|Halobacterium sp. strain NRC-1, each cell about 5 μm in length.|
- Note: The word "halobacterium" is also the singular form of the word "halobacteria".
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.
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.
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.
- See the NCBI webpage on Halobacterium. Data extracted from the "NCBI taxonomy resources". National Center for Biotechnology Information. Retrieved 2007-03-19.
- 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 1224876. PMID 16176594.
- 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.
- 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.
- 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
- 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.
- Judicial, Commission of the International Committee on Systematics of Prokaryotes: (2005). "The nomenclatural types of the orders Acholeplasmatales, Halanaerobiales, Halobacteriales, Methanobacteriales, Methanococcales, Methanomicrobiales, Planctomycetales, Prochlorales, Sulfolobales, Thermococcales, Thermoproteales and Verrucomicrobiales are the genera Acholeplasma, Halanaerobium, Halobacterium, Methanobacterium, Methanococcus, Methanomicrobium, Planctomyces, Prochloron, Sulfolobus, Thermococcus, Thermoproteus and Verrucomicrobium, respectively. Opinion 79". Int. J. Syst. Evol. Microbiol. 55 (Pt 1): 517–518. doi:10.1099/ijs.0.63548-0. PMID 15653928.
- Oren A, Ventosa A (2000). "International Committee on Systematic Bacteriology Subcommittee on the taxonomy of Halobacteriaceae. Minutes of the meetings, 16 August 1999, Sydney, Australia". Int. J. Syst. Evol. Microbiol. 50: 1405–1407. PMID 10843089.
- 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
- Gibbons, NE (1974). "Family V. Halobacteriaceae fam. nov.". In RE Buchanan and NE Gibbons, eds. Bergey's Manual of Determinative Bacteriology (8th ed.). Baltimore: The Williams & Wilkins Co.
- Elazari-Volcani, B (1957). "Genus XII. Halobacterium Elazari-Volcani, 1940". In RS Breed, EGD Murray, and NR Smith, eds. Bergey's Manual of Determinative Bacteriology (7th ed.). Baltimore: The Williams & Wilkins Co. pp. 207–212.
- Elazari-Volcani, B (1940). "Studies on the microflora of the Dead Sea". Doctoral dissertation, Hebrew University, Jerusalem: 1–116 and i–xiii.
- PIJPER A (1953). "Halobacterium halobium". J Gen Microbiol 8 (1): viii. PMID 13035061.
- 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.
- 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.
- Oesterhelt D, Stoeckenius W (1971). "Rhodopsin-like protein from the purple membrane of Halobacterium halobium". Nat New Biol 233 (39): 149–52. Bibcode:1971Natur.233..149.. doi:10.1038/233149b0. PMID 4940442.
- 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.
- 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 1334761. PMID 1182271.
- 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 17314. PMID 11016950.