https://en.wikipedia.org/wiki/Halophile
Halophiles are organisms that thrive in high salt concentrations. They are a type of extremophile organism. The name comes from the Greek word for "salt-loving". While most halophiles are classified into the Archaea domain, there are also bacterial halophiles and some eukaryota, such as the alga Dunaliella salina or fungus Wallemia ichthyophaga. Some well-known species give off a red color from carotenoid compounds, notably bacteriorhodopsin. 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.
Halophiles are organisms that thrive in high salt concentrations. They are a type of extremophile organism. The name comes from the Greek word for "salt-loving". While most halophiles are classified into the Archaea domain, there are also bacterial halophiles and some eukaryota, such as the alga Dunaliella salina or fungus Wallemia ichthyophaga. Some well-known species give off a red color from carotenoid compounds, notably bacteriorhodopsin. 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.
Classification
Halophiles are categorized as slight, moderate, or extreme, by the extent of their halotolerance.
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.
Lifestyle
High
salinity represents an extreme environment to which relatively few
organisms have been able to adapt and occupy. 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. In the first (which is employed by the majority of halophilic bacteria, some archaea, yeasts, algae and fungi), organic compounds are accumulated 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 the selective influx of potassium (K+) ions into the cytoplasm. This adaptation is restricted to the moderately halophilic bacterial order Halanaerobiales, 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 passed on through massive lateral
gene transfer.
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.
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 deep salterns,
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 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 environments. Indeed, many cells
are so fragile that when placed in distilled water, they immediately lyse from the change in osmotic conditions.
Halophiles may use a variety of energy sources. They can be
aerobic or anaerobic. Anaerobic halophiles include phototrophic,
fermentative, sulfate-reducing, homoacetogenic, and methanogenic
species.
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.
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 16 Svedberg
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, but commonly appears in isolation studies.
Genomic and proteomic signature
The
comparative genomic and proteomic analysis showed distinct molecular
signatures exist 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. The core of these proteins is
less hydrophobic, such as DHFR, that was found to have narrower β-strands.
At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.
Examples
Halobacterium
is a genus of the Archaea that 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 a specific genus of the family Halobacteriaceae.
Some hypersaline lakes are a 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 halobacteria, 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−1
and can withstand strong osmotic shocks thanks 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 fresh water sources on the island and to their only food source is seaweed. They have adapted to handle salt concentrations that would kill other breeds of sheep.
