Halobacterium

Halobacterium
Halobacterium sp. strain NRC-1, each cell about 5 μm in length.
Scientific classification
Domain:	Archaea
Kingdom:	Euryarchaeota
Phylum:	Euryarchaeota
Class:	Halobacteria
Order:	Halobacteriales
Family:	Halobacteriaceae
Genus:	Halobacterium
Binomial name
Halobacterium Elazari-Volcani 1957
Species
H. jilantaiense H. noricense H. salinarum H. piscisalsi
Synonyms
Flavobacterium (subgen. Halobacterium) Elazari-Volcani 1940 Halobacter Anderson 1954 Halobacter not "Halobacterium" Schoop 1935 (nomen nudum)

Note: The word "halobacterium" is also the singular form of the word "halobacteria".

In taxonomy, Halobacterium is a genus of the Halobacteriaceae.

The genus Halobacterium ("salt" or "ocean bacterium") consists of several species of archaea with an aerobic metabolism which require an environment with a high concentration of salt; many of their proteins will not function in low-salt environments. They grow on amino acids in their aerobic conditions. Their cell walls are also quite different from those of bacteria, as ordinary lipoprotein membranes fail in high salt concentrations. In shape, they may be either rods or cocci, and in color, either red or purple. They reproduce using binary fission (by constriction), and are motile. Halobacterium grows best in a 42 °C environment. The genome of an unspecified Halobacterium species has been sequenced and comprises 2,571,010 bp (base pairs) of DNA compiled into three circular strands: one large chromosome with 2,014,239 bp, and two smaller ones with 191,346 and 365,425 bp. This species, called Halobacterium sp. NRC-1, has been extensively used for post-genomic analysis. Halobacterium species can be found in the Great Salt Lake, the Dead Sea, Lake Magadi, and any other waters with high salt concentration. Purple Halobacterium species owe their color to bacteriorhodopsin, a light-sensitive protein which provides chemical energy for the cell by using sunlight to pump protons out of the cell. The resulting proton gradient across the cell membrane is used to drive the synthesis of the energy carrier ATP. Thus, when these protons flow back in, they are used in the synthesis of ATP (this proton flow can be emulated with a decrease in pH outside the cell, causing a flow of H⁺ ions). The bacteriorhodopsin protein is chemically very similar to the light-detecting pigment rhodopsin, found in the vertebrate retina.

Species of Halobacterium

• Halobacterium cutirubrum > Halobacterium salinarum
• Halobacterium denitrificans > Haloferax denitrificans
• Halobacterium distributum > Halorubrum distributum
• Halobacterium halobium > Halobacterium salinarum
• Halobacterium jilantaiense
• Halobacterium lacusprofundi > Halorubrum lacusprofundi
• Halobacterium mediterranei > Haloferax mediterranei
• Halobacterium noricense
• Halobacterium pharaonis > Natronomonas pharaonis
• Halobacterium piscisalsi
• Halobacterium saccharovorum > Halorubrum saccharovoru
• Halobacterium salinarum
• Halobacterium sodomense > Halorubrum sodomense
• Halobacterium trapanicum > Halorubrum trapanicum
• Halobacterium vallismortis > Haloarcula vallismortis
• Halobacterium volcanii > Halobacterium volcanii

Description and significance

Halobacteria are halophilic microorganisms, which means they grow in extremely high salinity environments. This archaeon can act as a good model for some aspects of eukaryotic biology, such as DNA replication, transcription, and translation. Comparing a halophile genome to that of other prokaryotes should give insight into microbial adaptation to extreme conditions.

Genome structure

The Halobacterium NRC-1 genome is 2,571,010 bp compiled into 3 circular replicons. More specifically, it is divided into one large chromosome with 2,014,239 bp and 2 small replicons pNRC100 (191,346 bp) and pNRC200 (365,425 bp). While much smaller than the large chromosome, the two plasmids account for most of the 91 insertion sequences and include genes for a DNA polymerase, seven transcription factors, genes in potassium and phosphate uptake, and cell division. The genome was discovered to contain a high G+C content at 67.9% on the large chromosome and 57.9% and 59.2% on the two plasmids. The genome also contained 91 insertion sequence elements constituting 12 families, including 29 on pNRC100, 40 on pNRC200, and 22 on the large chromosome. This helps explain the genetic plasticity that has been observed in Halobacteria. Of the archaea, Halobacteria are viewed as being involved in the most lateral genetics (gene transfer between domains) and a proof that this transfer does take place.

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.

Ecology

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 adds to its reddish hue as well. An optimal temperature for growth has been observed at 37 °C.

On an interesting note, however, 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. 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). In addition, Halobacteria make pigments called bacterioruberins that are thought to protect cells from damage by ultraviolet light. 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.

Recombination and mating

UV irradiation of Halobacterium sp. strain NRC-1 induces several gene products employed in homologous recombination. For instance, a homolog of the rad51/recA gene, which plays a key role in recombination, is induced 7-fold by UV. Homologous recombination may rescue stalled replication forks, and/or facilitate recombinational repair of DNA damage. In its natural habitat, homologous recombination is likely induced by the UV irradiation in sunlight.

Halobacterium volcanii has a distinctive mating system in which cytoplasmic bridges between cells appear to be used for transfer of DNA from one cell to another. In wild populations of Halorubrum, genetic exchange and recombination occur frequently. This exchange may be a primitive form of sexual interaction, similar to the more well studied bacterial transformation that is also a process for transferring DNA between cells leading to homologous recombinational repair of DNA damage.

Further reading

Scientific journals

• DasSarma, S., B.R. Berquist, J.A. Coker, P. DasSarma, J.A. Müller. 2006. Post-genomics of the model haloarchaeon Halobacterium sp. NRC-1. Saline Systems 2:3.

Scientific books

• DasSarma, S. 2004. Genome sequence of an extremely halophilic archaeon, in Microbial Genomes, pp. 383–399, C.M. Fraser, T. Read, and K.E. Nelson (eds.), Humana Press, Inc, Totowa, NJ.
• Lynn Margulis, Karlene V.Schwartz, Five Kingdoms. An Illustrated Guide to the Phyla of Life on Earth (W.H.Freeman, San Francisco, 1982) pp. 36–37

Scientific databases

• PubMed references for Halobacterium
• PubMed Central references for Halobacterium
• Google Scholar references for Halobacterium

External links

• NCBI taxonomy page for Halobacterium
• Search Tree of Life taxonomy pages for Halobacterium
• Search Species2000 page for Halobacterium
• MicrobeWiki page for Halobacterium
• LSPN page for Halobacterium
• The Halobacterium Genome