Читать книгу Quantum Evolution: Life in the Multiverse - Johnjoe McFadden - Страница 10

The average temperature in London is about 13° Centigrade, rarely going above thirty degrees or dropping much below zero. Most higher plants and animals are happiest within a similar range of temperatures, so it is hardly surprising that life is particularly abundant in these latitudes. Humans do live in far more extreme environments. In Timbuktu, the Saharan temperature can rise to 50°C, whilst the inhabitants of Dawson in the Yukon valley endure nights where temperatures drop to –30°C. However, even mad dogs and Englishmen would succumb to heatstroke under a Saharan midday sun and frostbite would soon freeze anyone foolish enough to brave the winter nights of Alaska. Man survives these extremes of temperature by building shelters to provide warmth or shade, thus creating a more equable microenvironment protecting him from the heat and cold outside. The range of temperature that humans can endure (without resort to ingenuity) is actually quite narrow, lying somewhere between 5°C and 30°C.

Many animals survive more extreme environments. Often considered a barren wasteland, during its summer months the Antactic is teeming with life. Millions of seabirds and sea mammals nest on its coasts and fringe of drifting pack ice. Even the snow harbours life. Warmed by the summer sun, the interior of the pack ice becomes laced with channels of slushy brine filled with photosynthetic bacteria and algae. Antarctic mites burrow through the snow to graze upon on the microscopic bloom. The summer melt releases billions of these microbes into the ocean, to be harvested by the filter-feeding krill and channelled into the food chain supporting the seals, penguins and whales of Antarctica.

Within the interior, conditions are far harsher. The coldest temperature ever recorded was a chilly one hundred and twenty-nine degrees below zero at the Russia Vostok station in July 1983. Yet Antarctica is far from sterile. It harbours more than a thousand plant species, mostly mosses, fern and lichen. The topmost peaks of mountain ranges that rise above the ice are often colonized by lichen. Indeed, brown yellow and grey spots of lichen are ubiquitous on exposed rocks throughout the world. In Donegal, Ireland, where I was born, lichen has been scraped off rocks for centuries and used to colour wool for the cloth known as Donegal tweed. The same coloured lichen spots cling to the paving stones of disused paths and cover the crumbling ruins of ancient buildings. Lichen is actually two organisms: a fungus and an algae (or sometimes a bacterium) living in symbiosis. The photosynthetic algae provide nutrients that feed the fungus. What the fungus contributes is less clear, but it probably provides support and the ability to extract essential minerals from the rock. The success of this pairing allows lichen to colonize extreme environments barred to fungi or algae alone. However, even lichen cannot perform photosynthesis below zero. Although they survive the freezing temperatures of the Antarctic winter, they must await the warming sun to heat their rock substrate to a balmy 0–10°C before they can grow and reproduce. A similar freeze and burst strategy is followed by most of Antarctica’s flora which await rare warm spells to initiate a frantic flurry of growth and reproduction – generating millions of frost-tolerant spores or seeds – and closing down again when wintry conditions return.

The dry valleys of Antarctica are probably the most inhospitable regions on Earth. Bone-dry hurricane force winds race unimpeded across the Antarctic plateau, bringing temperatures dropping to –52°C. The winds quickly evaporate any traces of moisture from stray snow drifts. Bodies of long-dead seals and penguins lie perfectly preserved, desiccated, in conditions where even microbial decomposition is halted. A group of American scientists led by Diana Freckman of Colorado State University occupy a research station near the permanently frozen Lake Hoare in the McMurdo Dry Valley. Studying the ecology of the area, they have discovered a curiously simple ecosystem within the soil. Though the land is frozen half a mile deep, it is covered by a thin dry soil eroded from the rocks by the scouring wind. This soil harbours frost-tolerant bacteria and algae which are grazed upon by one or two species of nematode worm, themselves the prey of a third species of worm. The worms are mostly present in the soil as desiccated husks. Only when a rare trickle of snow meltwater moistens the soil do the microbes and nematodes spring into activity, hurriedly grazing, eating and reproducing before the freeze entombs them again. Nobody knows how long the worms can endure this Rip Van Winkle lifestyle, years certainly, but perhaps decades or even centuries.

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.