Academic Reading — Test 185

Reading passage

Coral reefs occupy less than one per cent of the ocean floor, yet they support an estimated quarter of all marine species at some stage of their life cycle. The vivid colours associated with healthy reefs do not come from the coral animals themselves, which are largely translucent, but from microscopic algae called zooxanthellae that live within the coral's tissue in a mutually beneficial partnership. The coral provides the algae with a sheltered habitat and the compounds needed for photosynthesis, while the algae supply the coral with the majority of its energy, in the form of sugars produced through photosynthesis, along with oxygen and the pigments that give reefs their characteristic hues. When this partnership breaks down, the phenomenon known as coral bleaching occurs, and reefs worldwide are increasingly experiencing it. Bleaching is fundamentally a stress response. Corals are remarkably sensitive to their surrounding water temperature, generally thriving within a narrow band of roughly 23 to 29 degrees Celsius. When sea temperatures rise even one or two degrees above the usual summer maximum for a particular reef, and remain elevated for several weeks, the zooxanthellae begin to malfunction. Heat disrupts the algae's photosynthetic machinery, causing it to generate excess reactive molecules that are toxic to both the algae and the coral host. In response, the coral expels its algal partners in large numbers as a protective measure. Because the algae carry most of the reef's pigmentation, their departure leaves the coral's white limestone skeleton visible through its now-transparent tissue, producing the ghostly white appearance from which the term bleaching derives. Crucially, a bleached coral is not yet a dead coral; it has lost its primary energy source but the underlying animal is often still alive and can recover if conditions improve quickly. Temperature is the dominant trigger, but it rarely acts alone. Bleaching events are frequently associated with prolonged periods of calm, sunny weather, since strong sunlight combined with warm water intensifies the stress placed on the zooxanthellae; ultraviolet radiation appears to accelerate the breakdown of the photosynthetic apparatus that heat alone initiates. Reduced water movement during still conditions also allows heat to build up in shallow lagoons and reef flats, areas that consequently bleach more severely than deeper, current-swept sections of the same reef. Large-scale climate patterns play a significant role as well. During El Nino events, warm surface waters spread across much of the tropical Pacific, and reefs across that ocean basin have repeatedly bleached in the months that follow such episodes. Some scientists also point to lowered salinity, often resulting from unusually heavy rainfall or freshwater runoff, as a secondary factor that can compound thermal stress in coastal reef systems. The consequences of repeated bleaching extend well beyond the loss of colour. A coral that has been severely bleached must rely on stored energy reserves and whatever nutrients it can capture directly from the water using its tentacles, a far less efficient strategy than photosynthesis. If favourable temperatures return within a few weeks, the coral can usually reacquire zooxanthellae and regain its normal appearance and function. However, if the thermal stress persists for a longer period, the coral's energy reserves become exhausted and it eventually starves, leaving behind a skeleton that is rapidly colonised by algae and other organisms. Even corals that survive a bleaching episode often show reduced growth rates and lowered reproductive output for several years afterwards, and they become considerably more vulnerable to disease. The frequency of mass bleaching events, in which large proportions of a reef system whiten simultaneously, has increased markedly since the phenomenon was first documented on a global scale in the 1980s. Whereas reefs once had roughly a decade or more between severe thermal stress episodes, a period sufficient for substantial recovery, many now experience damaging heat stress every few years. This shortened interval is widely attributed to the gradual warming of the world's oceans, which has raised the baseline temperature against which short-term heat spikes are measured, making it progressively easier for seasonal warming to push corals past their thermal threshold. Researchers monitoring reef health now use satellite-derived sea-surface temperature data to track accumulated heat stress, expressed as degree heating weeks, which has become a standard tool for forecasting where and when bleaching is likely to occur. Coral reefs are not without some capacity to adapt. Certain coral species host more heat-tolerant strains of zooxanthellae, and reefs that have experienced repeated, moderate warming sometimes show greater resistance to subsequent heat stress, a pattern researchers describe as acclimatisation. Some scientists are investigating whether deliberately exposing coral larvae to mild heat stress in controlled nursery conditions might help cultivate more resilient populations for future reef restoration. Nevertheless, most specialists agree that such adaptive mechanisms operate far more slowly than the current pace of ocean warming, meaning that the long-term survival of coral reef ecosystems will depend largely on limiting future increases in global sea temperature.