Pass the salt

You may like plenty of salt on your food, even though you realise that too much salt is bad for you. In fact, for most organisms, too much salt is fatal. High salt concentrations distort the structure of proteins causing them to stick together so they can no longer remain in solution. This destroys their biological function. Organisms may exclude the extra salt because their membranes prevent the salts from entering their cells. The problem then becomes one of the acquisition and retention of water.

A high concentration of salts in its environment means that the organism is in danger of losing water by osmosis (see Chapter 1 for an explanation of osmosis). Under these circumstances, organisms also find it difficult to gain the water they need from the environment since this involves the movement of water against the osmotic gradient (if the osmotic concentration outside the cells is higher than that inside, the water will move out of the cells and not into them). Despite these difficulties, there are some organisms that can thrive in environments which have very high salt concentrations. Seawater contains about 3 per cent salts and, since it is such a widespread environment, this can hardly be considered to be extreme. The salt concentration of seawater is fairly stable, except where it is diluted by inputs of freshwater from rivers. Salt concentrations higher than that of seawater occur mainly in terrestrial sites where the salts have become concentrated by the evaporation of water.

In some countries, salt is manufactured commercially from seawa-ter. The seawater is allowed to flow into ponds which are then isolated from the sea (salterns). A concentrated brine is produced by evaporation, impurities are removed and the salt crystallised. If you fly into San Francisco International Airport, you may notice the large salterns on the coast which are used for the harvesting of sea salt. Natural salterns develop in areas which are flooded periodically by the sea. Salt lakes are formed where the rivers and streams feeding them flow over soils and rocks which contain easily dissolved minerals. They have no outlet and so water evaporation can result in very high salt concentrations (see Chapter 2, 'Salt lakes and soda lakes'). If the water evaporates completely, salt flats and pans are formed. If salts derived from salt lakes or seawater become buried, they form deposits of rock salt.

There are few organisms that can grow where there are very high salt concentrations. Those that can tolerate such conditions but grow better at low salt concentrations are referred to as being halotolerant, while those that grow best at high concentrations are called halophilic. Those which grow in high osmotic concentrations caused by substances other than salts (such as sugars) are called osmophilic. There are very few halophilic organisms that grow at salt concentrations between 15-30 per cent. The Dead Sea, for example, has just one halo-philic alga (Dunaliella parva) and a few species of halophilic archaea (halobacteria). There are no plants or animals that can tolerate the con ditions of the Dead Sea, which has the highest concentration of salts of any salt lake in the world (about 30 per cent salts). Lakes and ponds with a lower concentration of salt (up to about 25 per cent salts) can support the growth of brine shrimps (Artemia species) and the larvae of brine flies (Ephydra species). Halobacteria can grow in saturated salt solutions (33 per cent sodium chloride) and may even become trapped and survive within salt crystals.

Halophilic organisms can function in environments with high salt concentrations because they accumulate substances within their cells which counterbalance the osmotic stress. The alga Dunaliella accumulates glycerol. Glycerol is an osmotically active substance (osmo-lyte) which takes up space in a solution, lowering the concentration of water and raising the internal osmotic pressure. This balances the concentration of water inside and outside the cells, preventing them from losing water by osmosis. Glycerol is used as an osmolyte by algae, yeasts, fungi and Artemia. Prokaryotes, such as bacteria, use a variety of sugars, sugar alcohols, amino acids and compounds derived from these (such as glycine betaine and ecotoine) as osmolytes. These substances are known as compatible solutes because they do not adversely affect the workings of cells.

For most organisms, salts are not compatible solutes since they have harmful effects on their cellular machinery at high concentrations. Some halobacteria, however, accumulate high concentrations of potassium chloride inside their cells, although they exclude sodium ions. This involves pumping ions across the membrane and the halo-bacteria gain the energy required to do so in a unique way. Their membranes contain light-sensitive purple pigments called rhodopsins. These convert light energy into chemical energy and, in the process, use it to power their ion pumps. The rhodopsins give halobacteria their colour and accumulations of salts in salterns and salt lakes are often stained pink by their presence. The enzymes, and other biological molecules, of the halobacteria need to be adapted to function in the presence of high concentrations of potassium. In fact, the cells of halo-bacteria cannot function without the high levels of potassium and the binding of sodium ions to their outer surface is essential for them to maintain the structure of their cell walls.

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