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Salinity

	Salinity- A wide range of salinity occurs in natural waters. For example, salinity is near zero in fresh water and almost at saturation level in salt lakes. A clear distinction can be made between the flora and fauna of fresh and salt water systems due to the variation in the salinity levels. The dissolved salt concentration of sea water varies from 33 to 37g per kg of water.
The maximum level of salinity has been noted in Red sea where it is 44 parts per thousand. Thus sea water characteristically contains a high salt content. Even within ocean variations may occur in salinity. Concentrations of salts are normally low in shallow offshore regions and near river mouths. Further rise in salinity in a water body may be due to the evaporation of water or ice formation. Decrease in salinity is also possible by the inflow of rain water or snow precipitation.
The most common salts responsible for the salinity are chlorides, sulfates and carbonates of sodium, potassium, calcium and magnesium. Most microorganisms inhabiting the sea are capable of growing at a high concentration of salt. These organisms are said to be halophilic. In contrast to the microorganisms from lakes and rivers, which can't grow at a salt concentration of more than 1 %, these marine halophiles are capable of growing best at 2.5 to 4.0% salt concentration. Examples of marine halophiles are Pseudomonas marina and Alteromonas haloplanktis. Another group, halotolerant, includes microorganisms which can tolerate a high salinity but grow better at low levels.

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