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== Lifestyle == High salinity represents an extreme environment in which relatively few organisms have been able to adapt and survive. Most halophilic and all [[halotolerance|halotolerant]] organisms expend energy to exclude salt from their [[cytoplasm]] to avoid protein aggregation ('[[salting out]]'). To survive the high salinities, halophiles employ two differing strategies to prevent [[desiccation]] through [[osmosis|osmotic]] movement of water out of their cytoplasm. Both strategies work by increasing the internal [[osmolarity]] of the cell. The first strategy is employed by some archaea, the majority of halophilic bacteria, [[yeast]]s, [[algae]], and [[fungi]]; the organism accumulates [[organic compounds]] in the cytoplasm—[[osmoprotectant]]s which are known as compatible solutes. These can be either synthesised or accumulated from the environment.<ref name="Santos">{{cite journal | vauthors = Santos H, da Costa MS | year = 2002 | title = Compatible solutes of organisms that live in hot saline environments | journal = Environmental Microbiology | volume = 4 | issue = 9| pages = 501–509 | doi=10.1046/j.1462-2920.2002.00335.x| pmid = 12220406 | hdl = 10316/8134 | hdl-access = free }}</ref> The most common compatible solutes are [[pH|neutral]] or [[zwitterion]]ic, and include [[amino acid]]s, [[sugar]]s, [[polyol]]s, [[betaine]]s, and [[ectoine]]s, as well as derivatives of some of these compounds. The second, more radical adaptation involves selectively absorbing [[potassium]] (K<sup>+</sup>) ions into the cytoplasm. This adaptation is restricted to the extremely halophilic archaeal family ''[[Halobacteriaceae]]'', the moderately halophilic bacterial order ''[[Halanaerobiales]]'', and 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 passed on through massive lateral gene transfer.<ref name=Santos/> 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.<ref name=Santos/> 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% [[Mass concentration (chemistry)#Usage in biology|w/v]] salts). These are the primary inhabitants of salt lakes, inland seas, and evaporating ponds of seawater, such as the deep [[saltern]]s, 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 charged [[amino acid]]s on their surfaces, allowing the retention of water molecules around these components. They are [[heterotroph]]s that 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 immediately [[cytolysis|lyse]] from 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.<ref name=Ollivier/><ref>{{cite journal | vauthors = Oren A | date = January 2002 | title = Diversity of halophilic microorganisms: environments, phylogeny, physiology, and applications | journal = Journal of Industrial Microbiology & Biotechnology | volume = 28 | issue = 1 | pages = 56–63 | pmid = 11938472 | doi = 10.1038/sj/jim/7000176 | s2cid = 24223243 }}</ref> The Haloarchaea, and particularly the family Halobacteriaceae, are members of the domain ''[[Archaea]]'', and comprise the majority of the prokaryotic population in [[hypersaline lake|hypersaline environments]].<ref name="Oren">{{cite journal |last1=Oren |first1=Aharon |year=2002 |title=Molecular ecology of extremely halophilic Archaea and Bacteria |journal=FEMS Microbiology Ecology |volume=39 |issue=1 |pages=1–7 |issn=0168-6496 |doi=10.1111/j.1574-6941.2002.tb00900.x |pmid=19709178 |doi-access=free}}</ref> Currently, 15 recognised genera are in the family.<ref>{{cite journal | vauthors = Gutierrez MC, Kamekura M, Holmes ML, Dyall-Smith ML, Ventosa A | date = December 2002 | title = Taxonomic characterization of Haloferax sp. (" H. alicantei") strain Aa 2.2: description of Haloferax lucentensis sp. nov | journal = Extremophiles | volume = 6 | issue = 6 | pages = 479–83 | pmid = 12486456 | doi = 10.1007/s00792-002-0282-7 | s2cid = 24337996 }}</ref> 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.<ref>{{cite journal | vauthors = Antón J, Rosselló-Mora R, Rodríguez-Valera F, Amann R | date = July 2000 | title = Extremely halophilic bacteria in crystallizer ponds from solar salterns | journal = Applied and Environmental Microbiology | volume = 66 | issue = 7 | pages = 3052–3057 | pmid = 10877805 | pmc = 92110 | doi = 10.1128/aem.66.7.3052-3057.2000 }}</ref> At times, the alga ''[[Dunaliella salina]]'' can also proliferate in this environment.<ref>{{cite journal | vauthors =Casamayor EO, Massana R, Benlloch S, Øvreås L, Díez B, Goddard VJ, Gasol JM, Joint I, Rodríguez-Valera F, Pedrós-Alió C | year = 2002 | title = Changes in archaeal, bacterial and eukaryal assemblages along a salinity gradient by comparison of genetic fingerprinting methods in a multipond solar saltern | journal = Environmental Microbiology | volume = 4 | issue = 6 | pages = 338–348 | doi=10.1046/j.1462-2920.2002.00297.x | pmid = 12071979 }}</ref> A comparatively wide range of taxa has been isolated from saltern crystalliser ponds, including members of these genera: ''Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula'', and ''Halobacterium''.<ref name=Oren /> 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 [[Polymerase chain reaction|PCR]]-based strategies that target 16[[Svedberg|S]] small subunit ribosomal ribonucleic acid (16S rRNA) genes.<ref>{{Cite journal |last=Ali |first=Ahmed Mohamed |last2=Abdel-Rahman |first2=Tahany M.A. |last3=Farahat |first3=Mohamed G. |date=2024-03-28 |title=Bioprospecting of Culturable Halophilic Bacteria Isolated from Mediterranean Solar Saltern for Extracellular Halotolerant Enzymes |url=http://www.mbl.or.kr/journal/view.html?doi=10.48022/mbl.2401.01010 |journal=Microbiology and Biotechnology Letters |language=en |volume=52 |issue=1 |pages=76–87 |doi=10.48022/mbl.2401.01010 |issn=1598-642X}}</ref> 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,<ref name="pmid11207773">{{cite journal | vauthors = Antón J, Llobet-Brossa E, Rodríguez-Valera F, Amann R | date = December 1999 | title = Fluorescence in situ hybridization analysis of the prokaryotic community inhabiting crystallizer ponds | journal = Environmental Microbiology | volume = 1 | issue = 6 | pages = 517–23 | pmid = 11207773 | doi = 10.1046/j.1462-2920.1999.00065.x }}</ref> but commonly appears in isolation studies.
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