Biology of coral bleaching

Bleached coral Photo: DPI

Corals belong to the group of animals called Cnidarians which include anemones, jellyfish, bluebottles and hydroids. Most corals are classified as a soft coral or hard coral. Both types of corals are colonies of small polyps. In hard corals, the polyp secretes a small limestone cup (or corallite) that surrounds it and protects the soft polyp tissue. Soft corals lack a limestone corallite structure but have tiny internal skeletal parts. Both hard and soft corals are affected by coral bleaching.

The coral animal is in a mutually beneficial partnership (or symbiosis) with tiny, single-celled algae called zooxanthellae. The zooxanthellae are packed into every cell of the coral tissue and can reach densities of 5 million per square centimetre of coral tissue. Like land plants, the zooxanthellae utilise light and dissolved bicarbonate ions in the process of photosynthesis to produce energy-rich compounds, some of which are transferred to the host coral. In return, the coral gives protection to the zooxanthellae. Because most corals rely on the zooxanthellae to supply much of their food, they are usually restricted to shallow seas where sufficient light can reach the coral and its zooxanthellae.

Like all living things, corals and zooxanthellae survive and grow well within a preferred range of environmental conditions. Temperatures outside their operating range are detrimental to them. Unlike fish that can move to more suitable environmental conditions, corals cannot escape areas of high temperature because they are attached to the reef structure. If temperatures are too hot for too long, the symbiotic relationship between the coral animal and its tenants, the zooxanthellae, collapses.

Bleached coral

When corals are stressed by high temperatures and light, some of the chemical reactions in photosynthesis break down. The Sun’s energy which is normally used to produce food for the algae and coral is diverted into the production of oxygen radicals. These oxygen radicals, similar to the chemicals implicated in human aging, are highly corrosive. They damage the parts of the zooxanthellae - the chloroplasts - where photosynthesis takes place. The coral polyps can also be damaged as a result of heat stress. The result is that large numbers of damaged zooxanthellae leave the coral.

The zooxanthellae in its tissue gives the coral its distinctive colour, so when the number of algae in the tissues are greatly reduced as a result of bleaching, the corals appear pale. The corals turn white if prolonged stress causes most of the algae to be lost.

If high temperatures are relatively short-lived, the zooxanthellae that remain within the coral tissue divide rapidly and the coral gradually regains its colour and survives. If the stressful conditions are prolonged or particularly severe, the zooxanthellae won’t reproduce, and many corals will die. There is great variation in the susceptibility to bleaching events between different coral species, and even colonies within a species.