The boiling point for water is 100°C (212° Fahrenheit). As with humans, the highest temperature at which most animals and plants can live is about 40°C. However, some insects and crustaceans are comfortable up to 50°C and some plants and fungi survive up to 60°C. Above this temperature the only organisms that can survive the heat are some groups of bacteria and archaea.
Cyanobacteria and microbial mats: 70°C
One group common in hot springs are cyanobacteria. They derive energy from the sun through photosynthesis, and produce oxygen much like plants. They will not grow in highly acidic waters. Their upper temperature limit is about 70°C; above this, photosynthesis cannot occur.
Cyanobacteria are usually green, and are found in most thermal areas throughout the world. Some cyanobacteria can be other colours because of pigments that mask the green chlorophyll. These pigments protect the bacteria from the sun’s ultraviolet radiation.
Cyanobacteria in New Zealand
Floating mats of cyanobacteria are present in hot pools in most of New Zealand’s geothermal areas. An exception is the Rotokawa region near Taupō, where most springs are highly acid, with very few cyanobacteria. The presence of cyanobacteria mats can therefore tell us something about the temperature and chemistry of a hot spring without having to measure it.
Thermus aquaticus: over 70°C
Above about 70°C, only non-photosynthesising bacteria can grow, and bacterial growths tend to be less colourful and more difficult to recognise. There are, however, many species of bacteria that prefer to live at these temperatures. One is Thermus aquaticus, originally identified in a hot spring at Yellowstone National Park in the USA. This thermophile, now manufactured artificially, supplies the enzyme used in the technique of replicating DNA from a wide variety of sources. The discovery of Taq polymerase, as the enzyme is called, has led to a revolution in genetic research. It is also used in DNA fingerprinting of humans for forensic and other purposes.
Hyperthermophiles: over 80°C
Hyperthermophiles have adapted to contend with extremely high temperatures, and will not grow at lower temperatures. Of the three broad divisions of life (bacteria, archaea and eucarya), relatively few bacteria can live at these temperatures; most hyperthermophilic organisms are archaea.
Many hyperthermophiles are found in hot springs and around deep-sea hydrothermal vents. The first hyperthermophile to be recognised was Sulfolobus acidocaldarius from Yellowstone National Park, and it was later found in New Zealand hot springs.
Strain 121: beyond boiling point
The organisms that are capable of surviving at the highest temperatures include Pyrolobus fumarii and Strain 121. Both of these species are archaea. Pyrolobus lives in the deep ocean around hydrothermal vents and is able to reproduce at a maximum temperature of 113°C. Strain 121, only recently discovered, is so far the record-holder with a maximum growth temperature of 121°C.
It is generally believed, although not proven, that the maximum temperature at which we might find living micro-organisms is about 150°C. In this heat the chemical bonds that make up important biomolecules such as amino acids begin to break down.