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 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. 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 . 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.
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 Dead Sea and solar salterns, where they tint the water column and sediments bright colors. In other words, they will most definitely perish if they are exposed to anything besides a very high, intense salt-conditioned environment. These prokaryotes require salt for growth. The high concentration of NaCl 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 environment. Indeed, many cells are so fragile that when placed in distilled water they immediately lyse from the change in osmotic conditions.
Haloarchaea, and particularly, the family Halobacteriaceae are members of the domain Archaea, and comprise the majority of the prokaryotic population in hypersaline environments. There are currently 15 recognised genera 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 have been isolated from saltern crystalliser ponds, including members of the following genera: Haloferax, Halogeometricum, Halococcus, Haloterrigena, Halorubrum, Haloarcula and Halobacterium families (Oren 2002). 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 16S 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.
It has been proposed that halophiles may be representative of life forms that may be present in niche ecologies on other planets. Geoffrey A. Landis of NASA Glenn Research Center, for example, has argued that liquid water, at the low temperature and pressures characteristic of the surface of Mars, is likely to be highly saline, and hence any extant lifeforms will be likely to be similar to terrestrial halophiles. Extremophiles are currently being extensively studied by the astrobiology program both as possible ancient forms of terrestrial life, and hence as clues about the origin and early forms of life, and also as possible analogues for extraterrestrial life.
The comparative genomic and proteomic analysis revealed that there is a distinct molecular signatures 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. At the DNA level, the halophiles exhibit distinct dinucleotide and codon usage.