Extreme Biology—Part 2
Amy Grunden
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This lecture is the second part of a two-part series on extreme biology. This section
covers acidic environments, high salt environments, high pH environments, survival under
extreme radiation exposure, and the importance of extremophiles.
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Extreme cold and extreme hot environments were covered in the first part of the series.
The second part covers hot and acidic conditions in sulfuric springs; high salt concentrations
found in salt lakes; and the high alkaline and salt conditions associated with soda lakes.
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Hot sulfur springs develop where there are large amounts of sulfur on the Earth’s surface
and a lava flow underneath the Earth’s crust. Bacteria and other organisms that live in these
sulfuric springs obtain energy by oxidizing elemental sulfur and hydrogen sulfide into sulfuric
acid. This creates very acidic conditions, which is why these springs are known as acidic sulfur
baths.
One of the organisms that exist in hot sulfur springs is known as Sulfolobus. Sulfolobus
can fix carbon dioxide using hydrogen sulfide as a reductant, producing sulfuric acid and
creating a pH as low as, in some cases, 1. Organisms living in very acidic environments must
have adaptations to their cell structure. If many protons from their environment were able to
enter into their cells, they would not survive. These organisms typically have membranes that are
highly impermeable to protons. They also have very efficient pumps that pump protons back out
of the cells, maintaining a neutral inner-cell environment.
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There are other acidic environments on Earth, such as those associated with active mines.
Miners use large amounts of acid to recover the metals that have been extracted, and there are
high levels of acid runoff from mines. A number of organisms are capable of living in this
runoff. For example, Picrophilus oshimae is capable of living at pHs as low as 0.7, and due to
adaptations in its membranes, it cannot grow at a pH greater than 4.
Another organism found in both terrestrial and aquatic acid environments is Cyanidarium
caldarium, which grows at a pH of 0.5. This organism is known to exist in every acid soil and
water system except those associated with the sulfur springs in Hawaii. It is thought that these
sulfur springs are too young for the Cyanidarium to have appeared. Again, these red algae
maintain a neutral pH with adaptations to their cell membrane.
Ferroplasma acidarmanus was discovered in a mine in the United Kingdom. This
organism thrives in acid mine drainage at a pH of 0. This organism has no cell wall, and thus no
supporting cell structure, which has been shown to be important in other bacteria that live in high
acidity. Many ongoing studies are trying to determine what adaptations exist within Ferroplasma
that allow it to grow at such high acidity without having a cell wall.
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The next type of extreme environment is characterized by high salt concentrations: for
instance, the Great Salt Lake, and the salt evaporation ponds found in and around San Francisco
Bay, where salt evaporates are maintained for salt mining. Note that these areas (in the top
photo) are red or pink; that is because the resident organisms have a number of photosynthetic
pigments that produce the pink and the red colors. A number of organisms have been identified
that are capable of living in both of these environments. Most belong either to the archaea or the
bacteria, but there are some algae as well.
Halophiles require high amounts of salt in order to live; most need at least a molarity of
salt of 1.5—equivalent to about 8% salt weight per volume. Many require 2 to 4 molar salt,
equivalent to up to 23% salt, and some can survive at the saturation point of salt, which is 36%.
To adapt to these conditions, these organisms have modified membranes, in this case
stabilized by sodium. When they are put into low salt environments their membranes fall apart
and they die. Typically, these organisms also contain efficient pumps that maintain very high
levels of inner cellular potassium chloride, to balance the sodium chloride that is outside the cell.
A number of halophiles also have unusual photosynthetic capabilities.
These organisms produce some interesting photopigments. The photopigments have
light-sensitive proteins, known as halorhodopsins, that import high levels of potassium chloride
(up to 4 to 5 molar) into the cell. Another pigment, known as bacteriorhodopsin, is involved in a
form of photosynthesis in the halobacterium.
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Algae can also be found in halophilic environments. Like other algae, they are
photosynthetic. They are red colored due to high concentrations of beta-carotene, which protects
the cells from intense light exposure. (The evaporation ponds receive a lot of sunlight.)
Halophilic algae, particularly Dunaliella salina, are capable of replacing sodium ions with
potassium ions. Another adaptation is the algae’s ability to modify its photosynthetic pathway to
stop producing sugar and start producing glycerol when it is in an environment where the salt
concentration is too high. Glycerol is water-soluble and prevents the cell from dehydrating.
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There are very low oxygen concentrations in the highly saline waters of evaporation
ponds and salt lakes. Respiration, which produces energy for cells, requires a lot of oxygen.
These cells, because they’re oxygen limited, need another way of making energy.
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To do this, they use a light pigment known as bacteriorhodopsin. Bacteriorhodopsin has a
chromophore known as retinol, a purple-pink pigment that is very similar to what is found in the
cones and rods in our eyes. When light strikes it, retinol changes conformation slightly, going
from a trans-form of a double bond to a cis-form. This enables it to pick up a proton, which it
moves outside the cell. This produces a large gradient, with many protons on the outside of the
cell and few protons on the inside, which is a driving force to produce ATP.
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The next category of extreme environment also has high salt, but it has another
complication as well: high alkaline conditions, or very high pH. These environments are
typically associated with industrial runoff from detergent and dye manufacturers, although there
are naturally occurring lakes in Africa, Australia, and the Middle East with cover carbonatebased rock, giving them very high levels of base.
There are very few known organisms—and no known eukaryotes—that can tolerate high
alkaline conditions. Prokaryotes have been isolated from these conditions, and one of these is a
kind of cyanobacteria known as Plectonema, which is able to thrive at a pH of 13.
One adaptation to high alkaline conditions is, again, an efficient pump, but this time to
move protons into the cell. This sodium proton pump balances out the base on the outside.
There are very high levels of cyanobacteria in the soda lakes in Africa, which are a food
source for large flocks of flamingos. One lake in particular is home to more than one million
flamingos. These flamingos eat the cyanobacteria and produce up to 15 tons of feces and urine a
day. This waste goes back into the lake system, providing food for other non-cyanotype
(photosynthetic) bacteria, such as natronobacter type bacteria. Soda lakes with flamingo
populations support a large amount of life.
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The last extreme environment involves extremes in radiation exposure. Deinococcus
radiodurans is a bacterium capable of surviving very high levels of radiation - 3 to 5 millions
rads, which is about 3,000 times more radiation exposure than a human can withstand. A photo
of Deinococcus radiodurans is on the left-hand side of the slide. It forms spherical clumps of
cells, and has photopigments that give it a red color. On the right is a graph of radiation
exposure. The top line with the squares represents exposure of Deinococcus radiodurans to
various levels of radiation. There is a slight drop off in survival after 6,000 Grays worth of
radiation; even after 10,000 Grays of radiation, half the cells are still living. Compare that to the
survival of the bacteria E. coli, which is shown by the line with diamonds. Just 1,000 Grays is
completely fatal- to E. Coli. Scientists have found that D. Radiodurans has very high rates of
DNA repair. Radiation exposure breaks apart the chromosome, and the fact that the Deinococcus
has the capability of repairing its DNA is what makes it capable of surviving these levels of
radiation exposure.
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This is a DNA gel showing Deinococcus radiodurans and E.coli DNA that has been
exposed to radiation. There are size markers on the left-hand side of the gels, which shows the
size of the DNA that had been chopped up by radiation.
The left-hand gel is from Deinococcus radiodurans. About 12 hours after radiation
exposure, the chromosomal DNA is essentially equivalent to the pre-radiation exposure. Contrast
this with the irradiated E.coli; after 9, 12, even 29 hours, the chromosomal DNA has not been
repaired. It is this repair mechanism that is thought to allow Deinococcus radiodurans to survive
extreme radiation exposure.
NASA is very interested in Deinococcus radiodurans for probing survivability in
extreme UV exposure and in space vacuum. They conducted rocket experiments in which filters
containing Deinococcus radiodurans were placed on a rocket, shot up about 300 km into space,
exposed to extreme UV for more than six minutes, and then brought back down. The
Deinococcus was capable of surviving the rocket trip, although the UV exposure combined with
a space vacuum posed more of a survivability challenge than just exposure to the vacuum alone
or the UV alone. NASA is interested in potentially using Deinococcus radiodurans to produce
pharmaceuticals and foodstuffs in space, because they know this organism can survive both the
vacuum and UV exposure of space. On Earth, there is great interest in using Deinococcus
radiodurans to reduce or eliminate radioactive waste. Deinococcus radiodurans are being
engineered to get rid of metals and other waste products that are associated with radioactive
waste sites.
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Other than expanding our basic knowledge of biology, extremophiles are useful because
they produce enzymes that can function under extreme conditions. For instance, enzymes can be
harvested from hyperthermophiles that are capable of converting sugars under high heat
conditions. These can be used in the conversion of cornstarch into high fructose corn syrup, for
instance, which is typically conducted under high heat conditions.
Another area of interest is in the modification of frozen foods. For instance, enzymes
have been isolated from psychrophiles that prevent ice crystal formation in ice cream, improving
its texture and flavor.
Acidophilic proteins are currently being used to remove sulfur from coal, oil and also
from herbal products.
From alkalophiles, both cellulases and proteinases are being harvested and used in
detergents to remove grass, grease, and ketchup stains.
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Extremophiles on Earth may also have implications for research on other planets, since
some extreme environments on this planet are very similar to what we might expect to find on
Mars or on Jupiter’s moon, Europa. Mars may have hydrothermal vent type systems and also
very cold areas that may have supported life when there was water on the planet. Europa has
what looks to be a liquid lake under a frozen ice cover, very similar to Lake Vostoc.
Understanding how life exists in extreme environments on Earth will help us model experiments
that we can then take to other planets.