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8. 15.1: Diverse Cell Forms of Methanogens

Diverse Cell Forms of Methanogens

There are over 50 described species of methanogens, sharing over 30 signature proteins.

 

LEARNING OBJECTIVES

 

Outline the physical characteristics associated with methanogens

 

KEY TAKEAWAYS

Key Points

 

  • Methanogens are usually either coccoid (spherical) or bacilli (rod shaped).
  • Methanogens have a cell wall that is composed of pseudopeptidoglycan, which offers lysozyme resistance.
  • There are many diverse strains of methanogens, including M. smithii (found in the human gut), M. kandleri (discovered on the wall of a black smoker), and M acetivorans (found in oil wells, trash dumps, and deep-sea hydrothermal vents ).

 

Key Terms

 

  • polysaccharide: Complex sugars. A polymer made of many saccharide units linked by glycosidic bonds.

Methanogens belong to the domain archaea, which are distinctly different than bacteria. There are over 50 described species of methanogens, sharing over 30 signature proteins. These species do not form a monophyletic group, but are split into three clades. Therefore, the large numbers of proteins uniquely shared by all methanogens may be due to lateral gene transfers.

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Methanopyrus kandleri: M. kandleri, a methanogen, is the only strain in the genus Methanopyrus. Methanopyrus kandleri can survive and reproduce at 122°C.

Methanogens are usually either coccoid (spherical) or bacilli (rod shaped). The cell walls of of Methanogens, like other Archaea, lack peptidoglycan, a polymer found in the cell walls of the bacteria. Instead, some methanogens have a cell wall that is composed of pseudopeptidoglycan. Pseudopeptidoglycan differs in chemical structure from bacterial peptidoglycan, but resembles eubacterial peptidoglycan in morphology, function, and physical structure. These differences makes these archaea resistant to the enzyme, lysozyme, which only breaks down β (1,4) sugar linkages like those found in peptidoglycan. Those that do not contain pseudopeptidoglycan have at least one paracrystalline array (S-layer) made up of proteins that fit together like a jigsaw puzzle.

There are many diverse strains of methanogens. Methanobrevibacter smithii is the dominant archaeon in the human gut. M. smithii is pivotal in the removal of excess hydrogen from the human gut. They are important for the efficient digestion of polysaccharides, allowing for an increase in the transformation of nutrients into calories.

Methanocaldococcus jannaschii thermophilic methanogen isolated from a hot spring at Woods hole. It was the first archaeon to have its complete genome sequenced, identifying many genes and synthesis pathways unique to the archaea.

Methanopyrus is a genus of methanogens, with a single described species, M. kandleri. M. kandleri is a hyperthermophile, discovered on the wall of a black smoker from the Gulf of California at a depth of 2000 m, at temperatures of 84-110 °C.

Methanosarcina acetivorans is a versatile methane producing microbe which is found in such diverse environments as oil wells, trash dumps, deep-sea hydrothermal vents, and oxygen-depleted sediments beneath kelp beds. Only M. acetivorans and microbes in the genus Methanosarcina use all three known metabolic pathways for methanogenesis.