3'3 Classification of Prokaryotes

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3.3 Classification of Prokaryotes

• In the 5K system all prokaryotes were placed in
  the Kingdom Monera because they have prokaryotic
  cells
• Until the late 20th century, prokaryotic taxonomy
  was based on phenotypic characters such as shape,
  motility, Gram stain, and nutritional mode
• Starting in the 1970s, a molecular systematics
  approach using SSU-rRNA DNA sequences as a marker
  for evolutionary relationships revealed some
  startling results
• The genetic diversity of prokaryotes is immense
• Horizontal gene transfer is is important in the
  evolution of prokaryotes

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Major Findings from Molecular Systematics

• There are two distinct evolutionary lineages of
  prokaryotes, the Archaea and the Bacteria
• Kingdom Monera is therefore obselete, and Archaea
  and Bacteria are both domains in the 3D system
• Archaea is actually more closely related to
  Eukarya than to Bacteria and shares many
  characteristics with it.

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Major Findings from Molecular Systematics

• Most traditional groupings do not hold up
• Eg. Gram-negative bacteria are not one lineage
• Cyanobacteria is a monophyletic group

Figure 27.12 - A tentative phylogeny of some of
the major taxa of prokaryotes
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Major Groups of Bacteria
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Diversity Within the Archaea

• The extremophiles - including the thermophiles,
  the halophiles and the methanogens
• The thermophiles thrive in very hot environments
• The halophiles live in highly saline environments
• The methanogens are strict anaerobes that use CO2
  to oxidize H2 as a source of energy, releasing
  CH4 as a waste product
• All of the halophiles and methanogens, and some
  of the thermophiles, are in the Kingdom
  Euryarchaeota
• Most of the thermophiles are in the Kingdom
  Crenarchaeota

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Methanogens
Some methanogens inhabit the anaerobic guts of
animals and aid in their digestion
Others live in swamps and marshes and produce
marsh gas!
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Extreme Halophiles

• These critters tolerate or even require a high
  salt environment
• 15-20 salinity found in evaporating ponds of San
  Francisco Bay
• The colour is due to bacteriorhodopsin - a
  photosynthetic pigment

Halobacterium halobium
(A photoheterotroph!)
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Extreme Thermophiles

• These little guys tolerate temperatures up to
  about 113oC, although the optimal temperature is
  usually 60-80oC
• They tend to inhabit hot springs
• Eg. Sulfolobus solfataricus lives in sulfur-rich
  volcanic springs, and obtains energy by using CO2
  to oxidize sulfur

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3.4 Example of A Heterotrophic BacteriumEscherich
ia coli

• Domain Bacteria, Kingdom Gamma Proteobacteria
• A colony-forming bacillus (rod-shaped) bacterium
• Gram-negative
• A nutritional generalist - requires only a single
  monosaccharide (eg. glucose) as a carbon energy
  source
• Part of the human intestinal fauna
• Normally beneficial, but certain strains can
  cause food poisoning

E. coli photo gallery
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3.5 Example of An Autotrophic BacteriumNostoc sp.

• Domain Bacteria, Kingdom Cyanobacteria
• A filamentous spherical (coccus) species
• Filaments are also called trichomes
• Trichomes form gelatinous colonies
• Contains the pigment chlorophyll for oxygenic
  photosynthesis
• Demonstrates metabolic cooperation
• Most cells perform photosynthesis
• Specialized cells called heterocysts perform
  nitrogen fixation
• The conversion of atmospheric N2 to NH3
• O2 deactivates enzymes for nitrogen fixation
• A very independent organism
• Needs only light energy from its environment,
  makes everything else

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Nostoc sp. Photo Gallery