Ahalophile (from the Greek word for 'salt-loving') is anextremophile that thrives in highsalt concentrations. In chemical terms, halophile refers to aLewis acidic species that has some ability to extract halides from other chemical species.
While most halophiles are classified into the domainArchaea, there are alsobacterial halophiles and someeukaryotic species, such as thealgaDunaliella salina andfungusWallemia ichthyophaga. Some well-known species give off a red color from carotenoid compounds, notablybacteriorhodopsin.
Halophiles can be found in water bodies with salt concentration more than five times greater than that of the ocean, such as theGreat Salt Lake in Utah,Owens Lake in California, theLake Urmia in Iran, theDead Sea, and inevaporation ponds. They are theorized to be a possible analogues for modeling extremophiles that might live in the salty subsurface water ocean of Jupiter'sEuropa and similar moons.[1]
Halophiles are categorized by the extent of theirhalotolerance: slight, moderate, or extreme. Slight halophiles prefer 0.3 to 0.8M (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.[2] Halophiles requiresodium 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 allhalotolerant organisms expend energy to exclude salt from theircytoplasm to avoid protein aggregation ('salting out'). To survive the high salinities, halophiles employ two differing strategies to preventdesiccation throughosmotic movement of water out of their cytoplasm. Both strategies work by increasing the internalosmolarity of the cell. The first strategy is employed by some archaea, the majority of halophilic bacteria,yeasts,algae, andfungi; the organism accumulatesorganic compounds in the cytoplasm—osmoprotectants which are known as compatible solutes. These can be either synthesised or accumulated from the environment.[3] The most common compatible solutes areneutral orzwitterionic, and includeamino acids,sugars,polyols,betaines, andectoines, as well as derivatives of some of these compounds.
The second, more radical adaptation involves selectively absorbingpotassium (K+) ions into the cytoplasm. This adaptation is restricted to the extremely halophilic archaeal familyHalobacteriaceae, the moderately halophilic bacterial orderHalanaerobiales, and the extremely halophilic bacteriumSalinibacter ruber. The presence of this adaptation in three distinct evolutionary lineages suggestsconvergent 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.[3] 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.[3]
Of particular note are the extreme halophiles orhaloarchaea (often known ashalobacteria), 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 deepsalterns, 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 chargedamino acids on their surfaces, allowing the retention of water molecules around these components. They areheterotrophs 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 immediatelylyse 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.[2][4]
The Haloarchaea, and particularly the family Halobacteriaceae, are members of the domainArchaea, and comprise the majority of the prokaryotic population inhypersaline environments.[5] Currently, 15 recognised genera are in the family.[6] The domainBacteria (mainlySalinibacter ruber) can comprise up to 25% of the prokaryotic community, but is more commonly a much lower percentage of the overall population.[7] At times, the algaDunaliella salina can also proliferate in this environment.[8]
A comparatively wide range of taxa has been isolated from saltern crystalliser ponds, including members of these genera:Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula, andHalobacterium.[5] 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 usingPCR-based strategies that target 16S small subunit ribosomal ribonucleic acid (16S rRNA) genes.[9] 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 thein situ community. This is seen in cases such as the genusHaloarcula, which is estimated to make up less than 0.1% of the in situ community,[10] 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 asDHFR, that was found to have narrower β-strands.[11]In one study, the net charges (at pH 7.4) of the ribosomal proteins (r-proteins) that comprise theS10-spc cluster were observed to have an inverse relationship with the halophilicity/halotolerance levels in both bacteria and archaea.[12] At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.[13]
Halobacteriaceae is a family that includes a large part of halophilic archaea.[14] The genusHalobacterium 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.
Somehypersaline lakes are habitat to numerous families of halophiles. For example, theMakgadikgadi Pans inBotswana form a vast, seasonal, high-salinity water body that manifests halophilic species within thediatom genusNitzschia in the familyBacillariaceae, as well as species within the genusLovenula in the familyDiaptomidae.[15] Owens Lake in California also contains a large population of the halophilic bacteriumHalobacterium halobium.
Wallemia ichthyophaga is abasidiomycetousfungus, which requires at least 1.5 Msodium chloride forin vitro growth, and it thrives even in media saturated with salt.[16] Obligate requirement for salt is an exception in fungi. Even species that can tolerate salt concentrations close to saturation (for exampleHortaea werneckii) in almost all cases grow well in standard microbiological media without the addition of salt.[17]
The fermentation of salty foods (such assoy sauce,Chinese fermented beans,salted cod, saltedanchovies,sauerkraut, etc.) often involves halophiles as either essential ingredients or accidental contaminants. One example isChromohalobacter beijerinckii, found in salted beans preserved in brine and in saltedherring.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)[18][19] 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 fromOrkney, Scotland. They have limited access to freshwater sources on the island and their only food source isseaweed. They have adapted to handle salt concentrations that would kill other breeds of sheep.[20]
