ARCHAEBACTERIA
The
Archaea constitute a domain
and kingdom
of single-celled
microorganisms.
These microbes are prokaryotes, meaning
that they have no true nucleus or any other membrane-bound organelles in their
cells. Archaea were initially classified as bacteria,
receiving the name Archaebacteria (in the Archaebacteria kingdom), but
this classification is outdated. Archaeal cells have unique properties
separating them from bacteria and from eukaryotes. The Archaea are further
divided into multiple recognized phyla. Classification is difficult
because majority of these has not been isolated in the laboratory and have only
been detected by analysis of their nucleic acids
in samples from their environment.
HABITATS:
Archae
live on wide range of habitats. Based on the habitat, Archae are divided into
the following groups –
1.
Extremophiles:- Extremophile archaea are members of four main physiological
groups. These are the halophiles, thermophiles, alkaliphiles, and acidophiles.
Halophiles,
including the genus Halobacterium,
live in extremely saline environments such as salt lakes
and outnumber their bacterial counterparts at salinities greater than 20–25%. Thermophiles grow best at temperatures
above 45 °C (113 °F), in places such as hot springs; hyperthermophilic
archaea grow optimally at temperatures greater than 80 °C (176 °F).
The archaeal Methanopyrus
kandleri Strain 116 can even reproduce at
122 °C (252 °F), the highest recorded temperature of any organism. Alkaliphiles and Acidophiles archaea exist in very alkaline and acidic conditions.
For example, one of the most extreme archaean acidophiles is Picrophilus torridus,
which grows at pH 0, which is equivalent to thriving in 1.2 molar
sulfuric acid.
2.
Mesophiles:- Archaea include mesophiles that grow
in mild conditions, in swamps
and marshland,
sewage, the oceans,
the intestinal tract
of animals and soils.
3.
Extra-terrestrial:- Resistance to extreme environments has made archaea the
focus of speculation about the possible properties of extraterrestrial
life. Some extremophile habitats are not
similar to those on Mars,
leading to the suggestion that viable microbes could be transferred between
planets in meteorites.
4.
Aquatic Archae:- Large numbers of Archaea found throughout the world's
oceans in non-extreme habitats among the plankton community
(as part of the picoplankton). Some
marine Crenarchaeota are capable of nitrification,
suggesting these organisms may affect the oceanic nitrogen cycle.
Vast numbers of archaea are also found in the sediments
that cover the sea floor, with
these organisms making up the majority of living cells at depths over 1 meter
below the ocean bottom. Some Archaea are common in cold oceanic environments
such as polar seas.
MORPHOLOGY:
1. Shape and Size:- Individual
archaea range from 0.1 – 1.5 μm in diameter, and occur in various shapes,
commonly as spheres, rods, spirals or plates. Other morphologies in the Crenarchaeota
include irregularly shaped lobed cells in Sulfolobus,
needle-like filaments that are less than half a micrometer in diameter in Thermofilum,
and almost perfectly rectangular rods in Thermoproteus
and Pyrobaculum.
Haloquadratum walsbyi
are flat, square archaea that live in hypersaline pools.
Some species form aggregates or
filaments of cells up to 200 μm long. These organisms can be prominent in biofilms. Notably,
aggregates of Thermococcus
coalescens cells fuse together in culture,
forming single giant cells. Archaea in
the genus Pyrodictium
produce an elaborate multicell colony involving arrays of long, thin hollow
tubes called cannulae that stick out from the cells' surfaces and
connect them into a dense bush-like agglomeration. Multi-species colonies exist,
such as the "string-of-pearls" community that was discovered in 2001
in a German that can range up to 15 centimetres (5.9 in) long; these
filaments are made of a particular bacteria species.
2. Cell Wall and Flagella:- Archaea
and bacteria have generally similar cell structure, but cell composition and
organization set the archaea apart. Like bacteria, archaea lack interior
membranes and organelles. Like
bacteria, archaea cell membranes
are usually bounded by a cell wall and they
swim using one or more flagella.
Structurally, archaea are most similar to gram-positive
bacteria.
Most archaea (but not Thermoplasma
and Ferroplasma)
possess a cell wall. In most archaea the wall is assembled from surface-layer
proteins, which form an S-layer. An
S-layer is a rigid array of protein molecules that cover the outside of the
cell (like chain mail). This
layer provides both chemical and physical protection, and can prevent macromolecules
from contacting the cell membrane. Unlike bacteria, archaea lack peptidoglycan
in their cell walls.[101] Methanobacteriales
do have cell walls containing pseudopeptidoglycan,
which resembles eubacterial peptidoglycan in morphology, function, and physical
structure, but pseudopeptidoglycan is distinct in chemical structure; it lacks D-amino acids
and N-acetylmuramic acid.
3. Membranes:- Archaeal
membranes are made of molecules that differ strongly from those in other life
forms, showing that archaea are related only distantly to bacteria and
eukaryotes. In all organisms, cell membranes
are made of molecules known as phospholipids.
These molecules possess both a polar part that dissolves in water (the phosphate
"head"), and a "greasy" non-polar part that does not (the
lipid tail). These dissimilar parts are connected by a glycerol moiety.
In water, phospholipids cluster, with the heads facing the water and the tails
facing away from it. The major structure in cell membranes is a double layer of
these phospholipids, which is called a lipid bilayer.
METABOLISM:
Archaea
exhibit a great variety of chemical reactions in their metabolism and use
many sources of energy. These reactions are classified into nutritional groups,
depending on energy and carbon sources. These are as follows –
1.
Lithotrophs:- Some archaea obtain energy from inorganic compounds
such as sulfur or ammonia (they are
lithotrophs). These
include nitrifiers,
methanogens and anaerobic
methane oxidisers. In these
reactions one compound passes electrons to another (in a redox
reaction), releasing energy to fuel the cell's activities. One compound acts as
an electron donor
and one as an electron acceptor.
The energy released generates adenosine
triphosphate (ATP) through chemiosmosis, in the
same basic process that happens in the mitochondrion
of eukaryotic cells.
2.
Phototrophs:- Other groups of archaea use sunlight as a source of energy
(they are phototrophs).
However, oxygen–generating photosynthesis
does not occur in any of these organisms. Many basic metabolic pathways
are shared between all forms of life; for example, archaea use a modified form
of glycolysis (the Entner–Doudoroff
pathway) and either a complete or partial citric acid cycle.
These similarities to other organisms probably reflect both early origins in
the history of life and their high level of efficiency.
Phototrophic
archaea use light to produce chemical energy in the form of ATP. In the Halobacteria,
light-activated ion pumps like bacteriorhodopsin
and halorhodopsin
generate ion gradients by pumping ions out of the cell across the plasma membrane.
The energy stored in these electrochemical
gradients is then converted into ATP by ATP synthase. This
process is a form of photophosphorylation.
The ability of these light-driven pumps to move ions across membranes depends
on light-driven changes in the structure of a retinol
cofactor
buried in the center of the protein.
3.
Authotrophs:- Other archaea use CO2 in the atmosphere as a
source of carbon, in a process called carbon fixation
(they are autotrophs). This
process involves either a highly modified form of the Calvin cycle or a
recently discovered metabolic pathway called the
3-hydroxypropionate/4-hydroxybutyrate cycle. The Crenarchaeota also use the reverse Krebs cycle
while the Euryarchaeota also use the reductive acetyl-CoA
pathway. Carbon–fixation is powered by
inorganic energy sources. No known archaea carry out photosynthesis.
Archaeal energy sources are extremely
diverse, and range from the oxidation of ammonia by the Nitrosopumilales to
the oxidation of hydrogen sulfide
or elemental sulfur by
species of Sulfolobus,
using either oxygen or metal ions as electron acceptors.
4.
Methanogens:- Some Euryarchaeota are methanogens living in
anaerobic
environments, such as swamps. This form of
metabolism evolved early, and it is even possible that the first free-living
organism was a methanogen. A common reaction involves the use of carbon dioxide
as an electron acceptor to oxidize hydrogen.
Methanogenesis involves a range of coenzymes that are
unique to these archaea, such as coenzyme M and methanofuran. Other
organic compounds such as alcohols, acetic acid or formic acid are used
as alternative electron acceptors
by methanogens. These reactions are common in gut-dwelling archaea. Acetic acid is also
broken down into methane and carbon dioxide directly, by acetotrophic
archaea. These acetotrophs are archaea in the order Methanosarcinales,
and are a major part of the communities of microorganisms that produce biogas.
REPRODUCTION:
Archaea
reproduce asexually by binary or multiple fission,
fragmentation, or budding. Meiosis does not
occur, so if a species of archaea exists in more than one form, all have the
same genetic material. Cell division
is controlled in a cell cycle; after
the cell's chromosome is
replicated and the two daughter chromosomes separate, the cell divides.
In genus Sulfolobus,
the cycle has characteristics that are similar to both bacterial and eukaryotic
systems. The chromosomes replicate from multiple starting-points (origins of
replication) using DNA polymerases
that resemble the equivalent eukaryotic enzymes.
In euryarchaea the cell division
protein FtsZ,
which forms a contracting ring around the cell, and the components of the septum that is
constructed across the center of the cell, are similar to their bacterial
equivalents. In crenarchaea and thaumarchaea, however, the cell division
machinery Cdv fulfills a similar role. This machinery is related to the
eukaryotic ESCRT-III machinery which, while best known for its role in cell
sorting, also has been seen to fulfill a role in separation between divided
cell, suggesting an ancestral role in cell division.
Both bacteria and eukaryotes, but
not archaea, make spores.
Some species of Haloarchaea undergo phenotypic switching
and grow as several different cell types, including thick-walled structures
that are resistant to osmotic shock
and allow the archaea to survive in water at low salt concentrations. These are
not reproductive structures and may instead help them reach new habitats.
CLASSIFICATION:
The
classification of archaea, and of prokaryotes in general, is a rapidly moving
and contentious field. Current classification systems aim to organize archaea
into groups of organisms that share structural features and common ancestors.
These classifications rely heavily on the use of the sequence of ribosomal RNA
genes to reveal relationships between organisms (molecular
phylogenetics). Most of the culturable and
well-investigated species of archaea are members of two main phyla, the Euryarchaeota
and Crenarchaeota.
Other groups have been tentatively created. For example, the peculiar species Nanoarchaeum
equitans, which was discovered in 2003, has
been given its own phylum, the Nanoarchaeota.
A new phylum Korarchaeota has also
been proposed. It contains a small group of unusual thermophilic species that
shares features of both of the main phyla, but is most closely related to the
Crenarchaeota. Other recently detected species of archaea are only distantly
related to any of these groups, such as the Archaeal Richmond Mine acidophilic nanoorganisms
(ARMAN), which were discovered in 2006 and are some of the smallest organisms
known.
A superphylum – TACK, has been
proposed that includes the Thaumarchaeota,
Aigarchaeota, Crenarchaeota,
and Korarchaeota. This
superphylum may be related to the origin of eukaryotes.
Concept
of Species:- The classification of archaea into
species is also controversial. Biology defines a species
as a group of related organisms. The familiar exclusive breeding criterion
(organisms that can breed with each other but not with others)
is of no help here because archaea reproduce asexually.
Archaea show high levels of horizontal gene
transfer between lineages. Some researchers
suggest that individuals can be grouped into species-like populations given
highly similar genomes and infrequent gene transfer to/from cells with
less-related genomes, as in the genus Ferroplasma.
On the other hand, studies in Halorubrum
found significant genetic transfer to/from less-related populations, limiting
the criterion's applicability. A second concern is to what extent such species
designations have practical meaning.
Current knowledge on genetic
diversity is fragmentary and the total number of archaeal species cannot be
estimated with any accuracy. Estimates of the number of phyla range from 18 to
23, out of which only 8 have representatives that have been cultured and
studied directly. Many of these hypothesized groups are known from a single
rRNA sequence, indicating that the diversity among these organisms remains
obscure. The Bacteria also contain many uncultured microbes with similar
implications for characterization.
IMPORTANCE
OF ARCHAEA:
1. Role
in chemical cycling:- Archaea
recycle elements such as carbon, nitrogen and sulfur through their various habitats. Although these
activities are vital for normal ecosystem
function, archaea can also contribute to human-made changes, and even cause pollution.
Archaea
carry out many steps in the nitrogen
cycle. This includes both reactions that remove nitrogen from ecosystems
(such as nitrate-based
respiration and denitrification) as well as processes that introduce
nitrogen (such as nitrate assimilation and nitrogen
fixation). Researchers recently discovered archaeal involvement in ammonia
oxidation reactions. These reactions are particularly important in the oceans.
The archaea also appear crucial for ammonia oxidation in soils. They produce nitrite, which
other microbes then oxidize to nitrate. Plants and other organisms consume the latter. In
the sulfur
cycle, archaea that grow by oxidizing sulfur compounds
release this element from rocks, making it available to other organisms.
However, the archaea that do this, such as Sulfolobus, produce sulfuric
acid as a waste product, and the growth of these organisms in abandoned
mines can contribute to acid mine drainage and other environmental
damage.
In
the carbon
cycle, methanogen archaea remove hydrogen and play an important role in the
decay of organic matter by the populations of microorganisms that act as decomposers
in anaerobic ecosystems, such as sediments, marshes and sewage-treatment
works.
Global
methane levels in 2011 had increased by a factor of 2.5 since pre-industrial
times: from 722 ppb to 1800 ppb, the highest value in at least 800,000 years.
Methane has an anthropogenic global warming potential (AGWP) of 29, which means
that it's 29 times stronger in heat-trapping than carbon dioxide is, over a
100-year time scale.
2. Interactions with
other organisms:- The well-characterized interactions between
archaea and other organisms are either mutual or commensal.
There are no clear examples of known archaeal pathogens or parasites.
However, some species of methanogens have been suggested to be involved in infections in the mouth, and Nanoarchaeum equitans may be a
parasite of another species of archaea, since it only survives and reproduces
within the cells of the Crenarchaeon Ignicoccus
hospitalis,[181]
and appears to offer no benefit to its host.
In contrast, Archaeal Richmond Mine
Acidophilic Nanoorganisms (ARMAN) occasionally connect with other archaeal
cells in acid mine drainage biofilms. The nature of this relationship is
unknown. However, it is distinct from that of Nanarchaeaum–Ignicoccus in
that the ultrasmall ARMAN cells are usually seen independent of the Thermoplasmatales
cells.
Mutualism
One
well-understood example of mutualism is the interaction between protozoa and methanogenic
archaea in the digestive tracts of animals that digest cellulose,
such as ruminants
and termites.
In these anaerobic environments, protozoa break down plant cellulose to obtain
energy. This process releases hydrogen as a waste product, but high levels of
hydrogen reduce energy production. When methanogens convert hydrogen to
methane, protozoa benefit from more energy.
In anaerobic protozoa, such as Plagiopyla
frontata, archaea reside inside the protozoa and consume hydrogen produced
in their hydrogenosomes. Archaea also associate with larger
organisms. For example, the marine archaean Cenarchaeum
symbiosum lives within (is an endosymbiont
of) the sponge Axinella
mexicana.
Commensalism
Archaea can also be commensals,
benefiting from an association without helping or harming the other organism.
For example, the methanogen Methanobrevibacter smithii is by
far the most common archaean in the human
flora, making up about one in ten of all the prokaryotes in the human gut.
In termites and in humans, these methanogens may in fact be mutualists,
interacting with other microbes in the gut to aid digestion. Archaean
communities also associate with a range of other organisms, such as on the
surface of corals,
and in the region of soil that surrounds plant roots (the rhizosphere).
3. Significance in technology and
industry:-
Extremophile
archaea, particularly those resistant either to heat or to extremes of acidity
and alkalinity, are a source of enzymes that function under these harsh conditions. These
enzymes have found many uses. For example, thermostable DNA
polymerases, such as the Pfu DNA polymerase from Pyrococcus furiosus, revolutionized molecular
biology by allowing the polymerase chain reaction to be used in
research as a simple and rapid technique for cloning DNA. In
industry, amylases,
galactosidases
and pullulanases
in other species of Pyrococcus that function at over 100 °C
(212 °F) allow food processing at high temperatures, such as the
production of low lactose milk and whey. Enzymes from these thermophilic archaea also tend to be
very stable in organic solvents, allowing their use in environmentally friendly
processes in green chemistry that synthesize organic compounds.
This stability makes them easier to use in structural biology. Consequently, the
counterparts of bacterial or eukaryotic enzymes from extremophile archaea are
often used in structural studies.
In contrast to the range of
applications of archaean enzymes, the use of the organisms themselves in
biotechnology is less developed. Methanogenic archaea are a vital part of sewage
treatment, since they are part of the community of microorganisms that
carry out anaerobic digestion and produce biogas. In mineral processing, acidophilic archaea display
promise for the extraction of metals from ores, including gold, cobalt and copper.
Archaea
host a new class of potentially useful antibiotics.
A few of these archaeocins have been characterized, but hundreds more are
believed to exist, especially within Haloarchaea
and Sulfolobus.
These compounds differ in structure from bacterial antibiotics, so they may
have novel modes of action. In addition, they may allow the creation of new selectable
markers for use in archaeal molecular biology.
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