Because of their high temperatures, hot springs have traditionally been regarded as devoid of life. Research during the last few decades has, however, shown that they are teeming with microbes – tiny survivors known as extremophiles.
Extremophiles are organisms that live in extreme conditions of temperature, acidity, salinity, pressure, or toxin concentration.
Most extremophiles are single-celled micro-organisms belonging to two domains of life – bacteria and archaea. These differ from fungi, plants, animals and other single-celled organisms because their genetic material is dispersed through the cell rather than being enclosed within a nucleus.
The main types of extremophile found in geothermal areas include:
For an organism to be classified as an extremophile, it must live its entire life at these unusual conditions. Many will actually die if conditions are less extreme.
In 1966 Thomas Brock showed that microscopic organisms thrived in hot springs at Yellowstone National Park, USA. Since then extremophiles have been found all over the world, and their study is one of the rapidly expanding areas of biological science. Because of its variety of thermal features, New Zealand is one of the best places to study these organisms.
Normal human body temperature is 37°C, and a comfortable bath temperature is about 40°C. Hotter than that, things get unpleasant and we labour to keep our temperature stable. After five seconds at 60°C, our skin will be permanently damaged. We are definitely not thermophiles.
From the genetic makeup of extremophiles, it is conjectured that the earliest life on earth evolved in a hot spring or deep-sea thermal vent several billion years ago. New Zealand’s geothermal environments may represent similar conditions. It is also possible that geothermal environments may be similar to the conditions on other planets or moons and, if life exists in these places, it may resemble the extremophiles we see on earth.
On a practical level, extremophilic organisms are of interest because they contain special molecules such as proteins that are resistant to high temperatures and have potential applications in biotechnology.
Unlike most organisms that require organic (carbon-containing) compounds for their energy or can carry out photosynthesis, some extremophiles can produce energy from inorganic compounds.
The hot water found in geothermal areas is formed as the result of heating of groundwater by deep heat sources. Very hot water is highly corrosive. As it moves through fractures deep in the earth it can dissolve minerals or convert them to other minerals.
When the water reaches the surface, it forms hot spring fluids. These may contain high concentrations of dissolved chemicals such as chloride, sulfate, sodium, potassium, bicarbonate and silica. Also present are minor dissolved chemicals including calcium, iron, aluminium, arsenic, ammonia, hydrogen and hydrogen sulfide. Some of these provide the basic energy source and nutrients for a number of extremophile micro-organisms.
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.
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.
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.
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 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.
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.
Many of New Zealand’s hot springs and volcanic craters are very acidic. There are many acid-loving extremophiles that thrive there, such as the alga Cyanidium caldarium. Unlike other algae, it is able to survive in water with pH values down to zero – close to the level of battery acid. Cyanidium is also a moderate thermophile and can grow in temperatures up to 56°C. Growths of Cyanidium are present in New Zealand geothermal areas where acid waters are found, including Rotokawa and White Island.
Scientists use the pH scale, from 0 to 14, to measure how acid or alkaline a solution is. Pure water is neutral, with a pH of 7. Values below 7 are acid, and those above are alkaline. For comparison, lemon juice has a pH of 2; sea-water is 8.2; and a concentrated ammonia solution is about 12.
Despite being able to survive extremely acid conditions, these organisms cannot tolerate such acidity inside the cell because essential molecules such as DNA become unstable. Acidophiles have evolved mechanisms to pump acids out of the cell in order to maintain weak to neutral acid conditions (pH 5–7). Many acidophiles also excrete protective, acid-resistant polysaccharides on their cell membranes.
Most acidophilic types of bacteria and archaea grow where sulfur compounds are present. This is not surprising given that the origin of very acid conditions is usually related to the chemical transformation of sulfur.
Examples of common acidophiles are Alicyclobacillus acidocaldarius (a moderately thermophilic, acidophilic bacterium) and the extremely thermophilic Sulfolobus acido caldarius, a member of the archaea domain.
When hot springs overflow they often form layers of sinter – a rock made of very fine-grained silica – that takes the form of flats, terraces and mounds. Sinter terraces are one of the most distinctive features of geothermal areas, and provide evidence for past geothermal activity.
Sinter deposits are covered with a wide variety of complex textural features such as spicules (spike-like growths of silica) and mini-terraces. Their surfaces are also extensively colonised by micro-organisms, which in many geothermal areas show their presence by colouring the sinter. The coloration of the Pink Terraces, obliterated in the 1886 eruption of Mt Tarawera, was probably due to the presence of extensive growths of a pigmented thermophilic bacterium such as Thermus ruber.
When high-temperature geothermal fluids reach the surface, they undergo drastic cooling. Much of the mineral material dissolved in these fluids can no longer remain in solution and begins to precipitate as the fluid cools.
The most common precipitate is amorphous silica. This is composed of silicon dioxide (SiO2), and has no regular crystal structure. Amorphous silica forms spheres so small they cannot be seen with a microscope. These spheres stick together to coat surfaces. They continue to increase in size, forming a continuous coating of silica much like a very thin layer of glass. Amorphous silica will coat any surface, including twigs, feathers, pine cones, newspaper, bottles and micro-organisms.
Other less common minerals that can be found in New Zealand deposits include calcite (calcium carbonate), gypsum (calcium sulfate), pyrite (iron sulfide) and other metal sulfides.
There is continuing debate among scientists about the contribution of micro-organisms to the growth and development of the many textural varieties of sinter deposits. Detailed study of sinters has shown that they are composed of thin layers of chemically precipitated silica interleaved with silicified mats and clots of thermophilic organisms. Do the organisms actively encourage silica precipitation, or are they just passive recipients of the silica?
The resolution of this controversy is of great importance. The discovery of biologically generated (biogenic) sinter textures in the ancient fossil record of the earth could help us to understand the origin and evolution of life. In addition, the discovery of preserved hydrothermal deposits containing biogenic textures on extraterrestrial bodies may provide us with evidence of life beyond earth.
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