Archaea: Microorganisms with multiple potential -By: Sunita Gaind

Archaea are a group of single-celled microorganisms that have no cell nucleus or any other membrane-bound organelles within their cells. Formerly, thought to be bacteria until DNA analysis, later categorized in Archaea domain. Their most positive characteristic lies in their existence under some extreme conditions of hot, acidic or alkaline environment. Archaea and bacteria are quite similar in size and shape, although a few archaea have flat and square-shaped cells e.g. Haloquadratum walsbyi. Most studied thermophilic archaea include Sulfolobus acidocaldarius, Acidianus brierleyi, and Metallosphaera sedula. However, Ferroplasma acidophilum and Ferroplasma acidarmanus are some examples of mesophilic archaea.

Classification

The domain Archaea includes four kingdoms
Crenarchaeota, Euryarchaeota, Korarchaeota, Nanoarchaeota.
(i) Crenarchaeota includes Sulfolobus and other organisms that grow in extremely hot or cold habitats.
(ii) Euryarchaeota includes methane producers and extreme halophiles.
(iii) Korarchaeota is primarily known from DNA sequences found in samples from hot springs eg. Aquificales
(iv) Nanoarchaeota includes the hyperthermophile.

Habitat

Although, many archaea occur in soils and surface ocean waters of moderate conditions, diverse archaea occupy habitats of very high salt content, acidity, methane levels, or temperatures that would kill most bacteria and eukaryotes. Methane producer Methanopyrus, grows best at deep-sea thermal vent sites where the temperature is 98°C. In fact, Methanopyrus is so closely adapted to its extremely hot environment that it will not grow when the temperature is less than 84°C. Some archaea (Sulfolobus) prefer habitats having both high temperatures and extremely low pH (3.0). Extreme halophiles ("salt-lovers") occupy evaporation ponds used to produce salt from seawater, often growing so abundantly that they color the ponds red.

Differences between Archaea and bacteria
ï'˜ Bacteria and Archaea differ in cell wall structure. All bacteria have peptidoglycan in cell wall whereas archaea do not have.
ï'˜ Cell membrane of Archaea contains ether linkages whereas cell membrane of bacteria have ester bond. Ether-linked membranes are resistant to damage by heat and other extreme conditions, which helps explain why many archaea are able to grow in extremely harsh environments.
ï'˜ Both are also different in RNA polymerase. Bacteria have only one RNA polymerase and react to antibiotics in a different way than archaea do. Archaea have three RNA polymerase as eukaryotes.
ï'˜ Archaea possess genes and several metabolic pathways that are more closely related to those of eukaryotes, notably the enzymes involved in transcription and translation.
ï'˜ Histone proteins are typically associated with the DNA of both archaea and eukaryotes, but they are absent from most bacteria.
ï'˜ Archaea use a much greater variety of energy sources of than eukaryotes: ranging from sugars to ammonia, metal ions or even hydrogen gas. Salt-tolerant archaea (the halobacteria) use sunlight as an energy source and other species of archaea fix carbon; however, unlike plants and cyanobacteria, no species of archaea is known to do both.
ï'˜ Some archaea scavenge elemental sulfur, often produced from the chemical oxidation of some sulfide minerals by ferric iron, oxidizing it to sulfuric acid. Archaea grow using ferrous iron and various sulfur substances. They obtain oxygen and carbon dioxide from air. They grow and reproduce in sulfuric acid environments approaching boiling temperatures with high dissolved metal content and low amounts of oxygen, carbon dioxide, ammonium, and phosphate.
ï'˜ Archaea reproduce asexually and divide by binary fission, fragmentation, or budding; unlike bacteria and eukaryotes, no known species form spores.
ï'˜ Pathogenic and parasitic archaea are not known,

Role of Archaea in biomining of metals
Biomining organisms directly attack the solid sulfide minerals, breaking the mineral's crystal structure, and then oxidize the sulfide constituent of the mineral to release the metals. Biomining microorganisms oxidize sulfide-sulfur (S2−-S) when the same is dissolved in acidic water. It is uncertain whether the organisms directly attack the solid mineral.
Biomining substantially improves gold and silver recovery from sulfidic-refractory ores. A mixture of sulfuric acid, water, and iron is added to these ores. The biomining microorganisms as archaea oxidize the ferrous iron to ferric iron that oxidizes the sulfide minerals in which the gold and silver is buried. As the sulfide mineral dissolves, the gold and silver are exposed. The treated ore is neutralized, mixed with water and small amounts of sodium cyanide or other chemicals that dissolve the precious metals. The dissolved gold and silver are then separated from the ore. The precious metals are reclaimed as pure products.

Role in carbon cycle

Methanogens are generally found in swampy wetlands, in deep-sea habitats, or in the digestive systems of cattles. Marsh gas produced in wetlands is largely composed of methane, and large quantities of methane produced gets trapped in deep-sea. Methane-consuming bacteria live in close association with wetland plants, which release the oxygen needed by these bacteria to metabolize methane. In the absence of methanotrophs, Earth's atmosphere would be much richer in the greenhouse gas methane, which would substantially increase global temperatures.
Archaea influence Earth's carbon cycle by producing and consuming methane (CH4). Methane-the major component of natural gas-is a powerful greenhouse gas, as are CO2 and H2O vapor. Atmospheric methane thus has the potential to alter the Earth's climate. Several groups of anaerobic archaea known as the methanogens convert CO2, methyl groups, or acetate to methane and release it from their cells

Industrial application

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.
ï'˜ Enzymes from Extremophile archaea have a wide range of uses. Thermostable DNA polymerases, such as the Pfu DNA polymerase from Pyrococcus furiosus, allows the polymerase chain reaction to be used as a simple and rapid technique for cloning DNA.
ï'˜ Amylases, galactosidases and pullulanases in other species of Pyrococcus that function at over 100 o C allow food processing at high temperatures, such as the production of low lactose milk and whey.
ï'˜ 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.
ï'˜ The stability of thermophilic enzymes also makes them easier to use in structural biology.
ï'˜ Methanogenic archaea are a vital part of sewage treatment, due to their ability to carry out anaerobic digestion and produce biogas. Methanogens are used in biogas production and sewage treatment,
ï'˜ In mineral processing, acidophilic archaea display promise for the extraction of metals from ores, including gold, cobalt and copper.
ï'˜ A new class of potentially useful antibiotics (archaeocins) has been discovered in archaea. A few of these archaeocins have been characterized, but hundreds more are believed to exist, especially within Haloarchaea and Sulfolobus. These compounds are important since they are different in structure to 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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