Psychrophiles sounds like a lot like "psychophiles", and if these microorganisms were smart enough to understand that they have chosen to live in ridiculously cold environments, they might come to realize how crazy they really are. But in their ignorance, these little critters live quite blissfully at the bottom of the sea, on the top of snow-covered mountains, in arctic ponds, deep alpine lakes, and in tundra soil near permafrost: environments below the temperature of 10 degrees Celsius (a temperature at which the vital processes of even cold-blooded animals grind to a near halt). Indeed, if psychrophiles are exposed to temperatures above 20 degrees (at which we humans are perfectly comfortable) they will probably die.
As all organisms require water to survive, even psychrophiles can't simply live in ice. Water in the soil freezes at just below 0 degrees Celsius and sea water freezes at around -1.9 degrees. Within ice, pockets of liquid water may remain and microorganisms can grow within these pockets to even lower temperatures. So how is it that these microorganisms survive, even thrive at temperatures in which the rest of us would cease to function? First of all, they need to somehow ensure that their bodily fluids remain fluid and don't freeze up. To cope with the danger of solidifying, psychrobacteria have developed two layers of skin. The outer layer is stiff and therefore resistant to freezing, but mesh-like enough to allow food and water to pass through it. The inner layer is more elastic and membranous; it allows the nutrients it needs in but blocks necessary cellular substances from leaking out. These cellular membranes are composed of a larger proportion of unsaturated fatty acids than are the cellular membranes of non-psychrophillic organisms. This is what enables their membranes to remain fluid at colder temperatures.
When water in soil freezes, the concentration of salts increases: the dehydration of cells caused by these salts is a major problem for microorganisms in cold environments. This problem of dehydration is dealt with more closely in our discussion of halophiles (salt-loving organisms), but the same problems and solutions come into play with psychrophiles. In the case of the latter, however, time can be a great factor in survival. If the freezing process of the soil water is a slow one, the cell may be able to recognize what is happening and build up a resistance to it by producing and encapsulating itself coats of mucus. Though psychrophiles are small, they've got plenty of defense mechanisms up their teeny little sleeves.
Again we wonder why any
type of organism would develop the ability to live in such an extreme environment
when there seems to be plenty of room in the comfortable places that we humans
inhabit. But the fact is that there isn't very much room on this planet, percentage-wise,
which humans would deem to be hospitable. As John Postgate notes in his book
The Outer Reaches of Life, "more than two thirds of the Earth's surface
is covered by the sea, and two thirds of that is unaffected by changes in surface
temperature and stays close to 2 degrees" (Postgate, The Outer Reaches
of Life, 26). When he computes it in a different way, Postgate notes that four
fifths of the inhabitable volume of our planet never rises above 5 degrees Celsius.
This is craziness, is what this is, but it's true. The environments that humans
label as "extreme" are a lot more common, even on our own planet,
than the environments that we deem to be normal. Certainly the majority of the
planets and moons in our solar system, in being further away from the sun than
Earth is, are cold. If we anticipate the discovery of life on worlds such as
that of Europa,
we should probably look to psychrophiles rather than Twilight Zone episodes
to get a hint of what that extraterrestrial life might look like.
Incredible. But please, show me more extremophiles.