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
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.[1]

The genus Halobacterium ("salt" or "ocean bacterium") consists of several species of the Archaea with an aerobic metabolism which requires 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, sequenced by Shiladitya DasSarma, 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 postgenomic 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

Description and significance

Halobacteria are halophilic microorganisms, which means they grow in extremely high-salinity environments. They can act as a 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 three circular replicons. More specifically, it is divided into one large chromosome with 2,014,239 bp and two 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 Halobacterium. 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. They 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. Though the cytoplasm retains an osmotic equilibrium with the hypersaline environment, the cell maintains a high potassium concentration 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.

Halobacterium may be 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, Halobacterium makes pigments called bacterioruberins that are thought to protect cells from damage by ultraviolet light. The obstacle they need to overcome is being able to grow at a low temperature during a presumably short time 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.[2] 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.[2] 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.[3] In wild populations of Halorubrum, genetic exchange and recombination occur frequently.[4] 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.[5]

Further reading

Scientific journals

Scientific books

Scientific databases

References

  1. ↑ See the NCBI webpage on Halobacterium. Data extracted from the "NCBI taxonomy resources". National Center for Biotechnology Information. Retrieved 2007-03-19.
  2. 1 2 McCready S, Müller JA, Boubriak I, Berquist BR, Ng WL, DasSarma S (2005). "UV irradiation induces homologous recombination genes in the model archaeon, Halobacterium sp. NRC-1". Saline Syst. 1: 3. doi:10.1186/1746-1448-1-3. PMC 1224876Freely accessible. PMID 16176594.
  3. ↑ Rosenshine I, Tchelet R, Mevarech M (1989). "The mechanism of DNA transfer in the mating system of an archaebacterium". Science. 245 (4924): 1387–9. doi:10.1126/science.2818746. PMID 2818746.
  4. ↑ Papke RT, Koenig JE, Rodríguez-Valera F, Doolittle WF (2004). "Frequent recombination in a saltern population of Halorubrum". Science. 306 (5703): 1928–9. doi:10.1126/science.1103289. PMID 15591201.
  5. ↑ Bernstein H and Bernstein C (2013). Evolutionary Origin and Adaptive Function of Meiosis. In Meiosis: Bernstein C and Bernstein H, editors. ISBN 978-953-51-1197-9, InTech, http://www.intechopen.com/books/meiosis/evolutionary-origin-and-adaptive-function-of-meiosis
  6. ↑ PIJPER A (1953). "Halobacterium halobium". J Gen Microbiol. 8 (1): viii. PMID 13035061.
  7. ↑ BAYLEY ST, KUSHNER DJ (1964). "THE RIBOSOMES OF THE EXTREMELY HALOPHILIC BACTERIUM, HALOBACTERIUM CUTIRUBRUM". J Mol Biol. 9 (3): 654–69. doi:10.1016/S0022-2836(64)80173-X. PMID 14216609.
  8. ↑ Oesterhelt D, Stoeckenius W (1974). "Isolation of the cell membrane of Halobacterium halobium and its fractionation into red and purple membrane". Methods Enzymol. Methods in Enzymology. 31 (Pt A): 667–78. doi:10.1016/0076-6879(74)31072-5. ISBN 978-0-12-181894-4. PMID 4418026.
  9. ↑ Oesterhelt D, Stoeckenius W (1971). "Rhodopsin-like protein from the purple membrane of Halobacterium halobium". Nat New Biol. 233 (39): 149–52. doi:10.1038/newbio233149a0. PMID 4940442.
  10. ↑ Stoeckenius W, Bogomolni RA (1982). "Bacteriorhodopsin and related pigments of halobacteria". Annu Rev Biochem. 51: 587–616. doi:10.1146/annurev.bi.51.070182.003103. PMID 6287921.
  11. ↑ Lozier RH, Bogomolni RA, Stoeckenius W (1975). "Bacteriorhodopsin: a light-driven proton pump in Halobacterium Halobium". Biophys J. 15 (9): 955–62. Bibcode:1975BpJ....15..955L. doi:10.1016/S0006-3495(75)85875-9. PMC 1334761Freely accessible. PMID 1182271.
  12. ↑ Ng WV, Kennedy SP, Mahairas GG, Berquist B, Pan M, Shukla HD, Lasky SR, Baliga NS, Thorsson V, Sbrogna J, Swartzell S, Weir D, Hall J, Dahl TA, Welti R, Goo YA, Leithauser B, Keller K, Cruz R, Danson MJ, Hough DW, Maddocks DG, Jablonski PE, Krebs MP, Angevine CM, Dale H, Isenbarger TA, Peck RF, Pohlschroder M, Spudich JL, Jung KW, Alam M, Freitas T, Hou S, Daniels CJ, Dennis PP, Omer AD, Ebhardt H, Lowe TM, Liang P, Riley M, Hood L, DasSarma S (2000). "Genome sequence of Halobacterium species NRC-1". Proc Natl Acad Sci U S A. 97 (22): 12176–81. Bibcode:2000PNAS...9712176N. doi:10.1073/pnas.190337797. PMC 17314Freely accessible. PMID 11016950.
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