Of all the environments on Earth, those that are defined by low temperatures are often not considered extreme environments. After all, when we think of cold environments we most often think of places like Antarctica where we know living organisms like penguins can be found. So why, then, do scientists consider places with temperatures much less than 15 °C extreme? And why are we so interested in psychrophiles: the extremophiles that thrive in temperatures lingering around the freezing point of water?
To answer our first question, consider for a moment a single celled organism. In order for that organism to survive, it must be able to transport nutrients in and out of its cell and it must be able to carry out the chemical reactions of life. All life on Earth uses water as its solvent; it dissolves useful nutrients and promotes movement of the molecules that interact in life's chemical reactions (metabolism). If the water in that single celled organism freezes, there will no longer be a way for the organism to access essential nutrients or complete the reactions necessary to gain energy to live. Not only that, but freezing the water inside of a cell could have drastic impacts on the cell membrane, literally tearing it apart. Consequently, living in temperatures well below the freezing point of water can be devastating to life on Earth. Devastating not just humans, but all life including bacteria.
So why might scientists be interested in the small subset of creatures, the psychrophiles, that thrive in environments of low temperature? One glance at the planetary inhabitants of our Solar System and our question can easily be answered. With a few exceptions, most of the planets that are of interest from an astrobiological perspective experience frigid temperatures. On Mars, for example, the temperatures linger well below the freezing point of water, except for rare occasions in equatorial regions where temperatures sometime jump a few degrees above zero. The red planet is actually considered temperate compared to planets and moons further out in the Solar System. A visit to Titan, although covered by a dense atmosphere of greenhouse gases, would yield temperatures as low as -120 °C. Europa, one of Jupiter's moons, has a crust made primarily of water ice a minimum of 1 km thick! As we explore our own cosmic backyard for evidence of past or present life, it will be essential to understand the effects of cold temperature on life on Earth so that we may better interpret evidence found on any of our frigid neighbors.
What have we learned so far about life in the extremely cold environments on Earth? Life in these extreme temperatures has adapted to avoid both the fate of freezing solid (thus eliminating the possibility of nutrient access and metabolism) and subsequent cell~membrane destruction if freezing were to occur. One trait that organisms have adapted is the inclusion of "anti-freeze" molecules into the cytoplasm. These molecules, usually special proteins or salt ions, actually prevent water inside the cell from freezing by decreasing the temperature at which water becomes a solid. In addition to incorporating different molecules into the cytoplasm, some organisms have a physically different cell~membrane composition. Cell membrane composition naturally varies between different types of organisms. However, organisms living in extremely cold environments incorporate many more unsaturated fatty acids, thereby increasing the fluidity of the membrane and decreasing the magnitude of harmful effects due to freezing. Finally, some organisms take an even more defensive stance; they sporulate, or protect themselves by releasing spores. A common psychrophile well known for its red spores is Chlamydomonas nivalis. It covers the surface of glacial snow with a pinkish layer.
Psychrophiles are not only specially adapted for extremely cold conditions, the microenvironments in which they live also promote their survival. Take, for example, the psychrophiles living around dust grains buried in a few inches of snow or ice in the dry valleys of Antarctica. Most of the year, the psychrophiles remain dormant in their frozen state. However, during the summer months, Sun shines continuously. Although the temperatures still don't rise above freezing, the dark dust grains in the ice absorb enough energy from the Sun to melt a tiny pocket of liquid water. Microorganisms living nearby utilize these bouts of solar powered melting to temporarily leave dormancy behind. These psychrophiles exist for years, benefiting from the thermal energy from a transient sun.
What does this say about our search for life beyond Earth? Like most scientific discoveries, the study of psychrophiles opens up the door of possibility, and stimulates an even greater set of questions. Surely if life thrives in temperatures well below the freezing point of water on Earth, there might be a small possibility that life exists in the icy crust of Europa, or the liquid ocean encased beneath. But will we be able to recognize the life forms living there? Will they have adapted an even more eloquent set of traits to increase their survival? Only time will tell, but in the meantime we can continue to explore the niches of Earth and strive to answer the questions that will help us uncover the mysteries of life beyond Earth.