When the sun sets on the Antarctic summer, the temperature plummets and everything freezes. Birds flee north and most seals seek warmer waters. An exception is the Wedell seal which remains a lonely outpost of mammalian life on the frozen pack ice (apart from the occasional naturalist). This hardy survivor winters over in the Antarctic by using its teeth to drill holes in the ice to the relative warmth of the ocean waters below (at temperatures a few degrees above freezing – far warmer than the air above) and its still plentiful food supply.
On the Antarctic landmass, covered by three miles of ice, there is no escape from the winter cold. The lichen, algae, nematodes and mites freeze within the snow, ice and soil to await the return of the sun. No living thing moves.
Except, that is, for the Emperor penguin. When all other animals flee north, the Emperor penguins head south to the freezing continental interior where they congregate in nesting sites on the central Antarctic plateau. The female lays her single egg and heads north herself, to the ocean, leaving the male penguin to perform perhaps nature’s most exemplary display of paternal duty. He gathers the egg in a pouch and, huddled together with as many as twenty-five thousand other penguins, he braves the coldest place on Earth. For three months the male penguin endures bitterly cold temperatures, searing winds and hunger before his mate finally returns with food for the newly hatched chick (but none for the stalwart male who must make his own way to the sea to find his next meal).
So do the activities of the Emperor penguin represent the lower temperature limit for active life on Earth? Not really, for the Emperors do not live at a low temperature. What the colony achieves is essentially equivalent to what the town of Dawson manages to do for its inhabitants – it maintains an equable microclimate. Each penguin shuffles continually around the colony, burning his fat store, generating heat which remains trapped within the huddled mass of feathers in the nesting site. The birds are thus able to maintain their internal body temperature close to the avian optimum of 42°C, well above the freezing temperatures outside the colony. Impressive though the penguins’ adaptation to the Antarctic winter is, their cells do not function at temperatures any lower than our own.
To find the lower limit for active life we must plunge into the waters below the ice of Antarctica. The world’s oceans occupy two thirds of the earth’s surface and approximately ninety per cent of the surface waters are colder than 5 degrees. Yet the oceans teem with life and are our most productive ecosystem. Most of the fish and invertebrates that live in the sea have body or cell temperatures that remain close to 5°C for most of their lifespan. Although the pace of life does tend to slow down at these temperatures (with concomitant longevity for many marine animals – sea turtles may live for more than two hundred years), life does thrive. Even temperatures a few degrees below the normal freezing point of water (salty water freezes below 0°C) are tolerated by Antarctic fish which incorporate a kind of antifreeze protein in their cells to prevent their tissues from freezing.
The coldest waters in the world are probably Antarctica’s brackish pools. Don Juan Lake is saturated with about forty-five per cent calcium chloride and does not freeze unless the water temperature falls below – 48°C. There is no photosynthetic activity in the lake, but live bacteria have been recovered from its waters. Whether these are true colonists or have merely drifted into the lake is unclear. They are certainly active when warmed to zero degrees. Liquid water is also found within the Antarctic Dry Valley lakes. Though the surfaces of these lakes are permanently frozen, geothermal activity can warm the deeper waters to temperatures as high as 25°C. A team of scientists drilled into these polar oases and extracted water samples that were found to contain a rich microbial flora with many unique bacterial species. The bacteria thrive just below the ice layer, where the temperatures may be as low as – 2°C, and in brackish waters may drop to – 12°C.
Freezing kills most living organisms. There are two ways ice damages living cells. Firstly, the mechanical shearing brought about by the formation of sharp ice crystals in cells, slices through the membranes, making the cells leaky, so that they die once thawed. Another problem arises because freezing expels dissolved salts that accumulate between the ice crystals, reaching concentrations toxic for living cells. There are, however, many organisms that can endure freezing. Even animals, particularly insects, frogs and lizards, can be frozen and thawed. Some species of frogs and turtles actually encourage the formation of ice crystals within their tissues. They make ice nucleation proteins which promote rapid freezing with the formation of smaller, less damaging ice crystals. Many microbes survive freezing due to the presence of cryoprotectants in their cells. Dormant life forms (seeds and spores) may survive for long periods, frozen, and have even been shown to endure temperatures close to absolute zero (–273°C, the temperature corresponding to a complete absence of heat – covered further in Chapter Six). The spores and seeds prevent ice formation by excluding free water from their cells. Many also produce large amounts of simple sugars that harden to form a glassy casing to protect the delicate enzymes and membranes inside. Even watery animal cells can also survive freezing. Human egg and sperm cells and even human embryos are routinely frozen during fertility treatments. The key to their survival seems to be freezing under carefully controlled conditions which minimize the damage to cells by promoting the formation of only very tiny ice crystals.
So although the frozen state is generally damaging to living cells, it does not necessarily destroy life. It does, however, prevent all living activity. Though ice and snow may shelter frozen seeds and spores and even frozen animals, nothing stirs in solid ice. Everything once alive is either dead or dormant. Active life is clearly incompatible with water in its solid state, ice. So the lower temperature limit for activity seems to be that experienced by marine and fresh water life in Antarctica, which may remain active down to about – 12°C.
HOW HOT IS TOO HOT?
To explore the upper extremes of temperature, our spacecraft must leave the Antarctic to travel to the baking deserts. Life is surprisingly abundant in many desert regions. Burrowing mammals survive by engineering air-conditioning systems to keep their tunnels and hence their bodies relatively cool during the day and restrict their hunting to the cool (often very cold) night hours. Their prey – small snakes, reptiles and arthropods – may tolerate temperatures up to 50°C. Temperatures as high as 60°C have been recorded in foraging ants as they race across the hot desert sands of the Sahara. But these are transient temperatures and cannot be tolerated for long by any animal. Plants including cacti and a number of desert grasses can tolerate quite high temperatures, but do not grow above about 45°C. Mosses and lichens may survive and grow at temperatures up to about 50°C. No plant or animal is known to be able to thrive above this temperature. This appears to be the upper limit for multicellular life on our planet.
It has long been known that bacteria are capable of growth at higher temperatures. Thermophilic (heat-loving) bacteria, growing at temperatures as high as 65–70°C, have been isolated from a number of hot habitats. Lichen and other microbes penetrate the surface of desert rocks. Even the sand is inhabited by microbes. The surface of sand drifts is often crusty from the presence of a tangled mesh of photosynthetic microbes and lichen that live in its top millimetre. These microbial mats can be quite productive, with roughly the same density of chlorophyll as a plant leaf, but are limited by the availability of moisture and must await the arrival of dew, fog or a rare shower before they are able to set their photosynthesis machinery into action. Thermophilic microbes can also be found in more mundane environments such as compost heaps, slag heaps (that reach temperatures as high as 60–70°C), and domestic hot-water systems.
It was generally thought that temperatures higher than about 75°C were incompatible with life. This view changed dramatically when, in the late 1960s, Thomas Brock, a microbiologist from the University of Wisconsin, was walking in Yellowstone National Park. The park, famous for its hot volcanic springs, lies within a volcanic