Halophilic and Halotolerant Micro-Organisms from Soils

Abstract

Hypersaline environments are extreme habitats in which several other factors, in addition to high salt content, may limit the growth of organisms. These additional factors include temperature, pH, pressure, oxygen, nutrient availability, and solar radiations (Rodriguez-Valera 1988). Hypersaline environments comprise hypersaline waters and soils . Hypersaline waters are defined as those environments that have higher concentrations of salts than seawater (Rodriguez-Valera 1988). However, depending on their origin, the salt composition may differ from that of seawater and on that basis hypersaline water habitats are categorized as thalassohaline , when the relative amounts of the different inorganic salts are approximately equal to those present in seawater, or as athalassohaline , if the proportions of the different salts are markedly different from those of seawater. The later environments are more heterogeneous and may have very different origins. Examples of thalassohaline water habitats, which are typically chroride types, are the Great Salt Lake or the solar salterns used for the industrial production of marine salt by evaporation of seawater; among the athalassohaline waters are the Dead Sea, the Wadi Natrun, Lake Magadi, and several other soda lakes. In contrast to the hypersaline waters, the hypersaline soils are not well defined and in fact there is no clear definition of a saline or hypersaline soil. They are widely represented in our planet. Because most soils contain small amounts of soluble salts, a soil would be considered as hypersaline when its salt concentration is above a certain threshold (Rodriguez-Valera 1988). According to Kaurichev (1980), soils containing more than 0.2% (w/v) soluble salt should be considered as saline soils. Micro-organisms show quite different responses to salt . According to the particular salt concentration required for their optimal growth, several physiological groups of micro-organisms are considered : (i) nonhalophiles require less than 1%