Halotolerance is theadaptation of livingorganisms to conditions of highsalinity.[1] Halotolerant species tend to live in areas such ashypersaline lakes,coastal dunes, salinedeserts,salt marshes, and inland saltseas andsprings.Halophiles are also organisms that not only live in highly saline environments but alsorequire thesalinity to survive. Halotolerant organisms on the other hand (belonging to differentdomains of life) can grow under saline conditions, but do not require elevated concentrations of salt for growth.Halophytes are salt-tolerant higher plants. Halotolerant microorganisms are of considerable biotechnological interest.[2]
Fields of scientific research relevant to halotolerance includebiochemistry,molecular biology,cell biology,physiology,ecology, andgenetics.
An understanding of halotolerance can be applicable to areas such asarid-zone agriculture,xeriscaping,aquaculture (of fish or algae), bioproduction of desirable compounds (such asphycobiliproteins orcarotenoids) using seawater to support growth, orremediation of salt-affected soils. In addition, many environmental stressors involve or induce osmotic changes, so knowledge gained about halotolerance can also be relevant to understanding tolerance to extremes in moisture or temperature.
Goals of studying halotolerance include increasing the agricultural productivity of lands affected bysoil salination or where only saline water is available. Conventional agricultural species could be made more halotolerant by gene transfer from naturally halotolerant species (by conventionalbreeding orgenetic engineering) or by applying treatments developed from an understanding of the mechanisms of halotolerance. In addition, naturally halotolerant plants or microorganisms could be developed into usefulagricultural crops orfermentation organisms.
Tolerance of high salt conditions can be obtained through several routes. High levels of salt entering the plant can trigger ionic imbalances which cause complications in respiration and photosynthesis, leading to reduced rates of growth, injury and death in severe cases. To be considered tolerant of saline conditions, theprotoplast must show methods of balancing the toxic andosmotic effects of the increased salt concentrations. Halophytic vascular plants can survive on soils with salt concentrations around 6%, or up to 20% in extreme cases (ocean salinity is around 3.5%). Tolerance of such conditions is reached through the use ofstress proteins and compatible cytoplasm osmotic solutes.[3]
To exist in such conditions, halophytes tend to be subject to the uptake of high levels of salt into their cells, and this is often required to maintain an osmotic potential lower than that of the soil to ensure water uptake. High salt concentrations within the cell can be damaging to sensitive organelles such as the chloroplast, so sequestration of salt is seen. Under this action, salt is stored within thevacuole to protect such delicate areas. If high salt concentrations are seen within the vacuole, a high concentration gradient will be established between the vacuole and the cytoplasm, leading to high levels of energy investment to maintain this state. Therefore, the accumulation of compatible cytoplasmic osmotic solutes can be seen to prevent this situation from occurring.Amino acids such as proline accumulate in halophyticBrassica species, quaternary ammonium bases such as Glycine Betaine and sugars have been shown to act in this role within halophytic members ofChenopodiaceae and members ofAsteraceae show the buildup of cyclites and soluble sugars. The buildup of these compounds allow for the balancing of the osmotic effect while preventing the establishment of toxic concentrations of salt or requiring the maintenance of high concentration gradients.[citation needed]
The extent of halotolerance varies widely amongst different species of bacteria.[4] A number ofcyanobacteria are halotolerant; an example location of occurrence for such cyanobacteria is in theMakgadikgadi Pans, a largehypersaline lake inBotswana.[5]
Fungi from habitats with high concentration of salt are mostly halotolerant (i.e. they do not require salt for growth) and not halophilic. Halophilic fungi are a rare exception.[6] Halotolerant fungi constitute a relatively large and constant part of hypersaline environment communities, such as those in thesolar salterns.[7] Well studied examples include the yeastDebaryomyces hanseniiandblack yeastsAureobasidium pullulans andHortaea werneckii.[8] The latter can grow in media without salt, as well as in almost saturatedNaCl solutions. To emphasize this unusually wideadaptability, some authors describeH. werneckiias "extremely halotolerant".[9]
