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18.15 Xerophiles

All life as we know it requires at least small amounts of liquid water for intermittent periods of time. There are many properties of water that make it the most well suited solvent for life. It is partially due to our dependency on water that explorers of our solar system are obsessed with the search for water. But how much water is necessary for life to exist? Scientists have discovered living organisms that will provide insights into the minimum amount of water required for life. These organisms live in extremely dry environments and are called xerophiles. Like their other extremophile relatives, xerophiles have adapted unique features that allow them to survive extreme desiccation. 

It doesn't take the wildest of imaginations to think of examples of dry conditions on Earth. Some of the driest regions on Earth exist in the Atacama Desert in Chile, where the annual precipitation is less than 0.004 inches a year. In fact, some areas of the desert have not see rain for over 400 years! Rivaling the Atacama in its lack of water are the dry valleys in Antarctica. Here there is very little annual precipitation, only a few centimeters a year at best. Although we think of Antarctica as a snow covered continent, the dry valleys are surprisingly absent of snow. 

It would seem that in an environment with such little water, the likelihood of finding life would be very slim. We do, in fact, find life in extremely dry environments. The problems resulting from desiccation can be quite prohibitive to organisms. Just a few of the obstacles faced are the accumulation of oxygen reactive species within cells, irreversible phase changes of the lipids that compose cell membranes, and denaturation or structural damage of proteins and nucleic acids. The life that has been found in these extreme environments possesses unique ways of combating the negative effects of desiccation. 

The primary mechanism of defending against desiccation is to increase the osmotic concentration inside the cell. This means that a cell will acquire more molecules in their cytoplasm that we call osmotica. In prokaryotes, the most common molecule used as osmotica is glycine betaine. Osmotica accumulates in the cytoplasm away from proteins, lipids, and nucleic acids. This forces the remaining water in the cell to aggregate around proteins, lipids, and nucleic acids and results in their stabilization during desiccation. 

With our more recent studies of the planet Mars, we have become increasingly aware of the lack of liquid water on our nearby neighbors. The discovery of life on Earth in extremely dry conditions gives us hope about future exploratory missions of Mars. At the very least, we may be better able to predict where to search Mars for evidence of past or present life.



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