Extremophiles are organisms that are capable of living, growing and reproducing under extreme conditions such as hot springs, hydrothermal vents, salt lakes, polar deserts and other stressful environments. They are a relatively recent discovery in the field of microbiology considering scientists only found out about them in the late 1960s. Extremophiles discovered are often grouped according to the type of extreme environment that they dwell in. Common groups of extremophiles include thermophiles, psychrophiles, halophiles and radiation-tolerant extremophiles.
Thermophiles can be loosely understood to be heat-loving bacteria. They can be divided into three groups, the moderate thermophiles, thermophiles and hyperthermophiles, growing at temperatures of 50C, 50C-80C and above 80C respectively.
Thermophiles were discovered in the 1960s by Thomas Brock and Hudson Freeze, who suspected the strange colours and structures as seen in hot springs of Yellowstone national Park was caused by bacteria. This was proved to be true and the bacteria found was named Thermus aquaticus, now used everywhere as a source for the polymerase chain reaction, a process used in mass multiplication of DNA. The discovery was a breakthrough at the time as it was previously thought that no life could survive beyond about 45C due to the stability of enzymes and membranes. That piece of information, however, was based on studies of mesophiles (normal temperature bacteria).
Apart from bacteria, cyanobacteria, which are photoautotrophs, are also adapted to living in these hot places. The dominant species of cyanobacterium in a niche is dependent on water chemistry as well as the temperature. The most common thermophilic cyanobacterium in the world is Synechococcus, which is found in almost every single hot spring and thrives at 75C, the highest temperature yet recorded for photosynthesis. Some archaea is also found in these high temperature locations including hot springs and deep sea hydrothermal vents.
Psychrophiles, on the other hand, are organisms that grow at low temperatures, in areas such as the Polar and high altitude regions, deep oceans and high latitude oceans. They can be divided into two groups, the obligate psychrophiles with optimum temperature below 15C and the facultative psychrophiles which are able to adapt to the cold rather than prefer the cold.
In these areas, ice algae growth is extremely prominent. However, cyanobacteria and heterotrophic bacteria growth are also found. An interesting discovery in these high altitude alpine or polar regions is what is now commonly called watermelon snow. The snow in the area is said to have a slight pink colour, especially when compressed by walking and it apparently also has a slight watermelon aroma! This is caused by growth of the chlorophyte Chlamydomonas nivalis and the pink colour is as a result of a carotenoid pigment, Astaxanthin.
Halophiles are organisms that can grow and reproduce in high salt environments and found in such conditions are usually archaea, but also some bacteria and a eukaryotic algal halophile, Dunaliella. These hypersaline habitats can be up to ten time more salty than seawater. This is usually a problem as the high salt content compared to body fluids would cause osmotic movement of water out of the body, thus drying out the organism.
The microorganisms however, employ two strategies to deal with the situation. The first one is the compatible solutes method where osmoprotectants are involved. The second method involves the ionic balance, achieved using potassium ions.
However, different from most extreme environments, there are actually animals, namely the brine shrimps, living in these hypersaline lakes, feeding on the halophilic microbes. The brine shrimps are in turn a food source for flamingos.
Radiation is often deemed harmful by humans and there are actually three types of radiation, namely non-ionizing, ionizing and particulate. It is mainly ionising radiation that we should be aware of as exposure of living tissues to ionising radiation can create positively and negatively charged ions, causing mutation to DNA, with horrible consequences.
However, harmful as it is to humans, the bacterium Deinococcus radiodurans can actually live quite comfortably even with a high radiation dose, with an upper radiation dose limit 2500 times that of humans. Deinococcus radiodurans was first isolated in 1956 from canned meat that was supposedly sterilised by gamma radiation and since then, it has been found in soils, foods and sewage. The organism is of particular importance today as some scientists see it as the key to discovering life on Mars, with its ability to endure not only high levels of radiation, but also high UV and severe desiccation.
The four groups introduced here is by no means a complete list. Other examples include acidophiles, alkaliphiles, metallo-tolerant and pollution-tolerant extremophiles. With all the research done on extremophiles at the moment, one can be quite sure that the list would continue to expand.