Generally speaking, freshwater species are routinely found in waters with <1000mgl-1 TDS (1%; 1600 mS cm-1), while few are found at concentrations greater than this. At low salt concentrations, organisms tend to show flexibility in their adaptation to various ion combinations. In saline lakes across Western Canada, for example, the rotifer Branchionus plicatilis and the harpactacoid copepod Cletocamp-tus spp., tend to prevail in lakes dominated by Cl while Leptodiaptomus sicilis and Diaptomus neva-densis dominate in SO4 and CO3 waters. Such flexibility, however, disappears as salinity increases. This is due to the fact that high salinity exerts an osmoregu-latory stress on endemic organisms. The term osmo-regulation, or osmotic regulation, refers to the regulation of the internal cell environment relative to the concentration in the external environment, across a semi-permeable membrane (cell membrane). Organisms adapted to life in salty environments (halophiles) do this by accumulating organic solutes like glycerol, sucrose and glycine within their cells. A great deal of energy is consequently expended to maintain properly functioning tissues. If cells did not do this, water would move across the cell membrane to the surrounding saline water (osmosis) in an attempt to equalize the salt concentration and cells would ultimately die. Because salt tolerant organisms expend a great deal of energy in osmoregulation, those halophiles that can produce energy in other ways (e.g., through photosynthesis) will tend to be favored in saline lakes. For example, the green alga Dunaliella spp. is present world wide as the main or sole primary producer in hypersaline environments.
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