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Terrestrial Life in Extreme Environments

This lecture begins to examine the subject of life in the Solar System, with a review of terrestrial life in extreme environments.

  1. Review: Food and Respiration
  2. The Extremophiles
  3. The Thermophiles
  4. The Acidophiles
  5. The Psychrophiles
  6. The Halophiles
  7. Prospects for Life Elsewhere

Review: Food and Respiration

In Lecture 4, the progressive development of life on Earth was discussed. The first living organisms, which developed from prebionts, were the prokaryotes. These lack an inner membrane around a nucleus of genetic material, and are similar to present-day bacteria. Evolutionary pressure on the prokaryotes led to changes in the ways in which they obtained their food, and the ways in which they converted their food into energy (i.e., respiration):

  • Initially, the prokaryotes were all heterotrophic: they obtained their food from surroundings. They were also anaerobic respirers: they converted their food into energy via chemical reactions (e.g., fermentation) which worked in the absence of free oxygen.
  • A shortage of food soon occurred, leading to the development of autotrophic prokaryotes, which synthesized their own food.
  • The first important process used by the autotrophs for food synthesis was anoxygenic photosynthesis, where carbon dioxide is combined with hydrogen sulphide and sunlight to produce sugar and sulphur.
  • A shortage of hydrogen sulphide led to the development of autotrophs which used oxygenic photosynthesis, where carbon dioxide is combined with water and sunlight to produce sugar and oxygen.
  • With the free oxygen produced by oxygenic photosynthesis, new organisms developed which used aerobic respiration instead, a far more efficient process.

It is tempting to suppose that all organisms went through thissequence of steps. However, recent evidence has indicated that there are groups of organisms which got stuck at a given stage, the particular stage being the best adapted for the environment in which they existed. Many of these archea (ancient prokaryotes) are still found today in extreme environments.

The Extremophiles

Extremophiles are organisms (almost always archea) which are adapted for living in very extreme environments, where other life wouldn't stand a chance. Examples of extremophiles include:

  • thermophiles: exist in hot environments
  • acidophiles: exist in acidic environments
  • psychrophiles: exist in cold environments
  • halophiles: exist in salty environments

The first use of the term `extremophile' appeared in a 1974 paper by R. MacElroy. Since then, interest in extremophiles has grown enormously (there is now a whole journal dedicated to them), due to their importance to industrial applications. But they can also provide important insights into life on other planets.

The Thermophiles

The thermophiles reproduce and grow readily in temperatures higher than 45C. Perhaps the most famous is Thermus aquaticus, discovered in hot springs in Yellowstone National Park.

Thermus aquaticus

Thermus aquaticus

A hot spring in Yellowstone National Park

A hot spring in Yellowstone National Park

The discovery was surprising, because at the temperatures within the springs (around 70C), normal organisms either die immediately or are unable to reproduce. The key ingredient in the survival of T. aquaticus was a special variant of the DNA polymerase enzyme. This enzyme is used during the process of reproduction to split DNA into two strands. Although it is usually destroyed by heat, T. aquaticus uses a variant which remains operational at high temperatures. This variant (known as Taq polymerase) is now a key ingredient in the polymerase chain reaction (PCR) technique, which is used for DNA fingerprinting.

To date, over 50 thermophiles are known. The most extreme is Pyrolobus fumarii, which can multiply in temperatures up to 113C. This grows on the walls of black smokers, undersea thermal vents which eject water at very high temperatures and pressures. The water is rich in minerals, and these (in combination with carbon dioxide) provide the food source for P. fumarii. For this reason, P. fumarii is classified as a chemoautotroph: it synthesizes its own food from surrounding chemicals.

Black smokers

Black smokers

The Acidophiles

The acidophiles are adapted for life in very acidic environments. Discovered in Yellowstone National Park at the same time as T. aquaticus, Sulfolobus acidocaldarius inhabits hot (85C) and sulphurous thermal springs. This environment can be characterized as boiling sulphuric acid! S. acidocaldarius is specially adapted to its environment, by keeping its insides much less acidic than the surroundings (intense acidity destroys DNA). S. acidocaldarius synthesizes its own food from hydrogen sulphide and carbon dioxide, in a process similar to anoxygenic photosynthesis. However, it does not require light, so it is classified as a chemoautotroph.

Sulfolobus acidocaldarius

Sulfolobus acidocaldarius

The Psychrophiles

The psychrophiles are adapted for life in cold environments, close to the freezing point of water. One example is Polaromonas vacuolata, which lives in antarctic sea ice. This organism reproduces best at temperatures of 4C; above 13C, it cannot reproduce at all!

Not all psychrophiles are archea; there are some eukaryotes amongst them, and even some multicellular organisms. In 1997, colonies of tubeworms were discovered living in methane hydrate deposits (a combination of natural gas and ice), 1,800 feet down on the bottom of the Gulf of Mexico. These ice worms are believed to get their food via symbiosis with colonies of chemoautotrophic bacteria living within them.

Ice worms

Ice worms

The Halophiles

Halophiles thrive in very salty environments, such as salt lakes and salt evaporation ponds. In a salty environment, there is a tendency for organisms to lose water through their cell walls through the process of osmosis, where water is drawn from low salt concentrations to high salt concentrations. The halophiles avoid this fate by retaining internally large amounts of specific salts, thus preventing dehydration.

One of the best-known examples of living halophiles can be found in Shark bay, Australia. The bay is full of stromatolites, rocky formations up to 1.5 metres high which were built by colonies of halophile cyanobacteria (the first ever oxygenic photosynthesizers). These cyanobacteria were able to survive until present day due to the fact that Shark bay is far too salty for any predators to exist in it.

Shark bay stromatolites

Shark bay stromatolites

Prospects for Life Elsewhere

The recent discoveries of many different types of extremophile has important implications for life elsewhere in the solar system. Life can take or synthesize food from a variety of sources; it can tolerate heat or cold; it can deal with highly acidic or alkalinic environments; it can cope with the presence of intense radiation. In fact, it appears that as long as there is liquid water available, life can flourish.


Updated 2009-10-13 12:37:56