Галофилы: Жизнь в условиях высокой солёности

Organisms that live in high salt concentrations
A halophile (from the Greek word for 'salt loving') is an extremophile that thrives in high salt concentrations. In chemical terms, halophile refers to a Lewis acidic species that has some ability to extract halides from other chemical species. While most halophiles are classified into the domain Archaea, there are also bacterial halophiles and some eukaryotic species, such as the alga Dunaliella salina and fungus Wallemia ichthyophaga. Some well known species give off a red color from carotenoid compounds, notably bacteriorhodopsin. Halophiles can be found in water bodies with salt concentration more than five times greater than that of the ocean, such as the Great Salt Lake in Utah, Owens Lake in California, the Lake Urmia in Iran, the Dead Sea, and in evaporation ponds. They are theorized to be a possible analogues for modeling extremophiles that might live in the salty subsurface water ocean of Jupiter's Europa and similar moons.

Halophiles are categorized by the extent of their halotolerance: slight, moderate, or extreme. Slight halophiles prefer 0.3 to 0.8 M (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. Halophiles require sodium chloride (salt) for growth, in contrast to halotolerant organisms, which do not require salt but can grow under saline conditions.

High salinity represents an extreme environment in which relatively few organisms have been able to adapt and survive. Most halophilic and all 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 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, yeasts, algae, and fungi; the organism accumulates organic compounds in the cytoplasm—osmoprotectants which are known as compatible solutes. These can be either synthesised or accumulated from the environment. 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 selectively absorbing potassium (K+) 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. The Haloarchaea, and particularly the family Halobacteriaceae, are members of the domain Archaea, and comprise the majority of the prokaryotic population in hypersaline environments. Currently, 15 recognised genera are in the family. 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. At times, the alga Dunaliella salina can also proliferate in this environment. 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. but commonly appears in isolation studies.

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 as DHFR, that was found to have narrower β strands. In one study, the net charges (at pH 7.4) of the ribosomal proteins (r proteins) that comprise the S10 spc cluster were observed to have an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea. At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.

Halobacteriaceae is a family that includes a large part of halophilic archaea. The genus Halobacterium under it has a high tolerance for elevated levels of salinity. Some species of halobacteria have acidic proteins that resist the denaturing effects of salts. Halococcus is another genus of the family Halobacteriaceae. Some hypersaline lakes are habitat to numerous families of halophiles. For example, the Makgadikgadi Pans in Botswana form a vast, seasonal, high salinity water body that manifests halophilic species within the diatom genus Nitzschia in the family Bacillariaceae, as well as species within the genus Lovenula in the family Diaptomidae. Owens Lake in California also contains a large population of the halophilic bacterium Halobacterium halobium. Wallemia ichthyophaga is a basidiomycetous fungus, which requires at least 1.5 M sodium chloride for in vitro growth, and it thrives even in media saturated with salt. Obligate requirement for salt is an exception in fungi. Even species that can tolerate salt concentrations close to saturation (for example Hortaea werneckii) in almost all cases grow well in standard microbiological media without the addition of salt. The fermentation of salty foods (such as soy sauce, Chinese fermented beans, salted cod, salted anchovies, sauerkraut, etc.) often involves halophiles as either essential ingredients or accidental contaminants. One example is Chromohalobacter beijerinckii, found in salted beans preserved in brine and in salted herring. Tetragenococcus halophilus is found in salted anchovies and soy sauce. Artemia is 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) 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 sheep are a breed of sheep originating from Orkney, Scotland. They have limited access to freshwater sources on the island and their only food source is seaweed. They have adapted to handle salt concentrations that would kill other breeds of sheep.