Halobacterium salinarum

Scientific Classification


Description of Phylum: Euryarchaeota
The more derived of the 2 kingdoms of Archaea, comprising a broad range of phenotypes including mechanogens, halophiles, and sulphate-reducing organisms. Members of the Euryarchaeota show less genetic similarity to those belonging to the domains Eucarya and Bacteria than do those of the Crenarchaeota.

The Euryarchaeota include the methanogens, which produce methane and are often found in intestines, the halobacteria, which survive extreme concentrations of salt, and some extremely thermophilic aerobes and anaerobes. They are separated from the other archaeans based mainly on rRNA sequences.

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Introduction to species

Halobacterium salinarum is an extremely halophilic marine gram-negative obligate aerobic archaeon. Despite its name, this microorganism is not a bacterium, but rather a member of the Domain Archaea. It is found in salted fish, hides, hypersaline lakes, and salterns. As these salterns reach the minimum salinity limits for extreme halophiles, their waters become purple or reddish color due to the algal bloom of halophilic Archaea. H. salinarum has also been found in high-salt food such as salt pork, marine fish, and sausages. The ability of H. salinarum to survive at such high salt concentrations has led to its classification as an extremeophile.
Halobacterium salinarum is a model organism for the halophilic branch of the archaea. It is rod-shaped, motile, lives in highly saline environments (4M salt and higher), and is one of the few species known that can live in saturated salt solutions. Mass cultures of Halobacterium salinarum as shown in the pictures below can be recognized by their typical color, which originates from bacterioruberins. Halobacterium salinarum is depicted in its natural environment and as a species that colonizes salines.

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(Halobacterium salinarum in its natural environment. The picture shows a salty pond in the Arabian desert, which is colored red due to the presence of Halobacterium salinarum)

It can live with light as only energy source due to the activity of the retinal protein bacteriorhodopsin, a light-driven proton pump, which has been studied in great detail and has become a paradigm of membrane proteins in general and transport proteins in particular. From this point, our focus has widened to study additional processes in which retinal proteins are involved: the energy metabolism of Halobacterium salinarum and the tactic responses with its associated signal transduction network.

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(Massive growth of Halobacterium salinarum in a saline)

Unique Morphological Features
Cell Structure and Metabolism
Halobacterium species are rod shaped and enveloped by a single lipid bilayer membrane surrounded by an S-layer made from the cell-surface glycoprotein. Halobacteria grow on amino acids in aerobic conditions. Although Halobacterium NRC-1 contains genes for glucose degradation as well as genes for enzymes of a fatty acid oxidation pathway, it does not seem able to use these as energy sources. Even though the cytoplasm retains an osmotic equilibrium with the hypersaline environment, the cell maintains a high potassium concentration. It does this by using many active transporters.Many Halobacterium species possess proteinaceous organelles called gas vesicles.

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Unique Anatomical/Physiological Features

Retinal proteins of Halobacterium salinarum

Halobacterium salinarum contains four retinal proteins, which are photosynthetic pigments with a retinal chromophore involved in light energy conversion and signal transduction. The four retinal proteins are
  • bacteriorhodopsin
    • the photosynthetic pigment that permits Halobacterium to grow with light as only energy source
    • a light-driven proton pump which converts light energy into a proton gradient. The energy stored in the proton gradient can be used in different ways, e.g. for generation of ATP via ATP synthase
  • halorhodopsin
    • a light-driven chloride pump that permits Halobacterium to maintain the high internal salt concentration upon growth
  • sensory rhodopsin I
    • involved in phototaxis, mediates the photophilic response to orange and also the photophobic response to UV light
    • forms a complex with the transducer protein htrI
  • sensory rhodopsin II
    • involved in phototaxis, mediates the photophobic response to blue light
    • forms a complex with the transducer protein htrII

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Adaptive Habitat Features

Adaptation to Extreme Conditions

High Salt
To survive in extremely salty environments, this archaeon—as with other halophilic Archaeal species—utilizes compatible solutes (in particular potassium chloride) to reduce osmotic stress. Potassium levels are not at equilibrium with the environment, so H. salinarum expresses multiple active transporters which pump potassium into the cell. At extremely high salt concentrations protein precipitation will occur. To prevent the salting out of proteins, H. salinarum encodes mainly acidic proteins. The average isoelectric point of H. salinarum proteins is 4.9. These highly acidic proteins are overwhelmingly negative in charge and are able to remain in solution even at high salt concentrations.
Low Oxygen
H. salinarum is an obligate aerobe but can survive in low-oxygen conditions. These organisms are able to survive in low-oxygen conditions by utilizing light-energy. H. salinarum express the membrane protein bacteriorhodopsin. Bacteriorhodopsin acts as a light-driven proton pump. It consists of two parts, the 7-transmembrane protein, bacterioopsin, and the light-sensitive cofactor, retinal. Upon absorption of a photon, retinal changes conformation, causing a conformational change in the bacterioopsin protein which drives proton transport.The proton gradient which is formed can then be used to generate chemical energy by ATP synthase.
High UV
These organisms are exposed to high amounts of UV radiation. To compensate, they have evolved a sophisticated DNA repair mechanism. H. salinarum encode DNA repair enzymes homologous to those in both bacteria and eukaryotes. This allows them to repair damage to DNA faster and more efficiently than other organisms and allows them to be much more UV tolerant.

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Range on Earth

(Halobacterium at a salt works near San Quentin, Baja California Norte, Mexico)external image 200px-Halobacterium_2.jpg

Halobacteria can be found in highly saline lakes such as the Great Salt Lake, the Dead Sea, and Lake Magadi. Halobacterium can be identified in bodies of water by the light-detecting pigment bacteriorhodopsin, which not only provides the archaeon with chemical energy, but gives it a reddish hue as well. An optimal temperature for growth has been observed at 37oC.
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(Dead Sea)

Interesting facts
On an interesting note, Halobacteria are a candidate for a life form present on Mars. One of the problems associated with the survival on Mars is the destructive ultraviolet light. Halobacteria have an advantage here. These microorganisms develop a thin crust of salt that can moderate some of the ultraviolet light. Sodium chloride is the most common salt and chloride salts are opaque to short-wave ultraviolet. Their photosynthetic pigment, bacteriorhodopsin, is actually opaque to the longer wavelength ultraviolet (its red color). The obstacle Halobacteria would need to overcome is being able to grow at a low temperature during a presumably short time span when a pool of water could be liquid.

H. salinarum is responsible for the bright pink or red appearance of the Dead Sea and other bodies of salt water. This red color is due primarily to the presence of bacterioruberin. Bacterioruberin, a 50 carbon carotenoid, is pigment present within the membrane of H. salinarum. It absorbs light in the UV range and can protect the cell against DNA damage incurred by UV light. This protection is not, however, due to the ability of bacterioruberin to absorb UV light. Bacterioruberin protects the DNA by acting as an antioxidant, rather than directly blocking UV light. It is able to protect the cell from reactive oxygen species produced from exposure to UV by acting as a target. The bacterioruberin radical produced is less reactive than the initial radical, and will likely react with another radical, resulting in termination of the radical chain reaction.

Oldest DNA Ever Recovered

A sample of a close genetic relative of H. Salinarum encapsulated in salt has allowed for the recovery of DNA fragments estimated at 121 million years old. Oddly, the material had been also recovered earlier, but it proved to be so similar to the modern descendants that scientists had believed the earlier samples were contaminated.

Scientists have previously recovered similar genetic material from the Michigan Basin, the same region where the latest discovery was made. But that DNA, discovered in a salt-cured buffalo hide in the 1930s, was so similar to that of modern microbes that many scientists believed the samples had been contaminated. The curing salt had been derived from a mine in Saskatchewan, the site of the most recent sample described by Jong Soo Park of Dalhousie University in Halifax, NS, Canada.

Russell Vreeland of Ancient Biomaterials Institute of West Chester University in Pennsylvania, USA, performed an analysis of all known halopathic bacteria, which yielded the finding that Park's bacteria contained six segments of DNA never seen before in the halopaths. Vreeland also tracked down the buffalo skin and determined that the salt came from the same mine as Park's sample. He has also discovered an even older halopath estimated at 250 million years old in New Mexico.

  • Kennedy, S. "Genome sequence of Halobacterium species NRC-1." PNAS 92.22 (2000): 12176-12181. PNAS. Web. 9 Nov. 2010.
  • Reilly, M. "World's oldest known DNA discovered - Technology & science - Science - DiscoveryNews.com - msnbc.com." Breaking News, Weather, Business, Health, Entertainment, Sports, Politics, Travel, Science, Technology, Local, US & World News- msnbc.com. msn, 17 Dec. 2009. Web. 9 Nov. 2010. <http://www.msnbc.msn.com/id/34467577/ns/technology_and_science-science/>.
  • Rudolph, J. "Car: a cytoplasmic sensor responsible for arginine." The EMBO Journal 18.5 (1999): 1146–1158. National Center for Biotechnology Information . Web. 9 Nov. 2010.