extremophiles that thrive in environments with very high concentrations of salt. The name comes from Greek for "salt-loving". While the term is perhaps most often applied to some halophiles classified into the Archaeadomain, there are also bacterial halophiles and some eukaryota, such as the alga Dunaliella salina. Some well-known species give off a red color from carotenoid compounds. Such species contain the photosynthetic pigment bacteriorhodopsin. Halophiles are categorized slight, moderate or extreme, by the extent of their halotolerance. Halophiles can be found anywhere with a concentration of salt 5 times greater than the salt concentration of the ocean, such as the Great Salt Lakein Utah, Owens Lakein California, the Dead Sea, and in evaporation ponds.scientific name: halophilic eubacterium
What halophiles do and how they work
High salinity represents an extreme environment that relatively few organisms have been able to adapt to and occupy. Most halophilic and all halotolerant organisms expend
energyto exclude salt from their cytoplasmto avoid protein aggregation (‘ salting out’). In order to survive the high salinities, halophiles employ two differing strategies to prevent desiccationthrough osmotic movement of water out of their cytoplasm. Both strategies work by increasing the internal osmolarityof the cell. In the first, that employed by the majority of bacteria, some archaea, yeasts, algaeand fungi, organic compoundsare accumulated in the cytoplasm – these osmoprotectants are known as compatible solutes. These can be synthesised or accumulated from the environmentSantos, H., and da Costa, M.S. (2002) Compatible solutes of organisms that live in hot saline environments. Environmental Microbiology 4: 501-509.] . The most common compatible solutes are neutral or zwitterionicand include amino acids, sugars, polyols, betaines and ectoines, as well as derivatives of some of these compounds.
The second, more radical, adaptation involves the selective influx of
potassium(K+) ions into the cytoplasm. This adaptation is restricted to the moderately halophilic bacterial Order Halanerobiales, the extremely halophilic archaeal Family Halobacteriaceaeand the extremely halophilic bacterium "Salinibacter ruber". The presence of this adaptation in three distinct evolutionary lineages suggests convergent evolution of this strategy, it being unlikely to be an ancient characteristic retained in only scattered groups or through massive lateral gene transfer . The primary reason for this is that the entire intracellular machinery (enzymes, structural proteins, etc.) must be adapted to high salt levels, whereas in the compatible solute adaptation little or no adjustment is required to intracellular macromolecules – in fact, the compatible solutes often act as more general stress protectants as well as just osmoprotectants.
Of particular note are the extreme halophiles or
haloarchaea(often known as halobacteria), a group of archaea, which require at least a 2 M salt concentration and are usually found in saturated solutions (about 36% w/v salts). These are the primary inhabitants of salt lakes, inland seas, and evaporating ponds of seawater, such as the Dead Sea and solar salterns, where they tint the water column and sediments bright colors. In other words, they will most definitely perish if they are exposed to anything besides a very high, intense salt-conditioned environment. These prokaryotes require salt for growth. The high concentration of NaCl in their environment limits the availability of oxygen for respiration. Their cellular machinery is adapted to high salt concentrations by having charged amino acids on their surfaces, allowing the retention of water molecules around these components. They are heterotrophs that normally respire by aerobic means. Most halophiles are unable to survive outside their high-salt native environment. Indeed, many cells are so fragile that when placed in distilled water they immediately lyse from the change in osmotic conditions.
Haloarchaea, and particularly, the family Halobacteriaceae are members of the domain Archaea, and comprise the majority of the prokaryotic population in hypersaline environments [Oren, A. (2002) Molecular ecology of extremely halophilic Archaea and Bacteria. FEMS Microbiology Ecology: 1-7.] . There are currently 15 recognised genera in the family [Gutierrez, M.C., Kamekura, M., Holmes, M.L., Dyall-Smith, M.L., and Ventosa, A. (2002) Taxonomic characterisation of Haloferax sp. ("H. alicantei") strain Aa 2.2: description of Haloferax lucentensis sp. nov. Extremophiles. 2002 Dec;6(6):479-83] . The domain Bacteria (mainly "Salinibacter ruber") can comprise up to 25% of the prokaryotic community, but is more commonly a much lower percentage of the overall population [Anton, J., Rossello-Mora, R., Rodriguez-Valera, F., and Amann, R. (2000) Extremely halophilic bacteria in crystallizer ponds from solar salterns. Applied and Environmental Microbiology 66: 3052-3057.] . At times, the alga "
Dunaliella salina" can also proliferate in this environment [Casamayor, E.O., Massana, R., Benlloch, S., Ovreas, L., Diez, B., Goddard, V.J., Gasol, J.M., Joint, I., Rodriguez-Valera, F., and Pedros-Alio, C. (2002) Changes in archaeal, bacterial and eukaryal assemblages along a salinity gradient by comparison of genetic fingerprinting methods in a multipond solar saltern. Environmental Microbiology 4: 338-348.] .
A comparatively wide range of taxa have been isolated from saltern crystalliser ponds, including members of the following genera: "Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula" and "Halobacterium" families (Oren 2002). However, the viable counts in these cultivation studies have been small when compared to total counts, and the numerical significance of these isolates has been unclear. Only recently has it become possible to determine the identities and relative abundances of organisms in natural populations, typically using
PCR-based strategies that target 16S small subunit ribosomal ribonucleic acid (16S rRNA) genes. While comparatively few studies of this type have been performed, results from these suggest that some of the most readily isolated and studied genera may not in fact be significant in the in-situ community. This is seen in cases such as the genus "Haloarcula", which is estimated to make up less than 0.1% of the in situ community [Anton, J., Llobet-Brossa, E., Rodriguez-Valera, F., and Amann, R. (1999) Fluorescence in situ hybridization analysis of the prokaryotic community inhabiting crystallizer ponds. Environmental Microbiology 1: 517-523.] but commonly appears in isolation studies.
Halophiles in astrobiology
It has been proposed that halophiles may be representative of life forms that may be present in niche ecologies on other planets.
Geoffrey A. Landisof NASA Glenn Research Center, for example, has argued that liquid water, at the low temperature and pressures characteristic of the surface of Mars, is likely to be highly saline, and hence any extant lifeforms will be likely to be similar to terrestrial halophiles [Landis, G. A., "Martian Water: Are there Extant Halobacteria on Mars?" " [http://www.liebertonline.com/toc/ast/1/2 Astrobiology] , Vol. 1," No. 2, 161-164 (2001).] . Extremophiles are currently being extensively studied by the astrobiologyprogram both as possible ancient forms of terrestrial life, and hence as clues about the origin and early forms of life, and also as possible analogues for extraterrestrial life.
Genomic and proteomic signature of halophiles
The comparative genomic and proteomic analysis revealed that there is a distinct molecular signatures for environmental adaptation of halophiles. At the protein level, the halophilic species are characterized by low hydrophobicity, overrepresentation of acidic residues, underrepresentation of Cys, lower propensities for helix formation and higher propensities for coil structure. At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage [Paul, S., Bag, S.K., Das, S., Harvill, E.T., Dutta, C.(2008) Molecular Signature of Hypersaline Adaptation: Insights from Genome and Proteome Composition of Halophilic Prokaryotes. Genome Biology 2008, 9:R70.] .
Examples of halophiles
* DasSarma, S. and P. DasSarma 2006. "Halophiles", [http://www.mrw.interscience.wiley.com/emrw/047001590X/home Encyclopedia of Life Sciences] , Wiley, London.
* Madigan, Michael T., and Barry L. Narrs, "Extremophiles" "Scientific American", April 1997: 82-88.
* [http://www.haloarchaea.com HaloArchaea.com]
* [http://textbookofbacteriology.net/procaryotes.html Important Groups of Prokaryotes] - Kenneth Todar
* [http://library.thinkquest.org/C003763/index.php?page=origin07 Astrobiology: extremophiles- life in extreme environments]
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halophile — [ alɔfil ] adj. • 1846; gr. hals, halos « sel » et phile ♦ Didact. Qui croît dans les milieux imprégnés de sel marin. Bactérie halophile. ● halophile adjectif Qui pousse naturellement dans les terrains imprégnés de sel. halophile adj. BIOL Se dit … Encyclopédie Universelle
halophile — [hal′ə fīl΄] n. [ HALO + PHILE] an organism living in a salty environment halophilic [hal΄əfil′ik] adj. halophilous [hə läf′i ləs] … English World dictionary
Halophile — Als Halophile oder Halotolerante (abgeleitet vom griechischen hals, halos = Salz) werden Organismen bezeichnet, die in Umgebungen mit erhöhter Salzkonzentration leben. Als Salz gilt dabei nicht nur Kochsalz, sondern auch jedes andere Mineralsalz … Deutsch Wikipedia
Halophile — Un organisme halophile (du grec alos, sel et philein, aimer) est un organisme qui s accommode ou a besoin de fortes concentrations en sel dans son milieu pour vivre. Les organismes halophiles sont des extrémophiles appartenant aux domaines des… … Wikipédia en Français
halophile — (ha lo fi l ) adj. Terme de botanique. Qui aime le sel, qui croît dans les terrains imprégnés de sel. ÉTYMOLOGIE En grec, sel et qui aime. SUPPLÉMENT AU DICTIONNAIRE HALOPHILE. Ajoutez : Terme de zoologie. Qui aime l eau salée.… … Dictionnaire de la Langue Française d'Émile Littré
halophile — noun Etymology: International Scientific Vocabulary Date: 1923 an organism that flourishes in a salty environment • halophilic adjective … New Collegiate Dictionary
halophile — halophil … Dictionary of ichthyology
halophile — n. [Gr. hals, salt, sea; philos, love] An organism adapted to living in a salty environment … Dictionary of invertebrate zoology
halophile — (hal o faīl) A microorganism that requires high levels of sodium chloride for growth … Dictionary of microbiology
halophile — Literally, salt loving: organism that tolerates saline conditions, in extreme cases in concentrations considerably in excess of those found in normal sea water such as salt lakes. Some Archaebacteria (eg. Halobacterium halobium ) are notable for… … Dictionary of molecular biology