Coral Reef

Coral reefs are immense structures built over hundreds and thousands of years by tiny animals called corals. They are the largest structures built by organisms (other than humans) on Earth and offer an exquisite amount of beauty and animal diversity that can compete with any ecosystem from all environments. Over 100,000 marine species call coral reefs home and humans receive a range of both ecological and economic benefits due to the presence of coral reefs and the ecosystems they support.

Why are coral reefs important?

Coral reefs are important for a number of reasons. Firstly, they are one of the most diverse habitats on Earth. Over 4000 fish species and more than 750 species of coral are found in coral reefs around the world. They are also important sinks for carbon dioxide as corals use carbon to form calcium carbonate.

Coral reefs provide a long list of important benefits to humans such as food, tourism, medicine and protection from the ocean. They can also be important nursery grounds for commercially important fish species.

Where are coral reefs found?

Coral reefs are found in a relative restricted area due to the biology of the corals and the bacteria that live within them. Their distribution is generally limited to within 30 degrees either side of the equator where the water is warmest. For a coral reefs to exist, the water temperature must be above 18°C and the depth less than 100m.

The largest coral reef system is the Great Barrier Reef found of the north-east coast of Australia. Other significant reefs are found through the Pacific, south-east Asia, the Caribbean and the Indian Ocean.

How are coral reefs built?

Coral reefs are built through the deposition of calcium carbonate (limestone) by corals over very long periods of time. Corals are invertebrate animals from the phylum Cnidaria which also contains jellyfish and anemones. Reef building corals are generally very small but large colonies of corals can over time produce vast areas of limestone.

Corals extract bicarbonate ions (HCO₃^–) from the water and use it to create calcium carbonate (CaCO₃) which is used to build a limestone skeleton. Over time the limestone accumulates and creates magnificent structures that can span thousands of square miles. Each coral lays down the limestone in a different way which is why there is such a wide range of structures found within coral reefs.

The important relationship between coral and bacteria

Due to the biology of corals, they are found in waters close to the equator which are generally clear and very low in productivity. In order for corals to quickly and efficiently receive the nutrients they require, they have formed a symbiotic relationship with phytoplankton from the genus Symbiodinium, although they are more commonly known as zooxanthellae.

The zooxanthellae are photosynthetic, so are able to produce carbohydrates from carbon dioxide within the water. Corals receive most of their energy from the zooxanthellae and in return the coral provides the zooxanthellae with inorganic nutrients.

What are the threats to coral reefs?

Coral reefs are under serious threat due to lowering pH levels of the world’s oceans combined with various more local effects. The oceans act as a giant sink for CO₂ absorbing atmospheric CO₃ and increasing CO₂ concentrations within the water. The resulting change in the waters chemistry makes it harder for corals to form calcium carbonate, create limestone skeletons and therefore build reefs. Other factors such as pollution and sedimentation can reduce the light availability for zooxanthallae which depend on sufficient access to light in order to photosynthesize.

Last edited: 25 February 2018