Psychrophiles
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
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 oC. 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. 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
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.
References:
Caviccioli, R. (2002). Extremophiles
and the search for extraterrestrial life, Astrobiology, 2(3), 281-292.
Rothschild,
L.J. & Mancinelli, R. L. (2001). Life in extreme environments, Nature,
409, 1092-1101.
Recommendations:
Extreme Environments
Beyond Earth
Life on
Mars
The Nature
of Life
The Tree of
Life
Thermophiles
Acidophiles
Alkaliphiles
Xerophiles
Halophiles
Radiation
resistant
Barophiles
Anaerobes
Tree of
Life
Oh my god,
this is an AWESOME image of Chlamydomonas nivalis. It is a must have!
http://home.online.no/~ptrptr/mflowers/Image/Chlamydomonas_nivalis.jpg
Psychrophiles_image1.jpg
This is a whole snow field with snow algae. Again, a must have.
http://www.antdiv.gov.au/asset/images/627_ul-Red_snow.jpg
http://www.antdiv.gov.au/default.asp?casid=2437
psychrophiles_image2.jpg
The following pictures tie in well with this article. They show the little pockets of dirt in ice
core samples. These I am sure we could
get permission to use from John Priscu at
http://www.homepage.montana.edu/~lkbonney/DOCS/Ice%20Microbe%20Images.htm
pyschrophiles_image3.jpg
pyschrophiles_image4.jpg
