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The halophiles, named after the Greek word for "salt-loving", are extremophiles that thrive in high
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...
concentrations. While most halophiles are classified into the domain
Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...
, there are also
bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
l halophiles and some
eukaryotic Eukaryotes () are organisms whose cells have a nucleus. All animals, plants, fungi, and many unicellular organisms, are Eukaryotes. They belong to the group of organisms Eukaryota or Eukarya, which is one of the three domains of life. Bacte ...
species, such as the
alga Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
'' Dunaliella salina'' and
fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
'' Wallemia ichthyophaga''. Some well-known species give off a red color from carotenoid compounds, notably
bacteriorhodopsin Bacteriorhodopsin is a protein used by Archaea, most notably by haloarchaea, a class of the Euryarchaeota. It acts as a proton pump; that is, it captures light energy and uses it to move protons across the membrane out of the cell. The resulting ...
. 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 The Great Salt Lake is the largest saltwater lake in the Western Hemisphere and the eighth-largest terminal lake in the world. It lies in the northern part of the U.S. state of Utah and has a substantial impact upon the local climate, particu ...
in Utah, Owens Lake in California, the Urmia Lake in Iran, the
Dead Sea The Dead Sea ( he, יַם הַמֶּלַח, ''Yam hamMelaḥ''; ar, اَلْبَحْرُ الْمَيْتُ, ''Āl-Baḥrū l-Maytū''), also known by other names, is a salt lake bordered by Jordan to the east and Israel and the West Ban ...
, and in
evaporation pond Evaporation ponds are artificial ponds with very large surface areas that are designed to efficiently evaporate water by sunlight and expose water to the ambient temperatures. Evaporation ponds are inexpensive to design making it ideal for multip ...
s. They are theorized to be a possible analogues for modeling extremophiles that might live in the salty subsurface water ocean of Jupiter's
Europa Europa may refer to: Places * Europe * Europa (Roman province), a province within the Diocese of Thrace * Europa (Seville Metro), Seville, Spain; a station on the Seville Metro * Europa City, Paris, France; a planned development * Europa Clif ...
and similar moons.


Classification

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.


Lifestyle

High salinity represents an extreme environment in which relatively few organisms have been able to adapt and survive. Most halophilic and all
halotolerant Halotolerance is the adaptation of living organisms to conditions of high salinity. Halotolerant species tend to live in areas such as hypersaline lakes, coastal dunes, saline deserts, salt marshes, and inland salt seas and springs. Halophiles are ...
organisms expend energy to exclude salt from their
cytoplasm In cell biology, the cytoplasm is all of the material within a eukaryotic cell, enclosed by the cell membrane, except for the cell nucleus. The material inside the nucleus and contained within the nuclear membrane is termed the nucleoplasm. ...
to avoid protein aggregation ('
salting out Salting out (also known as salt-induced precipitation, salt fractionation, anti-solvent crystallization, precipitation crystallization, or drowning out) is a purification technique that utilizes the reduced solubility of certain molecules in a s ...
'). To survive the high salinities, halophiles employ two differing strategies to prevent
desiccation Desiccation () is the state of extreme dryness, or the process of extreme drying. A desiccant is a hygroscopic (attracts and holds water) substance that induces or sustains such a state in its local vicinity in a moderately sealed container. ...
through
osmotic Osmosis (, ) is the spontaneous net movement or diffusion of solvent molecules through a selectively-permeable membrane from a region of high water potential (region of lower solute concentration) to a region of low water potential (region ...
movement of water out of their cytoplasm. Both strategies work by increasing the internal
osmolarity Osmotic concentration, formerly known as osmolarity, is the measure of solute concentration, defined as the number of osmoles (Osm) of solute per litre (L) of solution (osmol/L or Osm/L). The osmolarity of a solution is usually expressed as Osm/L ...
of the cell. The first strategy is employed by some archaea, the majority of halophilic bacteria,
yeast Yeasts are eukaryotic, single-celled microorganisms classified as members of the fungus kingdom. The first yeast originated hundreds of millions of years ago, and at least 1,500 species are currently recognized. They are estimated to constit ...
s,
algae Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular micr ...
, and
fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
; the organism accumulates
organic compounds In chemistry, organic compounds are generally any chemical compounds that contain carbon-hydrogen or carbon-carbon bonds. Due to carbon's ability to catenate (form chains with other carbon atoms), millions of organic compounds are known. The s ...
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
zwitterion In chemistry, a zwitterion ( ; ), also called an inner salt or dipolar ion, is a molecule that contains an equal number of positively- and negatively-charged functional groups. : With amino acids, for example, in solution a chemical equilibrium wil ...
ic, and include
amino acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s,
sugar Sugar is the generic name for sweet-tasting, soluble carbohydrates, many of which are used in food. Simple sugars, also called monosaccharides, include glucose, fructose, and galactose. Compound sugars, also called disaccharides or do ...
s,
polyol In organic chemistry, a polyol is an organic compound containing multiple hydroxyl groups (). The term "polyol" can have slightly different meanings depending on whether it is used in food science or polymer chemistry. Polyols containing two, thr ...
s, betaines, and ectoines, as well as derivatives of some of these compounds. The second, more radical adaptation involves selectively absorbing
potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmos ...
(K+) ions into the cytoplasm. This adaptation is restricted to the extremely halophilic archaeal family ''
Halobacteriaceae In taxonomy, the Halobacteriaceae are a family of the Halobacteriales in the domain Archaea.See the NCBIbr>webpage on Halobacteriaceae Data extracted from the ''Halobacteriaceae'' represent a large part of halophilic Archaea, along with mem ...
'', the moderately halophilic bacterial order '' Halanaerobiales'', and the extremely halophilic bacterium ''
Salinibacter ruber ''Salinibacter ruber'' is an extremely halophilic red bacterium, first found in Spain in 2002. Habitat ''Salinibacter ruber'' was found in saltern crystallizer ponds in Alicante and Mallorca, Spain in 2002 by Antón et al. This environment h ...
''. The presence of this adaptation in three distinct evolutionary lineages suggests
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last com ...
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 Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. Halobacteria are now recognized as archaea rather than bacteria and are one of t ...
(often known as
halobacteria Haloarchaea (halophilic archaea, halophilic archaebacteria, halobacteria) are a class of the Euryarchaeota, found in water saturated or nearly saturated with salt. Halobacteria are now recognized as archaea rather than bacteria and are one of ...
), 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 acid Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha ...
s on their surfaces, allowing the retention of water molecules around these components. They are
heterotroph A heterotroph (; ) is an organism that cannot produce its own food, instead taking nutrition from other sources of organic carbon, mainly plant or animal matter. In the food chain, heterotrophs are primary, secondary and tertiary consumers, but ...
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 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. The Haloarchaea, and particularly the family Halobacteriaceae, are members of the domain ''
Archaea Archaea ( ; singular archaeon ) is a domain of single-celled organisms. These microorganisms lack cell nuclei and are therefore prokaryotes. Archaea were initially classified as bacteria, receiving the name archaebacteria (in the Archaeba ...
'', and comprise the majority of the prokaryotic population in hypersaline environments. Currently, 15 recognised genera are in the family. The domain
Bacteria Bacteria (; singular: bacterium) are ubiquitous, mostly free-living organisms often consisting of one biological cell. They constitute a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria were am ...
(mainly ''
Salinibacter ruber ''Salinibacter ruber'' is an extremely halophilic red bacterium, first found in Spain in 2002. Habitat ''Salinibacter ruber'' was found in saltern crystallizer ponds in Alicante and Mallorca, Spain in 2002 by Antón et al. This environment h ...
'') 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 S 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 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. At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.


Examples

''
Halobacteriaceae In taxonomy, the Halobacteriaceae are a family of the Halobacteriales in the domain Archaea.See the NCBIbr>webpage on Halobacteriaceae Data extracted from the ''Halobacteriaceae'' represent a large part of halophilic Archaea, along with mem ...
'' is a family that includes a large part of halophilic archaea. The genus ''
Halobacterium ''Halobacterium'' (common abbreviation ''Hbt.'') is a genus in the family Halobacteriaceae. The genus ''Halobacterium'' ("salt" or "ocean bacterium") consists of several species of Archaea with an aerobic metabolism which requires an environmen ...
'' 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 ''Halococcus'' (common abbreviation: ''Hcc.'') is a genus of the Halococcaceae. Ecology ''Halococcus'' is a genus of extreme halophilic archaea, meaning that they require high salt levels, sometimes as high as 32% NaCl, for optimal growth. H ...
'' is another genus of the family Halobacteriaceae. Some
hypersaline lake A hypersaline lake is a landlocked body of water that contains significant concentrations of sodium chloride, brines, and other salts, with saline levels surpassing that of ocean water (3.5%, i.e. ). Specific microbial species can thrive in ...
s are habitat to numerous families of halophiles. For example, the Makgadikgadi Pans in
Botswana Botswana (, ), officially the Republic of Botswana ( tn, Lefatshe la Botswana, label= Setswana, ), is a landlocked country in Southern Africa. Botswana is topographically flat, with approximately 70 percent of its territory being the Kal ...
form a vast, seasonal, high-salinity water body that manifests halophilic species within the
diatom A diatom ( Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising se ...
genus '' Nitzschia'' in the family
Bacillariaceae Bacillariaceae is a family of diatoms in the phylum Heterokontophyta, the only family in the order Bacillariales. Some species of genera such as '' Nitzchia'' are found in halophilic environments; for example, in the seasonally flooded Makga ...
, as well as species within the genus '' Lovenula'' in the family
Diaptomidae Diaptomidae is a family of freshwater pelagic copepods. It includes around 50 genera: *'' Acanthodiaptomus'' Kiefer, 1932 *'' Aglaodiaptomus'' Light, 1938 *'' Allodiaptomus'' Kiefer, 1936 *'' Arctodiaptomus'' Kiefer, 1932 *'' Argyrodiaptomus'' B ...
. Owens Lake in California also contains a large population of the halophilic bacterium ''Halobacterium halobium''. '' Wallemia ichthyophaga'' is a basidiomycetous
fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...
, which requires at least 1.5 M
sodium chloride Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35. ...
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 ''Hortaea werneckii'' is a species of yeast in the family Teratosphaeriaceae. It is a black yeast that is investigated for its remarkable halotolerance. While the addition of salt to the medium is not required for its cultivation, ''H. 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 Soy sauce (also called simply soy in American English and soya sauce in British English) is a liquid condiment of Chinese origin, traditionally made from a fermented paste of soybeans, roasted grain, brine, and ''Aspergillus oryzae'' or '' As ...
, Chinese fermented beans,
salted cod Dried and salted cod, sometimes referred to as salt cod or saltfish or salt dolly, is cod which has been preserved by drying after salting. Cod which has been dried without the addition of salt is stockfish. Salt cod was long a major export of ...
, salted
anchovies An anchovy is a small, common forage fish of the family Engraulidae. Most species are found in marine waters, but several will enter brackish water, and some in South America are restricted to fresh water. More than 140 species are placed in 1 ...
,
sauerkraut Sauerkraut (; , "sour cabbage") is finely cut raw cabbage that has been fermented by various lactic acid bacteria. It has a long shelf life and a distinctive sour flavor, both of which result from the lactic acid formed when the bacteria ...
, 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 Herring are forage fish, mostly belonging to the family of Clupeidae. Herring often move in large schools around fishing banks and near the coast, found particularly in shallow, temperate waters of the North Pacific and North Atlantic Ocean ...
. '' 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 Seaweed, or macroalgae, refers to thousands of species of macroscopic, multicellular, marine algae. The term includes some types of '' Rhodophyta'' (red), ''Phaeophyta'' (brown) and '' Chlorophyta'' (green) macroalgae. Seaweed species such as ...
. They have adapted to handle salt concentrations that would kill other breeds of sheep.


See also

*
Arid Forest Research Institute Arid Forest Research Institute (AFRI) is a research institute situated in Jodhpur, Rajasthan, India. The institute conducts scientific research in forestry in order to provide technologies to increase the vegetative cover and to conserve biod ...
*
Biosalinity Biosalinity is the study and practice of using saline (salty) water for irrigating agricultural crops. Many arid and semi-arid areas actually do have sources of water, but the available water is usually brackish (0.5–5g/L salt) or saline (30� ...
* Halotolerance


References


Further reading

* * * * * * * * * * * *


External links


HaloArchaea.com


- Kenneth Todar
Astrobiology: extremophiles- life in extreme environments
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