ARCHAEAL CELL STRUCTURE
Mr. Archaea
Ms. Bacteria
INTRODUCTION
• Each year cattle exhale about 200 million metric tons of methane.
• 18 million metric tons of methane produced through manure.
• Carbon dioxide - GHG
• Methane and nitrous oxide - GHGs.
• Methane has about 20 times the heat retaining capacity of carbon dioxide.
• Methanogens-that make methane from hydrogen gas, carbon dioxide, and acetate.
2Corganic + 2H2O = CO2 + CH4
Methanotrophs organisms
that can oxidise methane
• Archaeal taxonomy is currently in a state of flux, but two phyla are well
established:Crenarchaeota and Euryarchaeota.
A TYPICAL ARCHAEAL CELL
SHAPE, ARRANGEMENT AND SIZE
Shapes
• Cocci and rods are common.
• Some cocci form clusters and some rods form chains.
• Curved rods, spiral and pleomorphic.
• Some archaea exhibit unique shapes, such as the branched form and the flat, postage-stampshape - increased surface area-to-volume (S/V) ratio
Size
• Typical rods are 1 to 2 µm wide by 1.0 to 5.0 µm long.
Fig: Methanobrevibacter smithii-ovalto-short rod-shaped cells
Fig: Thermoproteus tenax -a branched
archaeal cell
Fig: Hatoquadratum watsbyi -a square archaean
Fig: Methanosarcina mazei
-a coccus that forms clusters
Fig: forms long filaments that are covered with a
bacterial biofilm.
ARCHAEAL CELL ENVELOPES
• The cell envelope is same as bacteria.
• Plasma membrane is unique.
• Some archaea have monolayer membranes that function like bilayers.
• This structural difference is due to the presence of unique lipids in archaeal plasma membranes.
• For many archaea, an S-layer is the major, and sometimes only, component of the cell wall.
ARCHAEAL PLASMA MEMBRANES AND
NUTRIENT UPTAKE
• Archaeal membranes are composed primarily of lipids that differ from bacterial and
eukaryotic lipids in two ways.
1. They contain hydrocarbons derived from isoprene units-five-carbon, branched molecules.
• This affects the way the lipids pack together, which in turn affects the fluidity of the
membrane and its permeability
2. Hydrocarbons are attached to glycerol by ether links rather than ester links.
• Ether linkages are more resistant to chemical attack and heat than are ester links.
MEMBRANE LIPIDS
• Two major types of archaeal lipids have been identified:
1. Glycerol diethers
• Glycerol diether lipids are formed when two hydrocarbons are attached to glycerol.
• Usually, the hydrocarbon chains in glycerol diethers are 20 carbons in length.
2. Diglycerol tetraethers.
• Diglycerol tetraether lipids are formed when two glycerol residues are linked by two long
hydrocarbons that are 40 carbons in length.
• Tetraethers are more rigid lipids than diethers.
MEMBRANE LIPIDS
• When C-20 diethers are used, a typical bilayer membrane is formed.
• When the membrane is constructed of C-40 tetraethers, a monolayer membrane with
much more rigidity is formed.
• The addition of pentacyclic rings (isoprene) further increases this rigidity.
• Extreme thermophiles such as Thermoplasma and Sulfolobus, which grow best at
temperatures over 85°C, are almost completely tetraether monolayers.
• Active transport of nutrients.
CYTOPLASM
• Inclusions-polyhydroxyalkonates, polyphosphate granules, glycogen granules, and gas
vacuoles,
• Ribosomes
• Nucleoid (may contain plasmids)
• Cytoskeletal proteins:
1. FtsZ (tubulin homologue)
2. MreB (actin homologue)
3. Crenactin, an actin homologue unique to certain members of the phylum Crenarchaeota.
RIBOSOMES
23S rRNA
5S rRNA
5.8S rRNA
70 S – 50S +30S
16S rRNA
• Archaeal ribosomes have more proteins:
about 68 rather than about 55.
NUCLEOID
• Circular, doublestranded deoxyribonucleic acid
(DNA).
• Some archaea are polyploid; they have multiple
copies of their chromosomes throughout their life
cycles.
• Some members also have histones form
nucleosomes that are similar to the nucleosomes
observed in eukaryotes.
EXTERNAL STRUCTURES
Pili, Cannulae, and Hami:
• Some archaeal pili are composed of pilin proteins that are homologues of bacterial type IV pili
proteins.
• Two
structures
appear
to
be
unique
to
archaea:
cannulae
and
hami.
CANNULAE & HEMI
• Cannulae: are hollow, tube-like structures observed on the surface of thermophilic
archaea belonging to the genus Pyrodictium.
• Its function is unknown, but it is known that daughter cells arising from a single round
of
cell
division
remain
connected
to
each
other
by
cannulae.
• Hami: Look like tiny grappling hooks, which suggests they might function to attach
cells to surfaces. Indeed, archaeal cells that produce hami are members of biofilm
communities generally consisting of a hami-producing archaean and a bacterium.
ARCHAEAL FLAGELLA AND MOTILITY
Differences:
• 10 to 13 nm rather than 20 nm.
• The flagellum is not hollow.
• Longer hooks than bacterial hooks.
• No basal body.
• Propeller motion.
• Flagellar rotation is powered by ATP hydrolysis rather than by proton motive force.
Fig: Archaeal Flagellum. The different shades of blue in the filament
illustrate that the filament is composed of more than one type of
flagellin. Clear images of the basal body have not been obtained,
although some electron micrographs show a knob at the cell end of the
flagellum, as illustrated here.