Halophiles are salty little bastards. Some people practically ingest olives and anchovies and Lays Classic potato chips, but if they think they know something about loving salt, put them in a room with a halophile and they'll be quick to realize that they know nothing about really loving their sodium. The Dead Sea, the Great Salt Lake, natural and man-made salturns (places where pools of salt water have collected and the water has evaporated leaving salt), these are extremely salty environments in which one would think it difficult if not impossible to live. But halophiles thrive in such places: halobacteria need an environment of at least 18 percent sodium chloride in order to grow their best; they flourish in saturated solutions and even on salt crystals themselves. To these crazy little guys, regular salt water, not to mention fresh water lakes, would be lethal.
Sodium sucks the life-giving water out of cells by the process of osmosis, leaving them dried and withered to nothingness. In order to live in such salty environments, halophiles have therefore developed internal mechanisms to counter the pull of osmosis. The algae Dunaliella replaces its natural sodium ions with potassium ions, this, in turn, signals its photosynthesis to stop making starch and instead engage in the production of glycerol. Glycerol is a "water-soluble, non-ionic molecule which counterbalances the dehydrating effect of the external salt" (Postgate, The Outer Reaches of Life, 46). It basically balances the concentration of water inside and outside of the cell so that osmotic pressure is eased and water doesn't flow out of the cellular membrane. In Halobacteria, cells protect themselves from osmosis (in which salts outside of the cell suck water out of the cell) by keeping even higher concentrations of sodium within their cellular cytoplasm than exist outside of it. This, however, merely shifts the problem from the cellular to the molecular level, "as within the cell the salt will compete with proteins and other biomolecules for the essential solvent, water" (Gross, Life on the Edge, 75). If confronted with this problem, cells of any sort other than those of adapted halophiles would simply witness the aggregation of their proteins into deadening lumps. Within the cells of halophiles, however, proteins are coated in chains of acidic amino acids such that each carries a singular negative charge. Since negatives repel negatives, proteins are unable to aggregate together and the problem is solved. Thus halophiles have the unique ability to keep their cells moist and fluid in the face of extreme osmotic pressure from their hyper-saline environments.
Check out these halophiles and others at a Palomar extremophile website.
There's more extremophiles, right?