Learn about the origin and evolution of life and the search for life beyond the Earth.
Life in Extreme Environments
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From the course by The University of Edinburgh
Astrobiology and the Search for Extraterrestrial Life
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From the lesson
Life on Earth
What was the environment of early Earth like when life first emerged and what do we know about life on the earliest Earth? How did life evolve to cope with survival in extreme environments? What have been the major evolutionary transitions of life on the Earth?
Meet the Instructors
Charles CockellProfessor
Astrobiology
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How has life managed to survive over three billion years on the earth and what
adaptation does it need to survive environmental
changes throughout its long tenure on the Earth?
Well, one of the best ways to answer this question is to go
and look at how life manages to survive and grow in extreme environments.
Well, what do we mean by extreme?
Some people think that it's a very anthropocentric
term, it's just a matter of taste.
What may be extreme to one organism, is not extreme to another.
But we think that there is something much more fundamental to it than that.
Biological systems, do only function in a continual, particular, physical
and chemical extreme, and as we go to these circle
of extremes, such as high temperature, low temperature, increased pressure,
we find that the diversity of life generally in these environments
tends to decrease.
So it does seem that there are real extremes.
Extremes beyond which life cannot go.
Boundaries to the biosphere that are
determined by physical and chemical extremities.
And studying the organisms at these extremities can tell us much, about
how life on earth has managed to cope, with conditions on our planet.
And, just as interestingly, whether life might be
able to cope with the physical and chemical extremes
to be found on other planetary bodies.
The organisms that inhabit these extreme environments we call extremophiles.
Literally, extreme lovers.
There mainly the prokaryotes, the bacteria and alkali that we saw in the
tree of life, but there are
some eukaryotes that live in extreme environments.
Even flies for example, that live at the edges of volcanic hot
springs in Yellowstone National Park, but by and large, once we get to
really great extremes on the Earth, we find that
the organism that's are inhabiting these environment are the prokaryotes.
What are the types of extreme environments that exist out there?
Well these are some of the environments we'll look at in a little more detail.
There are hot and cold places, there are places that are very salty or
very dry, there are also places that are very acidic or very alkaline or
basic, and there are also places
with intense pressure and also intense radiation.
Let's have a look to begin with at hot environments.
There are many hot environments on the Earth that harbor
life, a good example is deep sea hydrothermal vents; places
where reduced fluids containing sulfates, other toxic chemicals are spewing
out from the crust of the Earth into the oceans.
And these hydrothermal
vents can be producing water, well over 100 degrees
centigrade because of the high pressure in the deep oceans.
The organisms that inhabit these hot environments, which also include volcanic
hot springs in places like Yellowstone National Park, are called thermophiles,
if they grow between 50 and 80 degrees, or if they
grow at really high temperatures above 80 degrees, they're called hyperthermophiles.
Now it's important
to remember that these organisms aren't just capable of growing
at these temperatures, they actually need to grow at high temperatures.
If you take a hyperthermophile, for
example, whose optimum growth temperature is above
80 degrees, and you bring it down to room temperature, it will generally die.
These are microbes that actually have to
be growing at these very high temperatures.
A good example is methanopyrus kandleri. This is
an archea that inhabits black smokers, deep ocean
hydrothermal vents that are black, because they're producing sulfites,
minerals that are produced in the oceans as
these fluids gush out of these black smoker vents.
The organism can grow up to temperatures of 110 degrees centigrade.
Some of the challenges it faces like all
thermophiles or hyperthermophiles are the breakdown of biomolecules.
Those high temperatures impart
energies to, the, to the microorganisms.
They tend to cause the biomolecules to breakdown.
They also have a problem with membrane fluidity.
Very high temperatures, the energy causes the
membranes to start shaking apart almost quite literally,
and that fluidity in the membrane can cause
problems for the integrity of the cell membrane.
How does it deal with these challenges
with living in these high temperature environments?
Well two of the ways that deals with
this is by evolving thermostable proteins and enzymes.
Enzymes being biological catalysts involved in
carrying out chemical reactions in the cell.
These proteins have extra chemical bonds and other types of
features that maintain their stability at these very high temperatures.
It turns out that these proteins, or catalysts,
these enzymes have, have commercial uses as well.
For example, thermostable catalyst enzymes are used in biological washing powder.
One of the reasons why your washing powder can work at high temperatures is because
it contains proteins, enzymes form microorganisms that have
been isolated from hydrothermal springs, volcanic hot springs.
So you see we can use these thermostable enzymes
and proteins for some very prosaic, but commercially useful applications.
Other adaptations include changes to the cell membranes and composition
to allow those membranes to maintain stability at high temperatures.
Microorganisms have also been found in freezing environments,
such as the depths of Antarctic ice sheets.
This is an example of the deep lake in the Antarctic lake called Vostok.
And just above that lake are microorganisms in
the ice, in the accretion ice that forms above
that deeply buried lake.
In these sorts of ice sheets, microorganisms
adapted to cold conditions can grow, and these
are called psychrophiles, microorganisms that can grow
at temperatures at less than 15 degrees centigrade.
What are the challenges of living in a cold environment?
Well, one challenge of course is membrane damage from ice.
If ice crystals form in the cell, it can damage the membranes.
Another problem is decreased membrane fluidity.
In very cold temperatures, the lipids begin to solidify, a bit like if
you put butter in your fridge, it begins to get a lot more solid.
Those fatty acids, those lipids in the cell
membranes, begin to solidify at very cold temperatures, and
reduce the fluidity of the membrane that's necessary
for the cell to export materials and import nutrients.
Another obvious
problem in freezing environments is the availability of liquid
water, much of it is frozen up in ice.
And so, organisms can have trouble getting hold of the
liquid water that they need to carry out by chemical reactions.
So how is it that these microbes can adapt to these extremely cold conditions?
Well one way in which they adapt is by altering their membrane composition.
A bit like the microbes living at
high temperatures, they need to change the composition
of the membranes to maintain their fluidity at low temperatures.
One way in which they can do this is
to incorporate more unsaturated fatty acids into their membranes.
These unsaturated fatty acids have kinks in the membrane structure that pulls,
pushes apart the membrane and makes it more fluid under cold conditions.
Some of these microbes and other organisms
also have antifreeze agents such as sugars.
And these sugars prevent ice crystals from forming in
the cells and reduce the chances of ice crystals
forming and damaging the membranes, another way in which
they can circumvent problems of growing at very low temperatures.
So that's hot and cold environments.
What about salty and dry environments?
Well, microorganisms have been found that can inhabit very
salty environments, such as microbes that live in the
Dead Sea, and in deposits of salts around the Dead Sea.
These are called halophiles.
And these halophiles, literally salt lovers, can
grow at salt concentrations between 15 and 37%.
Microbes that live in deserts, also capable of
tolerating very dry conditions, and these are called xerophilic
microbes, you can find them in the deserts of
the Atacama, the Sahara and other dry and desiccated
environments of the earth.
The challenges of living in salty and dry conditions are actually quite similar.
One problem is osmotic pressure and water availability.
In very high salt concentrations the salt has the tendency to
pull water out of the cells by the process of osmosis.
So cells have a problem in hanging onto their liquid water.
And, of course, in dry deserts the problem is also the water tends to evaporate.
Which causes problems for the maintenance of water in
the cell, also osmotic effects in very dry conditions.
Another challenge faced by mi-, microbes in
both salty and dry environments is membrane integrity.
Maintaining the integrity of the membranes and preventing them from falling
apart under these very high salt
concentrations that tend to disrupt biomolecules.
And under very dry conditions, where the dry conditions also tend
to cause disruption to the membranes lack of water.
How do organisms adapt to these very extreme, salty, and dry conditions?
Well two ways they can adapt are to control water loss from the cells.
They can produce salts and other solutes within the cell that tend to hang
onto the water, make it more difficult for it to dissipate from the cell.
Another way in which they can deal with these extreme conditions is
to go into a state of dormancy.
When they're dormant, they're not active, but it
allows them to wait around until conditions improve.
There's a particular case for microbes living in deserts where
they may want to go dormant and wait until liquid water
becomes available and they can reproduce and grow again, and
then go into a stage of dormancy when it dries up.
Microbes have also been found that live at extremes of pH.
For example the Rio Tinto River in Spain whose pH is down to
0.4 high concentration of protons in the water makes it very, very acidic.
There are also places with very high pH,
such as Mono Lake in the United States that
has a pH up to 12.5 very alkaline
or basic environments, that pose great challenges to microbes.
The challenges that are faced to the organisms that live in these environments
include the breakdown of their cellular components.
For example, in acidic environments the very high proton concentration that's
responsible for that acidity can disrupt
biomolecules, causing them to become inoperative.
These very challenging pH conditions are
also a great challenge to metabolic processes.
Processes for gaining energy and carrying out chemical reactions inside the cell.
How the cells
adapt to these extreme pHs?
Well, one way they can adapt is to regulate the
pH inside the cell in order to make it neutral.
So for example, a microorganism living in very acidic environments will
pump out protons from the cell and maintain the inside the
cell at near neutral pH, and at neutral pH the biomolecules
are much more stable and metabolic processes can proceed without disruption.
In other words,
these cells really don't like to be in an acidic environment.
But they can change their internal conditions
such they can survive and grow in acidic
environments without the acid on the outside of
the cell affecting cellular reactions inside the cell.
This is a really ingenious way by which cells can survive and grow in
acidic conditions and similar sorts of processes
are also found in very alkaline environments.
Organisms have also adapted to life under high pressure.
In fact many of the habitats on earth for life are actually at high pressure.
Two examples are the deep oceans, and the deep crust of the earth.
In the deep oceans many of the trenches are at great depths, such as
the Mariana Trench at 11 kilometers depth,
and here pressures exceed a thousand atmospheres.
The microbes that can live in these
environments are called piezophiles microbes, literally pressure
loving microbes.
In fact at the current time we don't know what upper pressure limit for life
is, but certainly they can survive at high
pressures we found in these deep ocean trenches.
Microbes are also found in the deep crust living
in rocks kilometers underneath the surface of the Earth.
The challenges of living at high pressure are the pressures cause the
tight packing of molecules and a loss of fluidity in cell membranes.
Pressure can also cause impaired cellular functions in activities.
Particularly of enzymes that are necessary to carry out
the catalytic functions of chemical reactions, inside the cell.
How do cells, how do organisms adapt to live in
these high-pressure environments in the deep oceans or in the crust?
Well, one way in which they can adapt is by changing gene expression.
They can produce, for example, molecules that enhance the uptake
of nutrients and other elements that they need for growth across the
cell membrane and thereby adapt to
these challenging environments under high pressure.
They can also change their membrane structure.
For example, by introducing unsaturated fatty acids
to increase membrane fluidity under high pressures.
And you'll notice, that unsaturated fatty acids were also
the way in which microbes can adapt to low temperatures.
So some of the mechanisms that organisms use to adapt
to one extreme are also used to adapt to other extremes.
They can be common responses to different environmental extremes.
We might think about one of the most extreme environments known to man and that
is the conditions of outer space. Can microorganisms survive in outer space?
Space is characterized by extremes of
radiation, freezing temperatures, dessication, and no oxygen.
A few years ago, my own laboratory launched rocks into orbit, and these rocks
were bolted onto the outside of the International Space Station.
And we brought some back to earth a year and
a half later to see whether anything had survived in space.
And we found a single micro organism, a gloeocapsa,
which is a type, cyanobacterium, a photosynthetic micro organism
that was capable of surviving in the extreme conditions
of space for a full year and a half.
Of course it didn't grow in space, but it did survive.
These types of experiments
show how outer space is an environment
that can even be survived by some microorganisms.
Showing how hardy they are and how able they are to
resist environmental extremes, if only for a short length of time.
Many extremes that we find on the earth don't occur in isolation.
I've talked about high and low temperatures,
I've talked about salty environments, and highly acidic,
and alkaline environments, but in fact in
many natural environments there are frequently multiple extremes.
An astrobiologist are very interested in
polyextremophiles, extremophiles that can tolerate multiple extremes.
Here's just one example of a rather famous organism
called deinococcus radiodurans.
This is a microbe that can tolerate high levels of radiation, it's also
found in environments with cold temperatures, it's
found in deserts that are very dry.
Some of these organisms can tolerate vacuum conditions, and members of this
group, the deinococci, are also found in very acidic environments as well.
So we see how there are polyextremophiles that can survive multiple
extremes, and by studying these microorganisms,
we can get better understandings of how
microbes tolerate the boundaries of extremes, to
find the boundaries of the earth biosphere.
Why is this of any interest to astrobiologists?
Well, there are really two reasons why we're interested in studying
microbes at extremes, and particularly those
microbes that can tolerate multiple extremes.
First of all, of course, it tells us about the boundaries of the earth's biosphere.
What are the limits
of life on earth? When do we go beyond those limits?
And how might environmental changes throughout the history
of the Earth, even if environmental changes cause
by humans, affect life on earth, and those
microorganisms that inhabit the outer boundaries of the biosphere?
But the other reasons for being interested
in studying life in extreme environments is
because it might give us better ideas
about the habitability of other worlds such as
Mars, Europa, and Enceladus and Titan and
other planetary bodies of interest to astrobiologists.
Once we know the physical conditions on those planetary bodies, we want
to be able to assess whether they are within the boundaries for life.
Whether life might be able to persist on those planetary bodies.
The only way in which we can do that is by
studying microorganisms of life in general in extreme environments, and seeing
whether extremes we observe on other planetary bodies are extremes that
can be tolerated by life forms that we know on the Earth.
So the study of life in extreme environments is essential
for assessing habitability, the ability other planetary bodies to harbor life.
So what have we learnt in this lecture?
We've learnt that extremophiles are organisms
that tolerate or require physical and
chemical extremes in order to be able to survive and grow and reproduce.
We've learnt that, although extremophiles span all major domains of
life, most of them come from the prokaryotes bacteria in [UNKNOWN].
We've learned that adaptations to extreme conditions
sustain cell integrity and function, frequently via modifications
to the cell membrane and molecules such
as proteins and enzymes, catalytic molecules in cells.
We've learned that interactions between multiple
extremes may influence extremophile growth and survival.
The study of life in extreme environments
is pivotal to understanding the emergence of
life on the Earth and understanding the
boundaries for life in the universe at large.
And finally we've also learned that molecules isolated
from extremophiles may also have commercial and industrial use.
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