Chapter 20
The Archaea
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Archaea
• Highly diverse with respect to
morphology, physiology,
reproduction and ecology
• Best known for growth in anaerobic,
hypersaline and high-temperature
habitats
• Also found in marine arctic
temperature and tropical waters
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Archaeal Cell Walls
• Lack muramic acid and D-amino acids
but can stain gram+ or gram• Walls found in group are diverse
– pseudomurein (peptidoglycan-like polymer)
found in some methanogenic species
– complex polysaccharides, proteins or
glycoproteins found in some other species
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Archaeal Membrane Lipids
• Differ from Bacteria and Eucarya in
having branched chain hydrocarbons
attached to glycerol by ether linkages
• Polar phospholipids, sulfolipids and
glycolipids are also found in archaeal
membranes
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Archaeal Lipids and Membranes
Archaea
Bacteria/Eucaryotes
• branched chain
• fatty acids
hydrocarbons
attached to
attached
to
glycerol by ester
glycerol by ether
linkages
linkages
• some have
diglycerol
tetraethers
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Archaeal Genetics and Molecular
Biology
• Exhibit some similarities to Bacteria or
Eucarya
• ~30% of genes shared exclusive between
archeons and eucaryotes code for proteins
involved in transcription, translation or
DNA metabolism
• Many genes shared only with bacteria are
involved in metabolic pathways
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– there is evidence for lateral gene transfer
between archeons and bacteria
More about Archaeal Genetics and
Molecular Biology
• DNA replication and transcription
have both bacterial and eucaryotic
features
• Archael mRNA appears to be
similar to that of bacteria rather
than eucaryotic mRNA
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Figure 20.2
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Archaeal Metabolism
• Organotrophy, autotrophy, and
phototrophy have been observed
• Differ from other groups in glucose
catabolism, pathways for CO2
fixation and the ability of some to
synthesize methane
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Figure 20.3
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Archaeal Taxonomy
Currently divided into two phyla
– Crenarchaeota
– Euryarchaeota
• most are extremely thermophilic
• many are acidophilic and sulfur
dependent
• Some are halophiles
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Table 20.1
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Phylum Crenarchaeota
class - Thermoprotei
orders:
Figure 20.4
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Crenarchaeota…
• Most are extremely thermophilic
• Many are acidophiles
• Many are sulfur-dependent
– some use as electron acceptor in anaerobic
respiration
– some, use as electron source (chemolithotrophs)
• Almost all are strict anaerobes
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Crenarchaeota…
• grow in
geothermally
heated water
or soils that
contain
elemental
sulfur
Figure 20.5 (a)
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Figure 20.5 (b)
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An Extremely Hyperthermophilic
Crenarchaeote
Figure 20.6
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Crenarchaeota…
• include organotrophs and
lithotrophs (sulfur-oxidizing and
hydrogen-oxidizing)
• contains 25 genera
– two best studied are Sulfolobus and
Thermoproteus
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Sulfolobus and Thermoproteus
Figure 20.7
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Genus Sulfolobus
• Irregularly lobed, spherical shaped
– cell walls contain lipoproteins and
carbohydrates
• Thermoacidophiles
– 70-80°C
– pH 2-3
• Metabolism
– lithotrophic on sulfur using oxygen (usually)
or ferric iron as electron acceptor
– organotrophic on sugars and amino acids
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Genus Thermoproteus
• Long thin rod, bent or branched
– cell walls composed of glycoprotein
• Thermoacidophiles
– 70-97 °C
– pH 2.5-6.5
• Anaerobic metabolism
– lithotrophic on sulfur and hydrogen
– organotrophic on sugars, amino acids, alcohols, and
organic acids using elemental sulfur as electron
acceptor
• Autotrophic using CO or CO2 as carbon source
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Phylum Euryarchaeota
• Often divided informally into five major
groups
–
–
–
–
–
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methanogens
halobacteria
thermoplasms
extremely thermophilic S0-metabolizers
sulfate-reducers
The Phylum Euryarchaeota
Figure 20.8
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Table 20.2
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Selected Methanogens
Figure 20.9
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Methanogen
coenzymes
Figure 20.10
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Methane Synthesis
Figure 20.11
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involves several unique
cofactors
Habitats of methanogens
• Anaerobic environments rich in
organic mater
– e.g., animal rumens
– e.g., anaerobic sludge digesters
– e.g., within anaerobic protozoa
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Ecological and practical importance of
methanogens
• Important in wastewater treatment
• Produce significant amounts of methane
– can be used as clean burning fuel and energy
source
– is greenhouse gas and may contribute to
global warming
• Can oxidize iron
– contributes significantly to corrosion of iron
pipes
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The Halobacteria
• Extreme halophiles
– require at least 1.5 M NaCl
• cell wall disintegrates if [NaCl] < 1.5 M
– growth optima near 3-4 M NaCl
• Aerobic chemoheterotrophs with complex
nutritional requirements
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Examples of Halobacteria
Figure 20.12
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Halobacterium salinarium
•Has unique type of photosynthesis
-not chlorophyll based
-contains bacteriorhodopsin
-uses modified cell membrane (purple
membrane)
•Absorption of light by bacteriorhodopsin drives
proton transport, creating PMF for ATP
synthesis
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Rhodopsin
• Now known to be widely distributed
among procaryotes
– found in marine bacterioplankton
– also found in cyanobacteria
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Genus Thermoplasma
• Thermoacidophiles; grow in refuse piles of coal
mines
– 55-59°C
– pH 1-2
• Cell structure
– shape depends on temperature
• 59°C – irregular filament
• at lower temperatures – spherical
• May be flagellated and motile
• Cell membrane strengthened by diglycerol tetraethers,
lipopolysaccharides, and glycoproteins
• Nucleosome-like structures formed by association of
DNA with histonelike proteins
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Figure 20.14
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Genus Picrophilus
• Irregularly shaped cocci, 1 to 5 M
diameter
– large cytoplasmic cavities that are not membrane
bound
– no cell wall
– has S-layer outside plasma membrane
• Thermoacidophiles
– 47 - 65°C
– pH < 3.5 (optimum 0.7)
• Aerobic
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Extremely Thermophilic S0Metabolizers
• Two genera, Thermococcus and
Pyrococcus
• Motile by flagella
• Optimum growth temperatures 88 –
100°C
• Strictly anaerobic; reduce sulfur to
sulfide
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Sulfate-Reducing Archaea
• Genus, Archaeoglobus
• Irregular coccoid cells
– cell walls consist of glycoprotein subunits
• Extremely thermophilic
– optimum 83°C
– isolated from marine hydrothermal vents
• Metabolism
– can be lithotrophic (H2) or organotrophic (lactate or
glucose)
– use sulfate, sulfite, or thiosulfite as electron acceptor
– possess some methanogen coenzymes
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