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 carbonate-based 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.