Read more: Extreme survival: The toughest life forms on Earth
Meet the bacterium you can boil, the ant that braves the Sahara鈥檚 midday sun and a worm that sticks its tail to hot rocks
Heat is a major challenge for life. On land, too much heat means that water evaporates or boils away, and without water nothing can survive.
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That is obviously not a problem under the sea. Temperatures can reach 400 掳C in deep-sea hydrothermal vents, where water is heated by the Earth鈥檚 interior. The upper temperature limit that organisms can endure becomes the point at which complex molecules, like DNA and proteins, start to break down; the surfeit of energy literally shakes apart their chemical bonds.
The hottest recorded temperature at which life has been able to grow is 121 掳C. This record is held by a microbe called simply Strain 121, which normally lives at temperatures of around 100 掳C in hydrothermal vents; it barely seemed to notice when it was heated to 121 掳C in the lab, in 2003 (). Even at 130 掳C the bacterium was still hanging in there, but it could not replicate until the temperature dropped.
Extreme 鈥渢hermophiles鈥� like Strain 121 have similar cellular chemistry to you and me. The difference is that their proteins and DNA are more tightly packed, so they can withstand more heat energy before unravelling. However, at temperatures over 100 掳C, essential metabolites such as ATP break down in seconds. So the upper temperature limit on life is set by how quickly a cell can replace these chemicals.
Multicellular life finds it harder to stand the heat, although it is not really known why. Most such organisms hit problems above 40 掳C, and no eukaryotes 鈥� organisms with a membrane-bound nucleus 鈥� live full-time above 60 掳C, with one exception, kind of. The Pompeii worm, Alvinella pompejana, was discovered at hydrothermal vents off the coast of the Galapagos Islands in the 1980s. Unusually, their tails experience scalding water of up to 80 掳C, as they are attached to the vent walls. The rest of their body is far enough away from the very hot water.
How the worms鈥� tails cope is not understood, partly because the animals don鈥檛 live very long in the lab. One factor may be their high level of collagen, a protein that is relatively stable at high temperatures. The hard tubes they build to live in and the symbiotic bacteria that grow in furry clumps on their bodies also offer some protection.
On land, the animal that can cope with the highest temperatures is a desert-dweller: the Saharan silver ant, Cataglyphis bombycina. This can withstand temperatures above 53 掳C for a few minutes while foraging for other creatures that have perished in the midday sun. How do the ants escape heat exhaustion themselves? Before leaving the nest they stock up on heat shock proteins, which help other proteins keep their shape. While out they tend to climb up anything tall they find to cool off in the breeze, whether it be plants, or scientists there to observe them.
