You know the Dead Sea? That hypersaline lake located between Israel and Jordan, were even the worst swimmer can remain buoyant? The lake that’s so salty that it’s supposed to be entirely dead, since no life can thrive in such salty conditions?
False! While you won’t find fish swimming around in the Dead Sea, there’s definitely life that has adapted to these hypersaline circumstances (and I do not mean the floating tourists). You will have to bring your microscope to find them though, since you won’t find anything alive that’s larger than a microbe. Microorganisms that have adapted to live a salty life are better known to scientists as halophiles, which literally means “salt-lovers”. Halophiles can be found both within the branches of bacteria and archaea. Some of these bacteria are so specialized that it’s impossible for them to live under normal conditions.
To understand what makes it so hard to live in a hypersaline lake, you’ll need to know something about osmosis. Suppose that we have a bacterial cell, living in a glass of tap water. Our little microscopic life-form is perfectly in balance with its surroundings: the concentration of salts is the same in its cytosol, as it is in the tap water. Now, we add some salt to the water. We have created a concentration difference: the tap water has become saltier than the cytosol of the bacterium, with only its cell membrane separating it. If water is able to pass the membrane while the salt ions cannot, water will leave the bacterium, to reduce the concentration difference. If the water would be less salty than the cytosol of the cell, water will enter the cell. In a nutshell, this is the process that is known as osmosis.
You can see how this provides an obstacle in any hypersaline environment: the surroundings of cells are so salty that normal bacteria would lose all their water as soon as they would be placed in the Dead Sea. They would simply shrivel away, poor things! How then, have halophiles overcome this osmotic challenge? It turns out, life has found two separate solutions to live in the great salty open.
Keeping the Salt Out
The first strategy is to keep the salt out. This seems to be the strategy preferred by the bacterial branch of the halophiles. Partly this is done by actively pumping out salt ions, so that the salt concentrations within the cell stay low. However, it is impossible to counteract the osmotic pressure of a hypersaline environment in this way. It simply takes too much energy! It would be like trying to remove the water from a leaky boat: the water keeps pouring back in. Therefore, these bacteria also synthesize high amounts of organic solutes. These organic solutes are polar, and osmotically active. This means that these compounds effectively reduce the concentration difference between the outside and the inside of the cell. They make the inside of the cell appear more salty, so that the osmotic pressure is lessened.
Letting the Salt In
Many archaea follow a different strategy. They have decided to let the salt pour into their cells, and deal with the consequences. They still shuttle some of ions around (which costs energy), but are able to tolerate intracellular sodium, potassium and chloride concentrations into the molar range! These concentrations would be devastating to the proteins of any normal bacterium. The charges of the ions would destroy normal protein structure and interfere with macromolecular interactions. Therefore, special system-wide adaptations are necessary so that proteins can function in the presence of so much salt. This also means that these archaea are less flexible: they’re not only adapted to a salty environment, they require the high salt concentrations for their proteins to function!
This is an interesting trade-off: in less saline conditions, a “salt out” bacterium is able to reduce its production of organic solutes, whereas a “salt in” archaeum cannot change its entire protein repertoire! The “salt in”-strategy costs less energy than the “salt out”-approach, but comes at a great cost of flexibility and adaptability. It’s really fascinating to see how these two strategies have arisen in this extreme environment. Life has infiltrated every nook and cranny of this planet, and found different ways of doing it!
Oren A (2008). Microbial life at high salt concentrations: phylogenetic and metabolic diversity. Saline systems, 4 PMID: 18412960
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