Halophile

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Halophiles are extremophile organisms that thrive in environments with very high concentrations of salt. The name comes from the Greek for "salt-loving". While the term is perhaps most often applied to some halophiles classified into the Archaea domain, 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 five times greater than the salt concentration of the ocean, such as the Great Salt Lake in Utah, Owens Lake in California, the Dead Sea, and in evaporation ponds.

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Life-style

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 energy to exclude salt from their cytoplasm to avoid protein aggregation (‘salting out’). In order to survive the high salinities, halophiles employ two differing strategies to prevent desiccation through osmotic movement of water out of their cytoplasm. Both strategies work by increasing the internal osmolarity of the cell. In the first (that is employed by the majority of bacteria, some archaea, yeasts, algae and fungi), organic compounds are accumulated in the cytoplasm – these osmoprotectants are known as compatible solutes. These can be synthesised or accumulated from the environment.[1] The most common compatible solutes are neutral or zwitterionic and 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 Halobacteriaceae 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 through massive lateral gene transfer[1] . 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.[1]

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