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.
Characteristics of 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.
Types of extremophile
The main types of extremophile found in geothermal areas include:
- thermophiles (heat-loving)
- acidophiles (acid-loving)
- thermoacidophiles (heat- and acid-loving).
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.
How extremophiles survive
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.