Extreme Biology — Part 1
Amy Grunden
Slide 1
This is the first part of a two-part lecture on extreme biology.
Slide 2
The topics to be covered in the first part include the types of extreme environments that
are present on Earth, the adaptations that cells must have in order to survive in them, and the
organisms that live in extreme cold and extreme hot environments.
Slide 3
One reason why extreme biology is of such importance is that scientists hope that an
understanding of how life can adapt to these extreme environments will help us appreciate the
possibilities of life on other planets, and also appreciate how our Earth developed and how life
on Earth began. Before discussing extreme environments and the organisms that exist in them, it
is necessary to be familiar with the types of organisms that are seen in these environments.
This lecture will primarily discuss organisms in the bacterial and archaeal families. The
organisms found in extreme environments are generally from these families because simple
organisms tend to fare better under extreme conditions.
Earth is thought to have formed under very hot conditions, and life—known as the
universal ancestor—was thought to have emerged about 3.8 billion years ago under these very
hot, reducing conditions. This life form eventually gave rise to three distinct types of organisms,
the bacteria, the archaea, and the eukarya. Organisms in the bacteria and the archaea branches are
known as prokaryotes. These are unicellular organisms that have no membrane-bound
organelles, such as nuclei or mitochondria. Eukaryotes can be multicellular and are known to
have membrane-bound organelles.
It is thought that the Bacteria and Archaeal lineages arose first, and then at some point in
history, bacteria that had engulfed smaller photosynthetic or aerobic heterotrophic bacteria were
able to form stable associations with the engulfed bacteria in a process called endosymbiosis.
Endosymbiosis allowed for the permanent maintenance of the organelles chloroplasts and
mitochondria within cells and ultimately gave rise to the Eukaryotic cell lineage. Types of
bacteria and archaea exist in extreme environments, as well as some algae and fungi, but no
animals or plants exist in extreme environments. They tend to exist on the edges of these
environments, making use of the bacteria and the archaea.
Slide 4
Which Earth environments are considered extreme? Five will be covered in this lecture
series, including sea ice (a very cold environment); hydrothermal vent systems (a very hot
environment, pictured in number two); sulfuric springs, which have two extreme conditions, both
high heat and highly acidic conditions; salt lakes, which have very high salt concentrations; and
soda lakes, which again have two extreme factors, both a high pH and a high concentration of
salt. Slide 5
Before these environments are discussed, it is important to understand what kind of an
adaptation cells need in order to survive in extreme environments. This diagram represents a
typical prokaryotic cell (from the bacteria and the archaea). Moving from the outward in: a
typical cell has a cell wall that provides both structure and support for the cell, and is typically
composed of sugars and protein. Inside the wall is the cytoplasmic membrane, which is typically
composed of a lipid bilayer and fatty acids, and is responsible for controlling what enters into the
cells. Its integrity is very important in sustaining life within the cell. Inside the cytoplasmic
membrane is the cytoplasm itself, consisting mostly (about 70%) of water, proteins, various
nutrients, and waste products that eventually are excreted out of the cell. Within the cell there is
also a nucleoid, which is DNA that has been extremely aggregated. Typically it is tightly packed
in supercoils that contain all the genetic information for the cell.
There needs to be various adaptations in the various parts of the cell in order for them to
survive extremes of cold, heat, acidity, alkalinity, and salt concentrations, because any of these
conditions by themselves could destroy the various structures of the cells. As the various
environments are discussed, the particular adaptations required for that particular environment
will be covered.
Slide 6
The first of the extreme environments is an extreme cold environment. This is a
particularly popular topic because over 75% of the Earth is permanently cold, meaning less than,
on average, 5°C, and in many places it is below the freezing point (0°C) for the entire year. Part
of the reason this is of great interest is because it is thought that there may be ice on
extraterrestrial bodies. Understanding how life exists in ice on Earth could potentially help the
search for life on extraterrestrial planets.
There is more life in cold environments than was initially thought. Recent studies have
shown that there are up to 10,000 to 100,000 cells per ml (milliliter) in cold arctic waters.
Scientists have primarily identified life through fluorescent tagging of cells, using ribosomal
RNA and probes that have color associated with them. Note the blue and the green dots in the
lower left-hand corner of the slide: these are discrete cells that have been tagged. Scientists are
starting to be able to enumerate the bacteria within cold waters.
The photo in the upper left is of the corner of an icebreaker, a type of ship used for
research in icy waters. In the bottom center is a photo of a cell known as Methanogenium
frigidum. This cell has recently been isolated, and its genome is now being sequenced. M.
frigidum will help scientists understand how cells are able to live in extreme cold environments;
at the moment, their physiology is somewhat undetermined.
Consider what challenges very cold conditions present to a cell. Ice crystals are very
sharp and can puncture cellular parts, killing the cell, so cells require liquid water. Membranes
are also an issue. Think about butter versus oil, and recall that some membranes are composed of
a lipid bilayer, containing fatty acids. At low temperatures those fatty acids – and thus the
membrane itself - get gelatinous, and nutrients cannot cross. Cells must adapt so that the
membrane stays fluid at low temperature.
Slide 7
This slide shows what the cells do in order to maintain the fluidity of the membrane.
There are two types of fatty acids that can exist in the lipid bilayer: one is known as a completely
saturated fatty acid, in which all the carbons in the fatty acid have a full complement of
hydrogen. This creates a straight chain. On the far righthand side of the slide is a drawing of
completely saturated fatty acids, tightly packed. This will gel at lower temperatures because they
are packed so closely, so cells incorporate fatty acids with double bonds. The double bonds cause
the fatty acid to curve, preventing the tight packing. Cells that are at low temperatures start
incorporating more and more of these unsaturated fatty acids, allowing more fluidity in the
membrane. Conversely, when cells are in higher temperatures they remove unsaturated fatty
acids and bring in saturated fatty acids so that the membranes do not become too fluid.
Slide 8
Moving on to other organisms found in cold environments, specifically on ice. In the
Arctic or Antarctic you can see patches of color on the ice—these are primarily algae. The color
is from photopigments that provide energy for the cell and protect the cell from extreme UV
exposure. An example is seen in the top left-hand picture—the red color is algae.
The picture in the top right is from a mountainous area in the Arctic, and it too is covered
with red. This is lichen. Lichen are a symbiotic relationship between algae and fungi. The fungi
provide the support structure for the algae, and the algae, which are photosynthetic, provide food
for the lichen. Within the arctic environment there are very strong winds, and algae alone on this
bluff would not survive.
Another type of organism present in cold environments is phytoplankton. These are
bacteria and archaea that serve as the main food source for krill, which in turn serve as a main
food source for larger organisms such as whales.
Slide 9
Another extreme cold environment has been discovered about 150 miles away from New
Orleans, in the Gulf of Mexico. On the bottom of the Gulf there are methane ice sheets, and
living on these ice sheets are polychaete worms (the pink worms shown in the slide). The layer
of yellow below the worms is methane ice. These polychaete worms are feeding on bacteria, that
are surviving on methane; in other words, the methane is serving as an energy source for
bacteria, and the worms eat the bacteria. There are many studies working on determining what
type of bacteria are there and what other organisms may be thriving in this methane-based
ecosystem.
Slide 10
Lake Vostoc is a lake in the Arctic region, composed of liquid water underneath
approximately 4 km of iceflow. There may be organisms trapped in this lake that have been there
since the beginning of life on Earth, and consequently there is a lot of interest in conducting
research there. Also, this lake mimics the type of water that might exist on Jupiter’s moon,
Europa, very closely. Experiments in Lake Vostoc may help scientists decide how to conduct
experiments on Europa.
Drilling in Lake Vostoc has begun, and ice cores have been taken from the ice flow.
Scientists are trying to use these ice cores to develop cultures; they have identified some bacteria
from these cultures, but as yet they have not been able to drill down to the liquid water.
However, they have reached the bottom of the ice flow, and are sampling the interface between
the lake water and the iceflow. From there they have identified about seven different types of
bacteria that are very related to modern forms of bacteria. Shown in the bottom right-hand corner
are two bacteria isolated from the ice cores; scientists are currently studying the physiology of
these organisms.
Slide 11
The second type of extreme environment we will cover is an extremely hot environment,
beginning with hydrothermal vent systems. Hydrothermal vent systems exist on the ocean floor,
where there are lava flows underneath the ocean’s crust. Sea water percolates down into the
ocean crust, and, as it does so, picks up minerals dissolved into the ocean crust. The water that
has been heated by the lava flow will shoot out of any cracks in the crust. Upon meeting the very
cold ocean water, the metals that have been dissolved in the hot water precipitate out, forming
tall chimney structures.
On the right is a video clip showing a plume of very hot liquid that is coming out of the
chimney system. As this continues, the chimney will continue to grow. When these chimneys
were initially identified, it was thought that there could be no life in these areas, because the
water coming out of the chimneys can be in excess of 350°C. However, there are bacteria in
these systems.
Another reason scientists thought there could not be much life associated with these
systems is because there is no sunlight. On Earth, photosynthesis forms the base of the food web.
On the bottom left of the slide is an illustration of how photosynthesis works: light energy is
used to fix carbon into sugar (shown here as number three), and the plant gives off oxygen. This
does not happen at the bottom of the ocean, because no sunlight filters down. However, in place
of sunlight, chemicals coming out of the chimney systems serve as an energy source. In this case,
it is mostly hydrogen sulfide that allows for the fixation of carbon dioxide: bacteria in the
chimney systems take hydrogen sulfide and CO2, produce sugars, and these sugars then serve as
a food source for other organisms.
Slide 12
The chemistry of a “black smoker.” After seawater seeps into the crust (1), oxygen and
potassium (2) and then calcium, sulfate, and magnesium (3) are removed from the water. As the
water begins to heat up (4), sodium, potassium, and calcium dissolve from the crust. Magma
superheats the water, dissolving iron, zinc, copper, and sulfur (5). The water then rises back to
the surface (6), where it mixes with the cold seawater, forming black metal-sulfide compounds
(7). Image courtesy Woods Hole Oceanographic Institution.
Slide 13
In summary, the non-biotic conditions in the hydrothermal vent systems include: very hot
temperatures (in excess of 350°C); extremely high pressures that keep the water in a liquid form
at these very hot temperatures; lots of metals, which serve as an energy source for bacteria; and,
because there are so many reducing type elements here, it is a very anaerobic environment.
Slide 14
There are a number of bacteria and archaea that exist as part of the chimney matrices.
There are organisms that can thrive up to 115°C in these systems, and it is thought that once
scientists have better ways of sampling chimney systems, we will find organisms which can
thrive at even higher temperatures, possibly up to 160°C (the point at which ATP starts
degrading, and what is believed to be the extreme upper limit for life at high temperature). What
is unique about the hydrothermal vent systems is that the bacteria are forming the basis for the
food web, supporting a diverse community of other animals.
Slide 15
This slide shows some of the bacteria and archaea that are associated with vent systems.
In the center of the slide is a vent contact slide - a little piece of glass that is dropped over the top
of a chimney vent, remains there for a period of time as bacterial and archaeal communities start
to grow, then retrieved and analyzed. Analyses from contact slides have allowed a number of
different archaea and bacteria to be identified. Some of the various organisms that have been
found include Methanococcus janaschii, which lives up to 85°C, and can fix carbon dioxide to
methane. The optimal temperature for Pyrococcus furiosus is 100°C, although it can survive
conditions up to 105°C. Archaeoglobus fulgidus is an archaea that lives in temperatures up to
83°C. The slide also shows the bacteria Thermotoga maritima and Aquifex aeolicus. These
bacteria and archaea exist in fairly high concentrations in chimney systems, but they represent
just a few of the many archaea and bacteria that are found there.
Slide 16
These organisms must be adapted to the chimney systems to survive. Membranes will put
in highly saturated fatty acids rather than the non-saturated fatty acids so they do not melt.
Another adaptation is found in the bond between the glycerol and the acid part of the fatty acid.
In a typical membrane this is an ester bond, as shown in the upper portion of the lipid structure.
High temperature organisms have an ether bond instead, which is stronger and helps keep the
fatty acid together. Some hyperthermophiles also have a monolayer instead of a bilayer, which
prevents the fatty acids from peeling apart.
At such high temperatures, proteins will normally denature, but some of the proteins in
these organisms are modified. Most of them have numerous amino acids within their centers.
The amino acids are highly non-polar or hydrophobic, causing the proteins to stick together.
Also, a number of charged amino acids exist on the surface of these proteins that are both
negatively and positively charged. These form ionic bridges that help keep the structure together.
There are also helper proteins, known as chaperonins, that help keep proteins folded at high
temperatures.
The DNA must also be stabilized and prevented from “unzipping”. High concentrations
of magnesium are present in many of these organisms. Magnesium that exists within the
phosphate bonds in DNA stabilizes the DNA. Also, most DNA in hyperthermophilic organisms
tends to be positively supercoiled instead of the negatively supercoiling seen in regular
organisms. This also helps stabilize DNA, although the mechanism of how this works is not
understood. Many archaea have large numbers of histones (the proteins that bind to DNA). The
drawing in the bottom left-hand corner represents a histone associated with a DNA molecule.
The last general category of hyperthermophile adaptations is what is known as
compatible solutes. These are generally either sugar-based or protein-based nutrients that are put
into the cell in high quantities. They help stabilize the proteins and the DNA.
Slide 17
There is an assemblage of animals that are often associated with these hot vents. In
contrast to adjacent cold-water deep bottoms, the rocky surfaces surrounding the hot vents are
covered with animals, including large limpets, clams, and mussels. Vent communities are
dominated by a large type of tube worm, often 1 m in length, that secretes tubes up to 3 m long.
These are called vestimentiferan tube worms. One interesting characteristic of vestimentiferan
worms is that they do not feed, and depend upon a mutual association with bacteria for their
nutrition.
The video on the left shows the tubeworms being attacked by a scale worm, causing one
of them to retract back into its tube. (Pause for video.)
The movie clip on the right is of tubeworms emerging from their tube. (Pause for video.)
One of the most interesting characteristics of these tubeworms is that they do not eat any
food. They have a mutual association with bacteria that provides all of their food.
Slide 18
The bacteria Thiomicrospira lives within the gut systems of these tubeworms.
Tubeworms produce hemoglobin very similar to the hemoglobin found in human blood. This
hemoglobin picks up oxygen, carbon dioxide, and hydrogen sulfide, and provides it to the
bacteria. The bacteria then take the hydrogen sulfide and use it to reduce carbon dioxide to
sugars. The bacteria use the sugars to produce new bacteria and also provide it to the tubeworm.
In short, the tubeworm provides protection to the bacteria, and the bacteria provide food to the
tubeworm – an endosymbiotic relationship.
Slide 19
These bacteria also exist in other organisms, such as bivalves. The mussels shown here
are associated with chimney systems. Calyptogena magnifica grow to a very large size, up to 20
cm; compare this to mussels outside the vent fields that typically grow to just a few millimeters.
Their proximity to the hydrothermal vent systems, and all the bacteria coming out of the systems,
provides these organisms with lots of food.
Slide 20
Snow flurries are big pieces of bacterial aggregates that come out of chimney systems.
This film clip shows bits and pieces of the floc material flowing out of the chimney system.
(Pause for video.)
Various organisms, such as mussels and crabs, eat these bits and pieces of bacterial
matter. In the picture to the right, there are a number of crabs lining up along a vent fissure. In
other parts of the ocean floor, you rarely see many crabs congregated together. But there is so
much bacterial matter available to eat in vent communities, that large numbers of crabs can be
sustained.
Slide 21
Large numbers of crabs will attract other organisms, such as octopi. The film clip shows
an octopus climbing up a chimney system. (Pause for video.)
This concludes the first part of the extreme biology lecture; both cold and hot extreme
environments have been covered, as well as various adaptations that cells make in these
environments.