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Reading Passage 2 – Second Lives Beneath the Waves

A When a ship reaches the end of its working life, it is usually towed to a breaker's yard, where it is cut up and its steel sold for scrap. A small number, however, meet a very different fate. Cleaned, stripped and then deliberately sunk, they come to rest on the seabed, where within months they begin to attract fish, corals and sponges. Artificial reefs of this kind have been created off the coasts of the United States, Australia, Malta and many other countries, and they have become popular destinations for divers. But the practice has always had its critics, and the question of whether a sunken ship is a gift to the ocean or simply a convenient way to dispose of waste has never been entirely settled.

B The idea of placing objects in the sea to attract fish is far from new. Fishermen in Japan were sinking wooden frames weighted with stones in the eighteenth century, and similar practices have been recorded in the Mediterranean. What has changed is the scale. In the second half of the twentieth century, governments began to sink not only ships but also old railway carriages, military vehicles and even concrete from demolished buildings. Some of these early projects were poorly planned: in one notorious case off the coast of Florida, around two million used car tyres were bound together and dropped into the sea in the 1970s, in the hope that they would become a reef. Instead, the bundles broke apart in storms, and loose tyres were thrown against existing natural reefs, damaging them. Clearing them up has cost millions of dollars.

C Lessons from such failures have shaped modern practice. Before a ship can be sunk today, it must be thoroughly cleaned of fuel, oil, paint containing toxic substances, and any loose material that could float away. Wiring and insulation, which may contain harmful chemicals, are removed, and openings are cut in the hull so that divers can enter and leave safely. This preparation is expensive and can take more than a year; in some cases it costs more than the value of the scrap metal the ship would otherwise have produced.

D Why do marine organisms colonise a wreck so quickly? Part of the answer is that, on much of the seabed, hard surfaces are scarce. Many coastal waters have floors of sand or mud, to which corals and sponges cannot attach themselves. A ship provides a large, stable structure rising from the seabed, with sheltered spaces of every size, from narrow gaps between metal plates to entire rooms. Small fish shelter in these spaces, and larger predators follow them. Biologist Elena Marsh, who has surveyed wrecks in the Gulf of Mexico for over a decade, has found that the number of fish around a ship can exceed that on a natural reef of similar size within five years.

E Not everyone is convinced that this represents a real benefit. A central question, raised by the fisheries scientist Tom Akers among others, is whether artificial reefs actually produce new fish or merely attract existing ones from the surrounding area. If the latter is true, a wreck may do more harm than good, because concentrating fish in one place makes them easier for fishing boats to catch. Akers argues that the answer probably varies from one species to another, and that without careful long-term monitoring it is impossible to know which effect dominates in any particular case.

F There are also concerns about what happens as a wreck ages. Steel corrodes in seawater, and while a large ship may remain structurally sound for many decades, its upper decks will eventually collapse. The engineer Rosa Lindqvist points out that the rate of corrosion depends heavily on depth and water temperature, and that ships sunk in shallow, warm waters may deteriorate much faster than their planners expected. As the structure breaks down, the habitat it provides changes too, and divers who enter older wrecks face increasing risks.

G Supporters respond that, whatever its effect on fish numbers, a well-managed artificial reef can relieve pressure on natural ones. Popular natural reefs suffer damage from the anchors and fins of large numbers of visiting divers. By offering an attractive alternative nearby, a sunken ship can draw some of these visitors away. The marine economist David Okoro has calculated that a single wreck off the coast of South Australia generates several million dollars a year in tourism, much of it for small businesses such as dive schools and guesthouses. For coastal communities, he suggests, this income may matter as much as any ecological argument.

Reading Passage 1 – Living Partnerships: The Hidden World of Lichens

A Walk through almost any landscape on Earth, from a city pavement to a windswept mountain summit, and you are likely to pass lichens without noticing them. They appear as grey-green crusts on rocks, leafy patches on tree bark or tiny branching tufts hanging from twigs. For centuries, naturalists classified them as a single kind of plant, and it was not until the second half of the nineteenth century that scientists began to understand that a lichen is not one organism at all, but a partnership.

B At the heart of every lichen is a fungus. Fungi cannot make their own food, and most obtain it by breaking down dead material or by living on other organisms. The lichen fungus has taken a different route: it houses a population of microscopic photosynthetic partners, either green algae or cyanobacteria, within its tissues. These partners use sunlight to produce sugars, a proportion of which pass to the fungus. In return, the fungus provides a sheltered structure that protects its partners from intense light and from drying out, and it absorbs water and minerals from rain, dust and the surface on which it grows. The body that results from this cooperation looks and behaves quite unlike either partner growing alone. Recent studies have suggested that some lichens also contain additional partners, such as yeasts, which makes the picture more complicated still.

C This arrangement allows lichens to survive in places where few other living things can. They are found on bare rock in hot deserts, on the frozen soils of Antarctica and on the exposed bark of trees in the tropics. The key to their resilience is an ability to tolerate almost complete drying out. When water is scarce, a lichen simply shuts down: its metabolism slows almost to a halt, and it can remain in this dormant state for long periods. As soon as it is moistened by rain, dew or even humid air, it resumes photosynthesis, sometimes within minutes. Experiments carried out on spacecraft have even shown that certain species can survive a period of exposure to the vacuum and radiation of space.

D The price of this toughness is extremely slow growth. Because lichens are active only when they are both wet and exposed to light, many crust-forming species expand by no more than a millimetre or two a year, and some grow far more slowly. Scientists have turned this characteristic into a practical tool. By measuring the diameter of the largest lichens on a surface such as a gravestone, a rock fall or ground left behind by a retreating glacier, and comparing it with lichens on surfaces of known age, researchers can estimate how long the surface has been exposed. The technique, known as lichenometry, has been used to date glacial deposits formed over the last few centuries, although its accuracy depends on careful calibration for each region, since growth rates vary with climate.

E Lichens are also remarkably sensitive to the quality of the air around them. Having no roots and no waxy outer layer of the kind that protects the leaves of many plants, they absorb much of what they need directly from rain and the atmosphere, and pollutants are taken in along with nutrients. Sulphur dioxide, released when coal and oil are burned, is particularly damaging to many species. In the industrial cities of nineteenth-century Europe, observers noticed that lichens were disappearing from trees and walls, and later studies linked this loss to polluted air. Today, surveys of which species are present in an area are widely used as an inexpensive way of monitoring air quality, and in several European cities some lichens have returned as levels of sulphur dioxide have fallen.

F People have found other uses for lichens too. In the Arctic, some species form a major part of the winter diet of reindeer and caribou, which dig through the snow to reach them. For centuries, lichens were also an important source of dyes: certain species were fermented to produce purple and red colours for wool, and the chemical indicator litmus, which turns red in acidic and blue in alkaline conditions, was originally obtained from lichens. In addition, lichens produce a wide range of chemical compounds, many of which are not known from any other group of organisms, and some of these are now being investigated for possible medical use.

G Despite their resilience, lichens face new pressures. Although many species have recovered from sulphur pollution, others are harmed by nitrogen compounds from traffic and intensive farming, which favour a small number of nitrogen-tolerant species at the expense of more sensitive ones. Because lichens grow so slowly, communities that are damaged may take decades to recover. Ecologists argue that these modest organisms deserve more attention, not only because of their value as indicators, but also because of the many roles they play in the ecosystems they inhabit, from beginning the slow process of turning rock into soil to providing food and nesting material for animals.

Reading Passage 2 – Farming Upwards

A At first sight the concept appears to belong to science fiction: lettuces, herbs and strawberries cultivated in tiers inside windowless warehouses, illuminated by pink and blue light and entirely without soil. Over the past fifteen years, however, so-called vertical farms have progressed from architects' sketches to functioning commercial operations. Their advocates portray them as a response to several of agriculture's most persistent difficulties, ranging from the depletion of freshwater resources to the lengthy supply chains on which urban populations depend. The sector's track record, nevertheless, has been strikingly inconsistent: a handful of profitable enterprises coexist with a series of costly and widely reported collapses.

B The underlying technology is relatively straightforward. In the majority of installations, crops are cultivated hydroponically, their roots immersed in a solution of water and dissolved minerals; in others, which rely on a technique termed aeroponics, the roots are left suspended in the air and periodically sprayed with a fine mist. Since the water is captured and recirculated rather than lost through evaporation and drainage, operators maintain that consumption can be as much as 95 per cent lower than on a conventional farm producing an equivalent crop. Because the growing environment is sealed, insects have considerable difficulty gaining access, with the result that pesticides are seldom required. Illumination is supplied by light-emitting diodes (LEDs), and it is here, according to the plant physiologist Professor Ines Duarte, that much of the real ingenuity resides. 'Photosynthesis does not respond equally to every wavelength,' she observes. 'LEDs allow us to tune the spectrum to the requirements of each species at each stage of its development – a degree of control that natural sunlight cannot provide.'

C For food retailers, though, the principal appeal lies less in resource efficiency than in freshness. A field-grown lettuce may spend several days in transit and storage before it is displayed for sale, deteriorating in both flavour and shelf life along the way. A vertical farm, by contrast, can be sited on the periphery of a city or even within it, so that produce picked in the early morning may be on supermarket shelves the same day. 'Consumers are not buying technology,' remarks Marcus Feld, who founded one of the earliest commercial vertical farms in northern Europe. 'What they are buying is a salad that keeps for a week in the fridge rather than three days.' Output is also highly predictable: an indoor facility is insulated from drought, flooding and unseasonal frost, and can therefore supply an identical volume throughout the year.

D The fundamental obstacle is energy. Whereas sunlight costs nothing, electricity does, and every hour of illumination received by an indoor crop must be paid for, as must the cooling required to dissipate the heat generated by the lamps. It is for this reason that vertical farms produce little apart from leafy greens, culinary herbs and a limited range of soft fruit – crops that mature rapidly, are harvested in their entirety and command a comparatively high price per kilogram. Staple cereals such as wheat and rice present an altogether different case. The agricultural economist Dr Hannah Kessler has estimated that the electricity needed to grow enough wheat indoors for a single loaf would cost many times the retail price of the loaf itself. 'For any crop, the decisive question is how much revenue each unit of light can generate,' she argues. 'In the case of basil, a considerable amount; in the case of grain, virtually none.'

E The consequences of these economics became painfully apparent when energy prices rose steeply in the early 2020s. A number of well-capitalised companies, some of which had attracted hundreds of millions of dollars in investment, were obliged to close facilities or file for bankruptcy. Critics contended that, in many instances, business plans had rested on unduly optimistic cost projections and on the assumption that rapid expansion would in itself generate economies of scale. Feld is frank about the industry's misjudgements. 'Many of us, quite simply, designed our operations for an era of cheap power,' he acknowledges. 'Once that era ended, the figures no longer added up.'

F Few commentators now anticipate that vertical farming will supplant conventional agriculture, yet this need not imply that it lacks a future. Some producers are adopting hybrid models, notably greenhouses that exploit natural daylight and activate supplementary LEDs only when sunlight is insufficient. Others are drawing on solar or wind generation, or on surplus heat from neighbouring industrial plants. Duarte, for her part, suspects that the technology's most significant contribution may lie in a different direction entirely. 'Consider polar research stations, cities in arid regions or, eventually, a lunar base,' she suggests. 'Where conventional cultivation is impossible, a vertical farm is under no obligation to compete with a field. It merely has to function.'