Universe a grand tour of modern science Phần 5 doc

Antarctica seemed more challenging. Penguins huddle together in the winter darkness to minimize their heat loss. On the other hand the nematode Panagrolaimus davidi, a worm almost too small to see, which lives among algae and moss on ice-free edges of Antarctica, regularly freezes solid each winter. It can chill out to minus 358C with virtually all its metabolism switched off, and then revive in the spring. In laboratory tests, it can go down to minus 808C without problems. Investigating the nematode’s survival strategy, Wharton found that the rate of cooling is critical. It survives the rather slow rate experienced in the wild but fast freezing in liquid nitrogen kills it. Cryptobiosis is the term used for such suspended or latent life. Various animals and plants can produce tough larvae, seeds or spores that seem essentially dead, but which can survive adversity for years or even millennia and then return to life when a thaw comes, or a shower of rain in the desert. To Wharton, these cryptobiotic organisms are not true extremophiles. Assessing the ability of larger animals to cope with extreme conditions, as compared with what archaea and bacteria can do, Wharton judged that only a few groups, mainly insects, birds and mammals, are much good at it. Insects resist dehydration with waxy coats. Warm-blooded birds and mammals contrive to keep their internal temperatures within strict limits, whether in polar cold or desert heat. On the other hand, fishes and most classes of invertebrate animals shun the most severe habitats—the big exception being the deep ocean floor. ‘We think of the deep sea as being an extreme environment because of the high pressures faced by the organisms that live there,’ Wharton commented, a quarter of a century after the discovery of the animals of the hydrothermal vents. ‘Now that the problems of sampling organisms from this environment have been overcome, we have realized that, rather than being a biological desert, as had been assumed, it is populated by a very diverse range of species. . . . Perhaps we should not consider the deep sea to be extreme.’ E For related subjects, see Global enzymes, Life’s origin, Tree of life and Extraterrestrial life. 296 extremophiles T he road from mumbai to pune, or Bombay to Poona as the British said during their Raj, takes you up India’s natural rampart of the Western Ghats. It’s not a journey to make after dark, when unlit bullock carts compete as hazards with the potholes and gullies made by the monsoon torrents. Natural terraces built of layer upon layer of volcanic rock give the scarp the appearance of a staircase, and Ghats is a Hindi word for steps. In the steep mountains and on the drier Deccan Plateau beyond them is the triangular heartland of the Indian peninsula. It is geologically odd, consisting mainly of black basalt, up to two kilometres thick, which normally belongs on the deep ocean floor. Preferring a Scandinavian word for steps, geologists call the terraced basalt ‘traps’. The surviving area of the Deccan Traps is 500,000 square kilometres, roughly the size of France. Originally the plateau was even wider, and rounder too. You have to picture this region as hell on Earth, 65 million years ago. Unimaginable quantities of molten rock poured through the crust, flooding the landscape with red-hot lava and spewing dust and noxious fumes into the air. It was not the only horrid event of its kind. Flood basalts of many different ages are scattered around the world’s continents, with their characteristic black bedrock. In the US states of Washington and Oregon, the Columbia River Plateau was made in a similar event 16 million years ago. The Parana flood basalt of south-east Brazil, 132 million years old, is more extensive than the Deccan and Columbia River basalts put together. Plumb in the middle of Russia are the Siberian Traps. Around 1990 several investigators confirmed that the flood basalt there appeared almost instantaneously, by geological standards. Through a thickness of up to 3500 metres, the date of deposition was everywhere put at 250 million years ago. This was not a rounded number. The technique used, called argon–argon dating, was accurate to about 1 million years. The basalt builds the Siberian Plateau, which is flanked to the east by a succession of unrelated mountain ranges. To the west is the low-lying West Siberian Basin, created by a stretching, thinning and sagging of the continental crust. During the 1990s, prospectors drilling in search of oil in the basin kept hitting basalt at depths of two kilometres or more. 297 Geologists at Leicester arranged with Russian colleagues to have the basalt from many of the West Siberian boreholes dated by argon-argon at a Scottish lab in East Kilbride. Again, it all came out at almost exactly 250 million years old. So a large part of the flood basalt from a single event had simply subsided out of sight. This meant that the original lava flood covered an area of almost 4 million square kilometres, half the size of Australia. The speed and magnitude of the event make it ghoulishly fascinating. In Iceland in 1783 the discharge of just 12 cubic kilometres of basalt in a miniature flood killed the sheep by fluoride vapour and caused ‘dry fog’ in London, 1800 kilometres away. In Siberia, you have to imagine that happening continuously for a million years. The Siberian affair’s most provocative aspect was that the huge volcanic event coincided precisely with the biggest disaster to befall life on the Earth in the entire era of conspicuous animals and plants. At the end of the Permian period, 250 million years ago, the planet almost died. About 96 per cent of all species of marine animals suddenly became extinct. Large land animals, which were then mammal-like reptiles, perished too. ‘The larger area of volcanism strengthens the link between the volcanism and the end-Permian mass extinction,’ the British–Russian team reported. Again the dating was good to within a million years. And it forced scientists to face up to the question: What on Ear th is all this black stuff really telling us? I A tangled web The facts and theories about flood basalts had become muddled. In respect of the recipe for the eruptions there were two conflicting hypotheses. According to one, a hot plume of rock gradually bored its way upwards from close to the molten core of the Earth, and through the main body, the mantle. When this mantle plume first penetrated the crust, its rocks melted and poured out as basalt. The other hypothesis was the pressure cooker. The rock below the crust is quite hot enough to melt, were it not squeezed by the great weight of overlying rock. Crack the crust, by whatever means, and the Earth will bleed. The relief of pressure will let the basalt gush out. That happens all the time, in a comparatively gentle way, at mid-ocean ridges where plates of the Earth’s outer shell are easing apart. Basalt comes up and slowly builds an ever-widening ocean floor. According to the pressure-cooker idea, just make a bigger crack at a point of weakness in a continent, and basalt will haemorrhage all over the place. There are old fault-lines everywhere, as well as many regions of stretched and thinned crust. The pressure cooker is much more flexible about candidate localities for flood-basalt events. With the mantle-plume hypothesis you need a pre-existing plume. 298 flood basalts Flood basalts often herald the break-up of a continent. Both in the eastern USA and West Africa are remnants of 200-million-year-old basalts released just before the Atlantic Ocean began to open between them, in the break-up of the former supercontinent of Pangaea. The South Atlantic between south-west Africa and Brazil originated later, and its immediate precursor was the 132-million-year-old flood basalt seen in Brazil’s Parana. A sector of the Atlantic that opened relatively late was between the British Isles and Greenland. The preceding basalt flood dates from 60 million years ago. Famous remnants of it include Northern Ireland’s Giant’s Causeway and Fingal’s Cave on the island of Staffa. The latter inspired Felix Mendelssohn to compose his Hebrides Overture, in unconscious tribute to the peculiarities of flood basalts. When the Deccan Traps formed, 65 million years ago, India was a small, free-range continent, drifting towards an eventual collision with Asia. The continental break-up that ensued was nothing more spectacular than the shedding of the Seychelles, as an independent microcontinent. Whether the effect on worldwide plate motions was large or small, in the mantle-plume theory the basaltic outbursts caused the continental break-ups. The pressure- cooker story said that a basalt flood was a symptom of a break-up occurring for other reasons. Another tangled web of ideas concerned the mass extinctions of life. In the 1980s, scientists arguing that the dinosaurs were wiped out by the impact of a comet or asteroid, 65 million years ago, had to deal with truculent biologists, and also with geologists who said you didn’t need an impact. The disappearance of the dinosaurs and many other creatures at the end of the Cretaceous Period coincided exactly with the great eruption that made the Deccan Tr aps of India. Climatic and chemical effects of so large a volcanic event could be quite enough to wreck life around the world. The issue did not go away when evidence in favour of the impact became overwhelming, with the discovery of the main cr ater, in Mexico. Instead, the question was whether the apparent simultaneity of impact and eruption was just a fluke. Or did the impact trigger the eruption, making it an accomplice in the bid to extinguish life? I Awkward coincidences Space scientists had no trouble linking impacts with flood basalts. The large dark patches that you can see on the Moon with the naked eye, called maria, are huge areas of basalt amidst the global peppering by impact craters large and small. And in 1974–75, when NASA’s Mariner 10 spacecraft flew past Mercury three times, it sent home pictures showing the small planet looking at first glance very like the Moon. 299 flood basalts The largest crater on Mercury is the Caloris Basin, 1500 kilometres wide. Diametrically opposite it, at the antipodes of the Caloris Basin, weird terrain caught the attention of the space scientists. It had hummocky mountain blocks of a kind not seen elsewhere. The Mariner 10 team inferred a knock-on effect from the impact that made the Caloris Basin. Seismic waves reverberating through the planet came to a strong focus at the antipodes, evidently with enough force to move mountains. Translated to terrestrial terms, a violent impact on Brazil could severely jolt the crust in Indonesia, or one on the North Pole, at the South Pole. This remote action enlarges the opportunities for releasing flood basalts. The original impact might do the job locally, especially if it landed near a pre-existing weak spot in the crust, such as an old fault-line. Or the focused earthquake waves, the shocks from the impact, might activate a weak spot on the opposite side of the planet. Either way, the impact might set continents in motion. Severe though it may be, an impactor hasn’t the power to drive the continents and the tectonic plates that they ride on, for millions of years. The energy for sustained tectonic action— earthquakes, volcanoes, continental collisions—comes from r adioactivity in the rocks inside the Earth. What impactors may be able to do is to start the process off. In effect they may decide where and when a continent should break. Advocates of impacting comets or asteroids, as the triggers of flood basalts, had plenty of scope, geographically. There was evidence for craters in different places with very similar ages, suggesting either the near-simultaneous arrival of a swarm of comets or a single impactor breaking up before hitting the Earth. So you could, for example, suggest that something hit India, or the Pacific seabed at the antipodes of India, 65 million years ago, to create the Deccan Traps, ir respective of what other craters might be known or found. In 1984, Michael Rampino and Richard Stothers of NASA’s Goddard Institute for Space Studies made the explicit suggestion, ‘that Earth’s tectonic processes are periodically punctuated, or at least modulated, by episodes of cometary impacts.’ Many mainstream geologists and geophysicists disliked this challenge, just as much as mainstream fossil-hunters and evolutionary theorists abhorred the idea of mass extinctions being due to impacts, or flood basalts. In both cases, they wished to tell the story of the Earth in terms of their own preferred mechanisms, whether of rock movements or biological evolution, concerning which they could claim masterful expertise. They wanted neither intruders from space nor musclers-in from other branches of science. The glove thrown down by Rampino and Stothers therefore lay on the floor for two decades, with just a few brave souls picking it up and dusting it from time to time. The crunch came with the new results on the Siberian Traps, and especially from the very precise dating that confirmed the match to the end-Permian 300 flood basalts catastrophe to life. There was no longer any slop in the chronological accounting, which previously left Ear th scientists free to choose whether or not they wished to see direct connections between events. The time had come for them to decide whether they were for or against cosmic impacts as a major factor in global geology as well as in the evolution of life. By 2002, the end-Permian event of 250 million years ago had a basalt flood and a mass extinction but no crater, although there were other hints of a possible impactor from outer space. A clearer prototype was the end-Cretaceous event of 65 million years ago, with a global mass extinction, a basalt flood in India, and a crater in Mexico. ‘To some Earth scientists, the need for a geophysically plausible unifying theory linking all three phenomena is already clear,’ declared Paul Renne of the Berkeley Geochronology Center. ‘Others still consider the evidence for impacts coincident with major extinctions too weak, except at the end of the Cretaceous. But few would dispute that proving the existence of an impact is far more challenging than documenting a flood basalt event. It is difficult to hide millions of cubic kilometres of lavas.’ There will be no easy verdict. Andrew Saunders of Leicester, spokesman for the dating effort on the buried part of the Siberian Traps, was among those sceptical about the idea that impacts can express themselves in basalt floods. ‘Some scientists would like to say that the West Siberian Basin itself is a huge impact crater,’ Saunders said, ‘but except for the presence of basalt it looks like a normal sedimentary basin. And if crust cracking is all you need for flood basalts, why don’t we see them in the biggest impact craters that we have?’ The controversy echoes a broader dispute among Earth scientists about the role of mantle plumes, which could provide an alternative explanation for the Siberian Traps. For that reason, the verdict about impacts and flood basalts will depend in part on better images of the Earth’s interior, expected from a new generation of satellites measuring the variations in gravity from region to region. Neither side in the argument is likely to yield much ground until those images are in, from Europe’s GOCE satellite launched in 2005. Meanwhile, the search for possible matches between crater dates and flood basalts will continue. E A closely related geological topic is Hotspots. For more about impacts, including the discovery of the 65-million-year-old crater in Mexico, see Impacts. Catastrophes for life are dealt with also under Extinctions. 301 flood basalts T he flowers on display in the 200-year-old research garden in Valencia, Jardı ´ Bota ` nic in the Catalan language, change with the seasons, as is usual in temperate zones. The Valencia oranges for which the eastern coast of Spain is famous flower early in spring, surrounded by blooming rockroses, but in summer the stars of the garden are the water hyacinths, flowering in the middle of the shade. In winter the strawberry trees Arbutus unedo will catch your eye. ‘All flowering plants seem to use the same molecular mechanisms to govern their dramatic switch from leaf-making to flower-making,’ noted Miguel Bla ´ zquez of the Universidad Polite ´ cnica de Valencia. ‘I want to know how the control system is organized, and linked to the seasons that best suit each species.’ For 10,000 years the question of when plants flower has been a practical concern for farmers and horticulturalists. Cultivated wheat and barley, for example, were first adapted to the seasons of river floods in the Middle East, but they had to adjust to spring rains and summer sunshine in Europe. The fact that such changes were possible speaks of genetic plasticity in plant behaviour. And year- round floral displays in well-planned gardens like Valencia’s confirm that some species and varieties take advantage even of winter, in the never-ending competition between plants for space and light. During the 20th century, painstaking research by physiologists and biochemists set out to clarify the internal mechanisms of plant life. Special attention to the small green chloroplasts in the cells of leaves, which capture sunlight and so power the growth and everyday life of plants, gradually revealed the molecular mechanisms. The physiologists also discovered responses to gravity, which use starch grains called statoliths as sensors that guide a seed to send roots down and stems up. They found out how growth hormones concentrate on the dark side to tip the stem towards the light. Similar mechanisms deploy leaves advantageously to catch the available light. To help it know when to flower, a plant possesses light meters made of proteins and pigments, called phytochromes for red light and cryptochromes and phototropins for blue light. By comparing chemical signals from the phytochromes and the cryptochromes with an internal clock, like that causing 302 jetlag in humans, the plant gauges the hours of darkness. So it always knows what the season is—by long nights in winter, short nights in summer, or diminishing or increasing night-length in between. Other systems monitor temperature, by making chemicals that can survive in the chill but break up as conditions get warmer. Having been first discovered in connection with the dormancy of seeds in winter and the transition to (vernal) springtime, this mechanism is called vernalization. The name still attaches to the molecular systems involved. When seen with hindsight, the research that told these tales was a bit like watching passing traffic without knowing the layout of the roads that brought it your way. Traditional physiology and biochemistry were never going to get to the bottom of the mysteries of plant behaviour. That is under the daily control of the genes of heredity, and of the proteins whose manufacture they command. I The Rosetta Stone for flowering time? Botany in the 21st century starts with a revolution, brought about by a great leap forward in plant genetics. It came with intensive, worldwide research on the small cress-like weed Arabidopsis thaliana. The entire genetic code—the genome—was published in 2000. This provided a framework within which biologists could with new confidence investigate the actions of genes acting in concert, or sometimes in opposition, to achieve various purposes in the life of the plant. Flowering is a case in point. The shaping of the flowers is under the control of sets of genes called MADS boxes, but that is relatively straightforward compared with the crucial decision a plant must make, about when to flower. Scientists have already distinguished nearly 40 genes involved in flowering time. Pooling their knowledge, they find the genes to be organized in four pathways. Ready to support flowering at any season is a so-called autonomous pathway, which activates the genes that convert a suitably positioned leaf bud into a flower. Before it can come into play it needs cues from two other genetic pathways that monitor the plant’s environment. The long-day pathway is linked to the calendar determined by the light meters and day-clock. The vernalization pathway responds to a long period of cold temperature—that is to say, to a winter. When conditions are right, the genes of the autonomous pathway are unleashed. This system is biased in favour of flowering during long days, whether in spring, summer or autumn. The option of winter flowering requires a fourth pathway that liberates the hormone gibberellin. The hormone can override the negative environmental signals coming to the long-day and vernalization pathways, and drive the plant to bloom. Miguel Bla ´ zquez identified the genes of 303 flowering the gibberellin pathway, when working with Detlef Weigel at the Salk Institute in California. ‘Nearly everything that we now know about the molecular mechanisms that control flowering time,’ Bla ´ zquez commented, ‘represents just five years’ research on just one small weed, arabidopsis. The basic picture has been amazingly quick to come, but to tell the full story of flowering in all its options and variations will keep us busy for many years.’ A quarter of a million species of flowering plants make their decisions in many different settings from the tropics to the Arctic. Each has evolved an appropriate strategy for successful reproduction. But one of the leaders of arabidopsis research in the UK, Caroline Dean of the John Innes Centre, was convinced that chasing after dozens of different genomes—everyone’s favourite plants—was not the right way forward. A better strategy was to learn as much as possible from arabidopsis first. ‘If we play our cards right,’ she argued, ‘we should be able to exploit the arabidopsis sequence to provide biological information that may very quickly reveal the inner workings of many different plants and how they have evolved.’ She meant much more than flowering, but that was an excellent test for her policy. Her team promptly identified a gene, VRN2, which enables arabidopsis to remember whether or not it has already experienced the cold conditions of winter. Writing with Gordon Simpson, Dean posed the question: ‘Will the model developed for arabidopsis unlock the complexities of flowering time control in all plants, as the Rosetta Stone did for Egyptian hieroglyphics?’ Their answer, in 2002, was probably. Whilst arabidopsis grows quickly to maturity, and responds strongly to the lengthening days of spring, many other plants use internal signals to prevent flowering until they are sufficiently mature. Rice, for example, does not flower until the days shorten towards the end of summer. The cues for flowering vary from species to species, and they include options of emergency flowering and seed setting in response to drought, overcrowding, and other stresses. Simpson and Dean nevertheless believed that much of this diversity could be explained by variations in the control mechanisms seen in arabidopsis, with changes in the predominance of the different genetic pathways. Arabidopsis itself adapts its flowering-time controls to suit its germination time, for example to avoid flowering in winter. Its basic strategy, a winter annual habit, relies on germination in autumn and flowering in late spring, and is suitable for places where summers are short or harsh. But some arabidopsis populations have evolved another strategy called rapid cycling, whereby the plant can ger minate and flower within a season. This is appropriate for mild 304 flowering conditions when more than one life cycle is possible within a year, and also in regions with very severe winters. Simpson and Dean were able to point to two different mutations found in arabidopsis in the wild, which create the rapid-cycling behaviour. Both occur in the gene called FRI and they have the effect of switching off the requirement for vernalization. ‘Rapid cycling thus appears to have evolved independently at least twice from late-flowering progenitors,’ they commented. Here is strong evidence that the flowering-time controls have been readily adjustable in the evolution of flowering plants. Where the variant genes involved are known in other species, these can often be seen to favour or disfavour particular pathways of the basic ar abidopsis system. There are exceptions, and vernalization in cereals may be a whole new story. I Counting the cold days Many crops in the world’s temperate zones, including the cereals, are winter varieties. That is to say, they are sown in the autumn and they flower in the spring or summer. It is vital that a plant should not mistake the equal night and day lengths of autumn for springtime, and flower too soon. The vernalization mechanism provides the necessary inhibition, by requiring that the plant should experience winter before it flowers. But where summers are short it is also essential that the plant should flower promptly in the spring. Places with short summers have relatively harsh winters, so the mechanism also has to act as an accelerator of flowering once winter has passed. Experiments with crops raised in controlled conditions illustrate vernalization in action. Grow winter barley in nothing but war mth and plenty of light, and it will look very strange. It just keeps on producing leaves, because it is waiting in vain for the obligator y cue of winter. Next expose the germinating seedlings to cold conditions, for a day, a week or a month, and then put them in the same perfect growth conditions. Those that had the longest time in the cold will flower faster. They will make fewer leaves before they switch to flower production. ‘Vernalization is quite amazing in its quantitative nature,’ Gordon Simpson said. ‘This response probably informs the plant as to the passage of winter, as opposed to just a frosty September night. But perhaps the vernalization requirement and response evolved independently and through different mechanisms in different plants. We’ll have a better idea of this when we work out the genes controlling vernalization in cereals like wheat.’ E For more about the now-famous weed, see Arabidopsis. For other features of plant life, see cross-references in Plants. Animal analogues are in Biological clocks. 305 flowering [...]... ´ Spain’s Laboratorio de Astrofısica Espacial y Fısica Fundamental, in charge of the optical monitor on Integral ‘Our hope is that we shall be watching at some other target in the sky and a gamma-ray burst may begin, peak and fade within the field of view of all our gamma-ray, X-ray and optical instruments For the science of gamma-ray bursts, that would be like winning the lottery.’ Astronomers have... Arabidopsis thaliana has 25, 500 genes, leaving the human gene count unexpectedly meagre by comparison, at about 32,000 One explanation on offer is that a single gene may be transcribed in many different ways, to code for different proteins, and animals make greater use of this versatility than plants do Another is that plants, being stationary, need a wider range of defences than animals that can run away from... the ancient Greeks, Galaxias meant milky, and astronomers adopted Galaxy ´ ´ as a name more posh and esoteric than the Via Lactea, Voie Lactee, Milchstrasse or Milky Way of everyday speech They figured out that the Galaxy is a flattened assembly of many billions of stars seen edge-on from inside it But by the 20th century they needed ‘galaxy’ as a general name for many similar star-swarms seen scattered... to galactic origins was to look in our own backyard, at the oldest stars of the Milky Way Galaxy itself, and at nearby galaxies Billions of years from now, the Magellanic Clouds and the Andromeda Galaxy may all crash into us If so, the spirals of the Milky Way and Andromeda will be destroyed and when the melee is over the ensemble will join the ranks of the ellipticals Nor are all these traffic accidents... could be seen as ordinary-looking supernovae But it does imply that, if each long-duration gamma-ray burst is a signal of the formation of a stellar black hole, then the Universe may be making dozens of them every day Supernovae manufacture chemical elements—you are made of such star-stuff— and confirmation of the supernova theory of gamma-ray bursts came from the detection of newly made elements Europe’s... long-lasting sources of gamma rays, Integral could see and analyse gamma-ray bursts occurring by chance in its field of view, about once a month And sometimes the same event would appear also in the narrower field of view of the optical monitor ‘We know that some of the visible flashes from gamma-ray bursts are bright ´ ´ enough for us to see, across the vast chasm of space,’ said Alvaro Gimenez of ´... ocean of space To distinguish our cosmic home a capital G was not enough, so they went back to the vernacular, not minding that Milky Way Galaxy was like saying Galaxy Galaxy The tautology has merit, because every naked-eye star in the sky belongs to the Galaxy even if it lies far from the high road of the Milky Way itself The only other naked-eye galaxies are the Large and Small Clouds of Magellan,... in gamma-ray bursts that go beyond the mechanisms that generate them The rate at which giant stars were born and perished has changed during the history of the Universe, and the numbers of gamma-ray bursts at different distances are a symptom of that evolution Quasars already provide bright beacons lighting up the distant realms and revealing intervening galaxies and clouds With better mastery of the... occasion an X-ray flash had appeared simultaneously in the Wide Field Camera on the spacecraft The physicist in charge of this X-ray telescope was John Heise of the Stichting Ruimte Onderzoek Nederland, the Dutch national space research institute in Utrecht He was away in Tokyo at a conference, but was always ready to react, by night or day, if his camera saw anything unusual in the depths of space Alerted... in a halo around our own Milky Way Galaxy, or stupendous explosions in other galaxies far away in the Universe The answer was that this gamma-ray burst, numbered GRB 970228 to denote its date, was associated with a faint galaxy I Camaraderie and competition Two years later a worldwide scramble to see another gamma-ray burst, also pinpointed by the X-ray camera on BeppoSAX, led to even better results . Moon with the naked eye, called maria, are huge areas of basalt amidst the global peppering by impact craters large and small. And in 1974– 75, when NASA’s Mariner 10 spacecraft flew past Mercury three. in our own backyard, at the oldest stars of the Milky Way Galaxy itself, and at nearby galaxies. Billions of years from now, the Magellanic Clouds and the Andromeda Galaxy may all crash into us galaxies far away in the Universe. The answer was that this gamma-ray burst, numbered GRB 970228 to denote its date, was associated with a faint galaxy. I Camaraderie and competition Two years