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Halophile

From Wikipedia, the free encyclopedia

Ahalophile(from the Greek word for 'salt-loving') is anextremophilethat thrives in highsaltconcentrations. In chemical terms, halophile refers to aLewis acidicspecies that has some ability to extract halides from other chemical species.

While most halophiles are classified into the domainArchaea,there are alsobacterialhalophiles and someeukaryoticspecies, such as thealgaDunaliella salinaandfungusWallemia ichthyophaga.Some well-known species give off a red color from carotenoid compounds, notablybacteriorhodopsin.

Halophiles can be found in water bodies with salt concentration more than five times greater than that of the ocean, such as theGreat Salt Lakein Utah,Owens Lakein California, theLake Urmiain Iran, theDead Sea,and inevaporation ponds.They are theorized to be a possible analogues for modeling extremophiles that might live in the salty subsurface water ocean of Jupiter'sEuropaand similar moons.[1]

Classification

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Halophiles are categorized by the extent of theirhalotolerance:slight, moderate, or extreme. Slight halophiles prefer 0.3 to 0.8M(1.7 to 4.8%—seawater is 0.6 M or 3.5%), moderate halophiles 0.8 to 3.4 M (4.7 to 20%), and extreme halophiles 3.4 to 5.1 M (20 to 30%) salt content.[2]Halophiles require sodium chloride (salt) for growth, in contrast to halotolerant organisms, which do not require salt but can grow under saline conditions.

Lifestyle

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High salinity represents an extreme environment in which relatively few organisms have been able to adapt and survive. Most halophilic and allhalotolerantorganisms expend energy to exclude salt from theircytoplasmto avoid protein aggregation ('salting out'). To survive the high salinities, halophiles employ two differing strategies to preventdesiccationthroughosmoticmovement of water out of their cytoplasm. Both strategies work by increasing the internalosmolarityof the cell. The first strategy is employed by some archaea, the majority of halophilic bacteria,yeasts,algae,andfungi;the organism accumulatesorganic compoundsin the cytoplasm—osmoprotectantswhich are known as compatible solutes. These can be either synthesised or accumulated from the environment.[3]The most common compatible solutes areneutralorzwitterionic,and includeamino acids,sugars,polyols,betaines,andectoines,as well as derivatives of some of these compounds.

The second, more radical adaptation involves selectively absorbingpotassium(K+) ions into the cytoplasm. This adaptation is restricted to the extremely halophilic archaeal familyHalobacteriaceae,the moderately halophilic bacterial orderHalanaerobiales,and the extremely halophilic bacteriumSalinibacter ruber.The presence of this adaptation in three distinct evolutionary lineages suggestsconvergent evolutionof this strategy, it being unlikely to be an ancient characteristic retained in only scattered groups or passed on through massive lateral gene transfer.[3]The primary reason for this is 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.[3]

Of particular note are the extreme halophiles orhaloarchaea(often known ashalobacteria), a group of archaea, which require at least a 2 M salt concentration and are usually found in saturated solutions (about 36%w/vsalts). These are the primary inhabitants of salt lakes, inland seas, and evaporating ponds of seawater, such as the deepsalterns,where they tint the water column and sediments bright colors. These species most likely perish if they are exposed to anything other than a very high-concentration, salt-conditioned environment. These prokaryotes require salt for growth. The high concentration of sodium chloride in their environment limits the availability of oxygen for respiration. Their cellular machinery is adapted to high salt concentrations by having chargedamino acidson their surfaces, allowing the retention of water molecules around these components. They areheterotrophsthat normally respire by aerobic means. Most halophiles are unable to survive outside their high-salt native environments. Many halophiles are so fragile that when they are placed in distilled water, they immediatelylysefrom the change in osmotic conditions.

Halophiles use a variety of energy sources and can be aerobic or anaerobic; anaerobic halophiles include phototrophic, fermentative, sulfate-reducing, homoacetogenic, and methanogenic species.[2][4]

The Haloarchaea, and particularly the family Halobacteriaceae, are members of the domainArchaea,and comprise the majority of the prokaryotic population inhypersaline environments.[5]Currently, 15 recognised genera are in the family.[6]The domainBacteria(mainlySalinibacter ruber) can comprise up to 25% of the prokaryotic community, but is more commonly a much lower percentage of the overall population.[7]At times, the algaDunaliella salinacan also proliferate in this environment.[8]

A comparatively wide range of taxa has been isolated from saltern crystalliser ponds, including members of these genera:Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula,andHalobacterium.[5]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 usingPCR-based strategies that target 16Ssmall subunit ribosomal ribonucleic acid (16S rRNA) genes.[9]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 thein situcommunity. This is seen in cases such as the genusHaloarcula,which is estimated to make up less than 0.1% of thein situcommunity,[10]but commonly appears in isolation studies.

Genomic and proteomic signature

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The comparative genomic and proteomic analysis showed distinct molecular signatures exist for the environmental adaptation of halophiles. At the protein level, the halophilic species are characterized by low hydrophobicity, an overrepresentation of acidic residues, underrepresentation of Cys, lower propensities for helix formation, and higher propensities for coil structure. The core of these proteins is less hydrophobic, such asDHFR,that was found to have narrower β-strands.[11] In one study, the net charges (at pH 7.4) of the ribosomal proteins (r-proteins) that comprise theS10-spccluster were observed to have an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea.[12]At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.[13]

Examples

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Halobacteriaceaeis a family that includes a large part of halophilic archaea.[14]The genusHalobacteriumunder it has a high tolerance for elevated levels of salinity. Some species of halobacteria have acidic proteins that resist the denaturing effects of salts.Halococcusis another genus of the family Halobacteriaceae.

Somehypersaline lakesare habitat to numerous families of halophiles. For example, theMakgadikgadi PansinBotswanaform a vast, seasonal, high-salinity water body that manifests halophilic species within thediatomgenusNitzschiain the familyBacillariaceae,as well as species within the genusLovenulain the familyDiaptomidae.[15]Owens Lake in California also contains a large population of the halophilic bacteriumHalobacterium halobium.

Wallemia ichthyophagais abasidiomycetousfungus,which requires at least 1.5 Msodium chlorideforin vitrogrowth, and it thrives even in media saturated with salt.[16]Obligate requirement for salt is an exception in fungi. Even species that can tolerate salt concentrations close to saturation (for exampleHortaea werneckii) in almost all cases grow well in standard microbiological media without the addition of salt.[17]

The fermentation of salty foods (such assoy sauce,Chinese fermented beans,salted cod,saltedanchovies,sauerkraut,etc.) often involves halophiles as either essential ingredients or accidental contaminants. One example isChromohalobacter beijerinckii,found in salted beans preserved in brine and in saltedherring.Tetragenococcus halophilusis found in salted anchovies and soy sauce.

Artemiais a ubiquitous genus of small halophilic crustaceans living in salt lakes (such as Great Salt Lake) and solar salterns that can exist in water approaching the precipitation point of NaCl (340 g/L)[18][19]and can withstand strong osmotic shocks due to its mitigating strategies for fluctuating salinity levels, such as its unique larval salt gland and osmoregulatory capacity.

North Ronaldsay sheepare a breed of sheep originating fromOrkney, Scotland.They have limited access to freshwater sources on the island and their only food source isseaweed.They have adapted to handle salt concentrations that would kill other breeds of sheep.[20]

See also

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References

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  1. ^Marion, Giles M.; Fritsen, Christian H.; Eicken, Hajo; Payne, Meredith C. (2003-12-01)."The search for life on Europa: Limiting environmental factors, potential habitats, and Earth analogues".Astrobiology.3(4): 785–811.Bibcode:2003AsBio...3..785M.doi:10.1089/153110703322736105.ISSN1531-1074.PMID14987483.
  2. ^abOllivier B, Caumette P, Garcia JL, Mah RA (March 1994)."Anaerobic bacteria from hypersaline environments".Microbiological Reviews.58(1): 27–38.doi:10.1128/MMBR.58.1.27-38.1994.PMC372951.PMID8177169.
  3. ^abcSantos H, da Costa MS (2002). "Compatible solutes of organisms that live in hot saline environments".Environmental Microbiology.4(9): 501–509.doi:10.1046/j.1462-2920.2002.00335.x.hdl:10316/8134.PMID12220406.
  4. ^Oren A (January 2002). "Diversity of halophilic microorganisms: environments, phylogeny, physiology, and applications".Journal of Industrial Microbiology & Biotechnology.28(1): 56–63.doi:10.1038/sj/jim/7000176.PMID11938472.S2CID24223243.
  5. ^abOren, Aharon (2002)."Molecular ecology of extremely halophilic Archaea and Bacteria".FEMS Microbiology Ecology.39(1): 1–7.doi:10.1111/j.1574-6941.2002.tb00900.x.ISSN0168-6496.PMID19709178.
  6. ^Gutierrez MC, Kamekura M, Holmes ML, Dyall-Smith ML, Ventosa A (December 2002). "Taxonomic characterization of Haloferax sp. (" H. alicantei ") strain Aa 2.2: description of Haloferax lucentensis sp. nov".Extremophiles.6(6): 479–83.doi:10.1007/s00792-002-0282-7.PMID12486456.S2CID24337996.
  7. ^Antón J, Rosselló-Mora R, Rodríguez-Valera F, Amann R (July 2000)."Extremely halophilic bacteria in crystallizer ponds from solar salterns".Applied and Environmental Microbiology.66(7): 3052–3057.doi:10.1128/aem.66.7.3052-3057.2000.PMC92110.PMID10877805.
  8. ^Casamayor EO, Massana R, Benlloch S, Øvreås L, Díez B, Goddard VJ, Gasol JM, Joint I, Rodríguez-Valera F, Pedrós-Alió 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(6): 338–348.doi:10.1046/j.1462-2920.2002.00297.x.PMID12071979.
  9. ^Ali, Ahmed Mohamed; Abdel-Rahman, Tahany M.A.; Farahat, Mohamed G. (2024-03-28)."Bioprospecting of Culturable Halophilic Bacteria Isolated from Mediterranean Solar Saltern for Extracellular Halotolerant Enzymes".Microbiology and Biotechnology Letters.52(1): 76–87.doi:10.48022/mbl.2401.01010.ISSN1598-642X.
  10. ^Antón J, Llobet-Brossa E, Rodríguez-Valera F, Amann R (December 1999). "Fluorescence in situ hybridization analysis of the prokaryotic community inhabiting crystallizer ponds".Environmental Microbiology.1(6): 517–23.doi:10.1046/j.1462-2920.1999.00065.x.PMID11207773.
  11. ^Kastritis PL, Papandreou NC, Hamodrakas SJ (October 2007). "Haloadaptation: Insights from comparative modeling studies of halophilic archaeal DHFRs".International Journal of Biological Macromolecules.41(4): 447–453.doi:10.1016/j.ijbiomac.2007.06.005.PMID17675150.
  12. ^Tirumalai MR, Anane-Bediakoh D, Rajesh S, Fox GE (December 2021)."Net Charges of the Ribosomal Proteins of the S10 and spc Clusters of Halophiles Are Inversely Related to the Degree of Halotolerance".Microbiol Spectr.9(3): e0178221.doi:10.1128/spectrum.01782-21.PMC8672879.PMID34908470.
  13. ^Paul S, Bag SK, Das S, Harvill ET, Dutta C (April 2008)."Molecular signature of hypersaline adaptation: insights from genome and proteome composition of halophilic prokaryotes".Genome Biology.9(4): R70.doi:10.1186/gb-2008-9-4-r70.PMC2643941.PMID18397532.
  14. ^Oren, Aharon (September 2014). "Taxonomy of halophilic Archaea: Current status and future challenges".Extremophiles.18(5): 825–834.doi:10.1007/s00792-014-0654-9.PMID25102811.S2CID5395569.
  15. ^Hogan, C. Michael (5 December 2008). Burnham, A. (ed.)."Makgadikgadi – ancient settlement in Botswana".The Megalithic Portal.— website hosts a collection of fossil and archeological find-site profiles.
  16. ^Zalar P, Sybren de Hoog G, Schroers HJ, Frank JM, Gunde-Cimerman N (May 2005). "Taxonomy and phylogeny of the xerophilic genus Wallemia (Wallemiomycetes and Wallemiales,cl. et ord. nov.) ".Antonie van Leeuwenhoek.87(4): 311–28.doi:10.1007/s10482-004-6783-x.PMID15928984.S2CID4821447.
  17. ^Gostincar C, Grube M, de Hoog S, Zalar P, Gunde-Cimerman N (January 2010)."Extremotolerance in fungi: evolution on the edge".FEMS Microbiology Ecology.71(1): 2–11.doi:10.1111/j.1574-6941.2009.00794.x.PMID19878320.
  18. ^Gajardo GM, Beardmore JA (2012)."The brine shrimp artemia: adapted to critical life conditions".Frontiers in Physiology.3:185.doi:10.3389/fphys.2012.00185.PMC3381296.PMID22737126.
  19. ^de Vos S, Van Stappen G, Vuylsteke M, Rombauts S, Bossier P (2018). "Identification of salt stress response genes using the Artemia transcriptome".Aquaculture.500:305–314.doi:10.1016/j.aquaculture.2018.09.067.S2CID92842322.
  20. ^Mirkena T, Duguma G, Haile A, Tibbo M, Okeyo AM, Wurzinger M, Sölkner J (2010). "Genetics of adaptation in domestic farm animals: A review".Livestock Science.132(1–3): 3.doi:10.1016/j.livsci.2010.05.003.

Further reading

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