4. THE DOMAIN ARCHAEA MICROBES
• As a group the Archaea [Greek archaios, ancient] are quite diverse, both in
morphology and physiology.
• They can stain either gram positive or gram negative and may be
spherical, rod-shaped, spiral, lobed, plate-shaped, irregularly shaped, or
pleomorphic.
• Some are single cells, whereas others form filaments or aggregates. They
range in diameter from 0.1 to over 15 μm, and some filaments can grow
up to 200 _m in length.
• Multiplication may be by binary fission, budding, fragmentation, or other
mechanisms.
• Archaea are just as diverse physiologically. They can be aerobic,
facultatively anaerobic, or strictly anaerobic. Nutritionally they range from
chemolithoautotrophs to organotrophs.
• Some are mesophiles; others are hyperthermophiles that can grow above
100°C.
5. CONTD…..
• Archaea often are found in extreme aquatic and terrestrial habitats.
• They are often present in anaerobic, hypersaline, or high temperature
environments.
• Recently archaea have been discovered in cold environments.
• It appears that they constitute up to 34% of the prokaryotic biomass in
coastal Antarctic surface waters.
• A few are symbionts in animal digestive systems.
6. Archaeal Cell Walls
• Although archaea can stain either gram positive or gram negative
depending on the thickness and mass of the cell wall, their wall structure
and chemistry differ from that of the bacteria.
• There is considerable variety in archaeal wall structure.
• Many gram- positive archaea have a wall with a single thick homogeneous
layer resembling that in gram-positive bacteria and thus stain gram
positive .
7. CONTD….
• Gram-negative archaea lack the outer membrane and complex combined
in various ways to yield membranes of different rigidity and thickness.
• For example, the C20 diethers can be used to make a regular bilayer
membrane.
• A much more rigid monolayer membrane may be constructed of C40
tetraether lipids . Of course archaeal membranes may contain a mix of
diethers, tetraethers, and other lipids.
• As might be expected from their need for stability, the membranes of
extreme thermophiles such as Thermoplasma and Sulfolobus are almost
completely tetraether monolayers.
8.
9. A much more rigid monolayer membrane may be
constructed of C40 tetraether lipids
10. Metabolism
• Some archaea are organotrophs; others are autotrophic.
• A few even carry out an unusual form of photosynthesis.
• Archaeal carbohydrate metabolism is best understood.
• The enzyme 6-phosphofructokinase has not been found in archaea, and
they do not appear to degrade glucose by way of the Embden-Meyerhof
pathway.
• Extreme halophiles and thermophiles catabolize glucose using a modified
form of the Entner-Doudoroff pathway in which the initial intermediates
are not phosphorylated.
11. CONTD…
• The halophiles have slightly different modifications of the pathway than do the
extreme thermophiles but still produce pyruvate and NADH or NADPH.
• Methanogens do not catabolize glucose to any significant extent.
• In contrast with glucose degradation, gluconeogenesis proceeds by a reversal of
the Embden-Meyerhof pathway in halophiles and methanogens.
• All archaea that have been studied can oxidize pyruvate to acetyl- CoA.
• They lack the pyruvate dehydrogenase complex present in eucaryotes and
respiratory bacteria and use the enzyme pyruvate oxidoreductase for this
purpose.
• Halophiles and the extreme thermophile Thermoplasma do seem to have a
functional tricarboxylic acid cycle.
• No methanogen has yet been found with a complete tricarboxylic acid cycle.
• Evidence for functional respiratory chains has been obtained in halophiles and
thermophiles.
14. • The reductive tricarboxylic acid cycle.
• The cycle is reversed with ATP and reducing equivalents [H] to form
acetyl-CoA from CO2.
• The acetyl-CoA may be carboxylated to yield pyruvate, which can then be
converted to glucose and other compounds.
• This sequence appears to function in Thermoproteus neutrophilus.
• The synthesis of acetyl-CoA and pyruvate from CO2 in Methanobacterium
thermoautotrophicum.
• One carbon comes from the reduction of CO2 to a methyl group, and the
second is produced by reducing CO2 to carbon monoxide through the
action of the enzyme CO dehydrogenase (E1).
• The two carbons are then combined to form an acetyl group. Corrin-E2
represents the cobamide-containing involved in methyl transfers
