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General Microbiology (MICR300)
Lecture 8
Microbial Diversity: Archaea
(Text Chapters: 13.1-13.12)
Archaea
Comparing Bacteria and Archaea
Characteristic
Prokaryotic cell structure
Cell wall
Membrane lipids
Ribosomes
Initiator tRNA
Operons
RNA polymerases
Methanogenesis
Growth above 1000C
Bacteria
Yes
Muramic acid present
Esther-linked
70S
Formylmethionine
Yes
One (4 subunits)
No
No
Archaea
Yes
Muramic acid absent
Ether-linked
70S
Methionine
Yes
Several (8-12 subunit each)
Yes
Yes
Phylogenetic Overview
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Archaea consists of four phyla, the
Euryarchaeota, the Crenarchaeota,
the Korarchaeota, and the
Nanoarchaeota, with the first two phyla
being the major ones.
Figure 13.1 shows a phylogenetic tree
of Archaea.
Metabolism

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With the exception of methanogenesis,
bioenergetics and intermediary metabolism in
species of Archaea are much the same as those
in various species of Bacteria.
Several Archaea are chemoorganotrophic and
thus use organic compounds as energy sources
for growth. Chemolithotrophy is also well
established in the Archaea, with H2 being a
common electron donor.
Metabolism

The capacity for autotrophy is widespread
in the Archaea and occurs by several
different pathways. In methanogens, and
presumably in most chemolithotrophic
hyperthermophiles, CO2 is incorporated
via the acetyl-CoA pathway or some
modification thereof.
Phylum Euryarchaeota
Extremely Halophilic Archaea

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Extremely halophilic Archaea require
large amounts of NaCl for growth. These
organisms accumulate high levels of KCl
in their cytoplasm as a compatible
solute.
These salts affect cell wall stability and
enzyme activity. The light-mediated
proton pump bacteriorhodopsin helps
extreme halophiles make ATP (Figure
13.4).
Methane-Producing
Archaea: Methanogens


A large number of Euryarchaeota produce
methane (CH4) as an integral part of their
energy metabolism. Such organisms are
called methanogens. Methanogenic
Archaea are strictly anaerobic
prokaryotes.
Habitats of methanogenic Archaea are
listed in Table 13.4.
Thermoplasmatales: Thermoplasma,
Ferroplasma, and Picrophilus


Thermoplasma, Ferroplasma, and
Picrophilus are extremely acidophilic
thermophiles that form their own
phylogenetic family of Archaea inhabiting
coal refuse piles and highly acidic
solfataras.
Cells of Thermoplasma and Ferroplasma
lack cell walls and thus resemble the
mycoplasmas in this regard.
Thermoplasma

To survive the osmotic stresses of life
without a cell wall and to withstand the
dual environmental extremes of low pH
and high temperature, Thermoplasma has
evolved a unique cell membrane structure
(Figure 13.11).
Phylum Crenarchaeota
Habitats and Energy Metabolism of
Crenarchaeotes


Table 13.7 summarizes the habitats of
Crenarchaeota. They include very hot and
very cold environments.
Most hyperthermophilic Archaea have
been isolated from geothermally heated
soils or waters containing elemental sulfur
and sulfides. Hyperthermophilic
Crenarchaeota inhabit the hottest
habitats currently known to support life.
Habitats and Energy Metabolism of
Crenarchaeotes


Cold-dwelling crenarchaeotes have been
identified from community sampling of
ribosomal RNA genes from many
nonthermal environments
Crenarchaeotes have been found in
marine waters worldwide and thrive even
in frigid waters and sea ice.
Hyperthermophiles from Terrestrial
Volcanic Habitats

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Sulfolobales and Thermoproteales are two
representative orders of hyperthermophilic
Archaea from Terrestrial volcanic habitats
Two phylogenetically related organisms isolated
from these environments include Sulfolobus and
Acidianus. These genera form the heart of an
order called the Sulfolobales.
Key genera within the Thermoproteales are
Thermoproteus, Thermofilum, and
Pyrobaculum.
Hyperthermophiles from Submarine
Volcanic Habitats


Submarine volcanic habitats are homes to
the most thermophilic of all known
Archaea. These habitats include both
shallow-water thermal springs and deepsea hydrothermal vents.
Pyrodictium and Pyrolobus are examples
of archaea whose growth temperature
optimum lies above 100ºC. The optimum
for Pyrodictium is 105ºC and for
Pyrolobus is 106ºC.
Hyperthermophiles from Submarine
Volcanic Habitats


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Cells of Pyrodictium are irregularly disc-shaped and
grow in culture in a mycelium-like layer attached to
crystals of elemental sulfur.
Other notable members of the Desulfurococcales
include Desulfurococcus and Ignicoccus.
Like Pyrodictium, Desulfurococcus is a strictly
anaerobic S0-reducing bacterium, but it differs from
Pyrodictium in that it is much less thermophilic,
growing optimally at about 85°C. Ignicoccus grows
optimally at 90ºC, and its metabolism is H2/S0
based.
Heat Stability of Biomolecules


Protein and DNA stability in hyperthermophiles
is critical to surviving high temperature.
Because most proteins denature at high
temperatures, much research has been done to
identify the properties of thermostable proteins.
Hyperthermophilic prokaryotes typically produce
special classes of chaperonins that function only
at the highest growth temperatures. In cells of
Pyrodictium, for example, the major chaperonin
is a protein complex called the thermosome.
Heat Stability of Biomolecules

All hyperthermophiles produce a DNA
topoisomerase called reverse DNA
gyrase. Reverse gyrase introduces
positive supercoils into DNA (in contrast
to the negative supercoils introduced by
DNA gyrase, found in all
nonhyperthermophilic prokaryotes).