Coral reefs are found throughout the world's tropical regions where they line coastlines in warm shallow ocean regions. But what exactly are corals and how do they form reefs?

What are coral reefs?

At the heart of coral reefs are corals, which are simple organisms that are related to sea anemones and jellyfish but which have developed an intimate symbiosis with tiny micro-algae called zooxanthellae. Together, coral and zooxanthellae trap the energy of sunlight and convert it into sugars and other organic compounds, which provide an important source of nutrition and energy for the corals. This symbiosis between animal and micro-algae is so successful that corals are also able to precipitate vast quantities of calcium carbonate. Calcium carbonate is a limestone-like material that many other tropical organisms produce and which builds up over time into the familiar structures associated with coral reefs. In addition to corals, many other organisms including jellyfish, clams, nudibranchs, flatworms, seaweeds, and tiny creatures known as foraminifera have also developed similar relationships with zooxanthellae. 
While coral reefs are the only living structure visible from space, they only occupy a tiny fraction of the ocean’s seascape ( less than 0.1%). Despite this small size, coral reefs are the most biologically diverse ecosystem in the ocean with scientists estimating that at least one million species live in and around coral reefs. On Australia's Great Barrier Reef, for example, there are over 500 species of corals, nearly 1,500 fish species and almost 3,000 species of molluscs. While these charismatic creatures are the most obvious organisms on coral reefs, the list of species on the Great Barrier Reef extends into hundreds of thousands of species when one takes into account the many other often cryptic organisms such as the crabs, shrimps, worms, echinoderms, marine algae, microbes and marine bacteria. At one end of this complex web of life are the primary producers which trap the energy of the sun, while at the other end are the apex predators such as sharks. Together these organisms make up a vibrant and interdependent ecosystem. 
Coral reefs have been a persistent feature of tropical regions of our planet for hundreds of millions of years. While some of the organisms have subtly changed at this time, the evidence of the reef building activity of coral reef ecosystems has been frozen in time as the huge limestone deposits across the planet. All through this time, coral reefs have dominated warm shallow seas, expanding and contracting with the slow changes in the environment around them. The Great Barrier Reef, for example, has formed during the warm periods and has disappeared during the glacial periods, when sea levels were 180m lower than they are today. This ebb and flow of coral reefs from the coastlines of the world is part of a natural cycle in which the Earth's distance from the Sun slightly shifted due to an eccentric wobble in its orbit over thousands of years.

The importance of Coral Reefs

Despite only occupying a small fraction of the earth's surface, coral reefs are incredibly important to humanity. Over 500 million people derive food and income from coral reefs that line the coasts of many tropical countries. This contribution to the livelihoods of these people is critical given that most of these populations are relatively impoverished and have nowhere else to go for their food and income. In developed countries, coral reefs also play important economic roles. For example, Australia's Great Barrier Reef generates over A$6 billion each year and provides employment for 63,000 people, which represents largely sustainable income from seafood and tourism. When evaluated across the world, coral reefs bring in hundreds of billions of dollars each year to national economies. In this regard, coral reefs are not only beautiful and precious from the biological perspective, they are important in underpinning the well-being of hundreds of millions of people throughout the world.

Destructive pressure on Coral Reefs

Despite their importance, coral reefs are degrading rapidly under the pressure of human activities. At local scales, destruction of coastal vegetation through mining, deforestation and agriculture is polluting coastal waters with sediments, toxins and nutrients. In many countries, coral reefs are also being overexploited such that key fish species and other organisms are disappearing, leading to large-scale ecological changes where corals disappear and other organisms such as seaweeds and cyanobacteria become dominant. Global scales, rising concentrations of greenhouse gases such as carbon dioxide in the atmosphere is leading to rapid changes in sea temperature and ocean acidification. This is producing environmental change which is hundreds of times faster than the ice age transitions of the past, which are fundamentally different to anything seen over the past 40 million years. These broad-scale changes have already had impacts on coral reefs through coral bleaching and disease, as well as a slowing of the ability of corals to form their calcium carbonate skeletons. Looking to the future, the projected changes to sea temperature and chemistry revealed that coral reefs may well disappear before the end of the current century if we don't take urgent action on the burning of fossil fuels and the contribution of carbon dioxide to Earth’s atmosphere.

Professor Ove Hoegh-Guldberg Chief Scientist of the Catlin Seaview Survey and Director of the Global Change Institute.