Extremophiles
Our
perception of the nature of life on Earth has changed over time. Before the invention of lenses, and the
subsequent development of microscopes, there was a depth to our world that we
could not grasp. As we have developed
more and more powerful microscopes and discovered a seemingly infinite amount
of microbial diversity, we have been forced to re-evaluate how we think about
the nature of life. The world of
microorganisms has spurred our desire to define a set of concrete boundaries
that encapsulate the limits for life on Earth.
It seems like the harder we try to establish the constraints; the more
that life surprises us. Not only are we
learning more about the diversity of life on Earth, but we are also learning
that life thrives in some very shocking conditions. The environments in which we are discovering
life are indeed extreme. And the
organisms that we find living in those environments are called extremophiles.
We must
strive to answer one question before proceeding any further; what is
extreme? Surely if we could travel back
in time, snatch one of Earth’s earliest living organisms, and bring it to
modern Earth, that organism would consider our environment very extreme. After all, early Earth’s environment would
have been nicely suited for that early organism; little oxygen would have been
in the atmosphere, the Sun would have been dimmer, and the climate on Earth
would have been completely different.
Our time-traveling critter might find it very difficult to survive in
its new oxygen-rich environment. To it,
modern Earth would represent a very extreme environment. Yet our environment seems rather temperate to
us. Using this logic, then, a definition
of extreme can’t be reached unless we define our system a bit more
concisely. It is most common to take a
more anthropocentric perspective when defining extreme. Almost any environment that would make it
difficult for humans to survive could be considered an extreme
environment. However, after we explore
the range of known extremophiles, a better feel for what constitutes an extreme
environment will likely be gained.
We should
pause here to make a general note about extremophiles. These are not organisms that merely tolerate
life in the extreme, these are organisms that are known to thrive in the
extreme and perish in the mediocre. Most
extremophiles are microorganisms, and in particular they generally belong to
the archaea domain of life. However,
there are multi-cellular, eukaryotic organisms that have been noted to survive
in harsh environments and may be considered extremophiles. Some of the most amazing photos of extreme
environments come from the deep sea near hydrothermal vents and include images
of eukaryotic extremophiles. This is
important to remember as we examine the extreme environments of Earth and the
creatures that inhabit them.
As humans,
we may perceive our planet Earth as a moderate and comfortable place to
live. However, there are a multitude of
environments which would prove far too harsh for our survival, especially if we
prohibited the use of technology.
Environmental extremes on Earth include niches that are very hot or very
cold, very acidic or very basic, extremely arid or dry, represent areas of
extreme salinity, exposed to high amounts of radiation, under great amounts of
pressure, and/or contain little to no oxygen.
This is not meant to represent a comprehensive list of extreme
environments, but rather is meant to provoke the reader to consider the wide
range of environments that are represented on Earth.
On a hot
summer day, we may exclaim that we are going to “die of the heat”, or on a cold
winter’s evening we make think to ourselves, “I am freezing to death!” but the
truth of the matter is that humans don’t experience temperature in the same way
as extremophiles. Can you imagine a hot
summer’s day that reached temperatures of 235oF (113oC)?
No? Well, this is currently the
highest temperature recorded in which life has been found living. The creatures that live there are a type of
extremophile known as thermophiles (thermo- meaning “heat” and –phile meaning
“loving”). On the other end of the
temperature spectrum we find psychrophiles – organisms that thrive in very cold
environments, down to -18oC (less than 0oF)!
The environments in which these two extremophiles live couldn’t be more
opposite. Thermophiles grow best at
temperatures greater than 176oF (80oC) which can be found in places such as boiling hot
springs or deep in the ocean near hydrothermal vents. On the other hand, psychrophiles prefer
temperatures much less than 15oC for optimum growth and have been discovered living in
snow, ice, and glaciers.
We’ve
already examined cases of extreme temperature.
These cases are perhaps the easiest to think about because as humans we
encounter temperature extremes in our daily lives. However, it is far less intuitive to think
about some of the extreme environments on Earth. For example, what might an environment with
extreme pH be like? The entire pH scale
only goes from 0 to 14. A neutral value
for pH is 7. Values less than 7 indicate
acidic conditions while values greater than 7are basic. Let’s begin by considering extremely low
values of pH. Acidophiles (acid-loving)
prefer environments that are acidic, generally with a pH value of less than
4. This would be like living in battery
acid. They are commonly found in acidic
It has been
proposed by astrobiologists that one of the environmental requirements for life
on Earth is the presence of liquid water.
As a result, we can identify organisms that thrive under conditions with
limited access to water as another type of extremophile, known as xerophiles. For centuries, human have been using the
knowledge that life doesn’t respond well to a lack of water to preserve our
food. Drying out fruits and meats
decreases the availability of water in those foods and prevents harmful
microorganisms from growing on them, thus preserving it for human consumption. Examples of natural environments that possess
extremely small amounts of liquid water would be the dry valleys in
Environments
that have a high salinity (high concentrations of salt) represent environments
similar to that occupied by xerophiles.
After all, if there is a high concentration of salt, it is likely that
the availability of water is decreased.
The main problem with high salinity, however, is an issue of osmotic
pressure. If a cell were to be placed in
a solution containing a great deal of salt, water would leave the cell due to
osmosis. This would cause the cell to
shrivel up and eventually die. Organisms
that live in environments of high salt concentration, like the Great Salt Flats
or the
Another
extreme environment on Earth is that consisting of high levels of
radiation. The word radiation can mean
many things. In this case, we are
considering radiation that is known to be detrimental to life. For example, if our Earth did not have a
protective ozone layer to shield the surface from large amounts of UV radiation
emitted from the Sun, it is unlikely that life would have ever evolved to live
on land. In addition to UV radiation,
exposure to ionizing radiation can also be damaging. A very surprising environment on Earth where
we have found life is in nuclear waste dumps and nuclear reactor water
cores. The extremophile that has been
identified living in this environment, Deinococcus radiodurans,
is considered a radiation-tolerant extremophile.
Now let’s
consider an environment not often visited by humans, the deep ocean. Although the technology has existed for divers
to explore the ocean up to a hundred meters or so, it has only been recently
that we have been able to investigate deep sea hydrothermal vents a few
thousand meters deep. As we plummet
deeper and deeper into the water, the cumulative weight of the water above
increases and so does the pressure.
Organisms that thrive at high pressures are called barophiles or
piezophiles. Whereas the atmospheric
pressure at sea level is 1 atmosphere (atm) of pressure, barophiles will thrive
at pressures up to 700 atmospheres. In
fact, if you were to place a barophile in an environment with only 1 atm of
pressure, it wouldn’t be able to live.
The last
extreme environment that we will discuss in detail is a good example of how
anthropocentrically we define extreme environments. This extreme environment is one that lacks
oxygen – otherwise known as anaerobic.
As we have already discussed, early organisms on Earth would not
consider an environment with little to no oxygen as extreme. Furthermore, there are many organisms
currently on Earth that do not require oxygen for survival. The types of organisms that live in anaerobic
environments can be split into two categories – facultative anaerobes and
strict anaerobes. Facultative anaerobes
are organisms that function well without oxygen but that can tolerate oxygen if
it is present in the environment. Strict
anaerobes abhor oxygen; if it is present in the environment it will kill them. There are tons of examples of anaerobic environments,
ranging from lake sediments to your digestive tract.
Although we
have not discussed all of the possible extreme environments or the creatures
that inhabit them, it would only take a little imagination to come up with
other types of extreme environments like those with excessive electric currents
or inside of rocks. The goal is to
identify extreme environments on Earth, study the nature of life within these
environments, and extend that knowledge to our search for life in the
Universe.
Recommendations:
Extreme
Environments Beyond Earth
Life on
Venus
Life on
Mars
Life on Gas
Giants
The Nature
of Life
The Tree of
Life
Thermophiles
Psychrophiles
Acidophiles
Alkaliphiles
Xerophiles
Halophiles
Radiation
resistant
Barophiles
Anaerobes
Tree of Life
This will
be a tough RLO to find images for. I
think that it would be cool to have images of all the extreme environments, but
I don’t know how many of them we will be able to get from NASA websites that
would be for free. I won’t spend much time
looking, but I may glance around a bit for some.
This was
taken from http://www.nps.gov/yell/press/images/thermalf/gprismat.jpg
and is free to use as long as a credit line containing the following is
included: Credit line must read: NPS Photo
By the way,
this is
There are
other suitable images at that website, so when it comes time for you to do
this, I’d like to help out a bit.
Extremophiles_image1.jpg
I’ve seen
this image credited on another website, so they may let us use it as well. I found it at http://www.desertusa.com/mag98/april/owens/owenslake.html. It is one of the best photos around for
halophilic environments.
extremophiles_image2.jpg
This is a
picture of a radiation tolerant extremophile from APOD(which you should already
have linked to Astropedia I believe).
http://antwrp.gsfc.nasa.gov/apod/ap040425.html
Extremophiles_image3.jpg
A picture
that would show the environment in which psychrophiles live. Again, from APOD.
Extremophiles_image4.jpg
http://antwrp.gsfc.nasa.gov/apod/image/0301/vida_lter_big.jpg
Acidic
river for showing environments for acidophiles.
This is the Rio Tinto in
Extremophiles_image4.jpg
http://microscope.mbl.edu/baypaul/microscope/general/page_01.htm
This is a
hydrothermal vent. I have seen this
image on about 5 different websites, so it must be easy to get permission to
use.
Extremophiles_image5.jpg
http://web.uvic.ca/sciweb/pics/hydrothermal-vent.jpg
Figure
below is blatantly stolen from but might be useful as an illustrative of the
breakdown of some extremophiles:
Rothschild,
L. J. & Mancinelli, R. L. (2001). Life in extreme environments, Nature, 409, 1092-1101.
