thermoplasma)
  • No cell wall (resemble Mycoplasmas)
    • single triple-layered membrane that contains glycoproteins plus lipopolysaccharide consisting of tetraether lipid with mannose and glucose
    • irregular filamentous shape at or above 59C, spherical (0.3-2 µm) below 59C
    • very small genome, with DNA surrounded by histone-like DNA-binding proteins
    • budding is mode of reproduction
  • Acidophilic thermophilic aerobic organotroph
  • Coal refuse piles which are heated and made acidic by (other) lithotrophic bacteria which oxidize iron pyrite (FeS) to sulfuric acid are the only habitats in which these Archaea are found in nature

Crenarchaeota

• most are extreme thermophiles with growth optima at temperatures greater than 80C and many of these are anaerobic lithotrophs (Pyrodictium), organolithotrophs or organotrophs (Thermococcus, Thermoproteus) that require elemental sulfur for optimal growth (Desulfurococcus)
• Sulfolobus is an aerobic organolithotroph
• there are also a number of as yet unidentified marine crenarchaeotes, including some cryophiles

Korarchaeota

• newly discovered extreme thermophiles
• these are the most primitive life forms discovered to date