Rainbow reefs - will 'sunscreen' save corals?

Scuba divers in different parts of the world may have noticed that instead of coral fading due to rising temperatures and warming oceans, some corals have started to turn brilliant rainbow colours. Now scientists at Southampton University believe they have been able to discover why this is happening.

The study, conducted by researchers at an English university, indicates that this phenomenon is a sign that corals are fighting for survival. What is more, the results show that corals are likely to be victorious in this fight.

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It turns out that corals carry symbiotic algae that are embedded in their cells. However, all it takes is a temperature rise of 1˚C above the usual summer maximum for this relationship to break down. If the algae disappear, the white calcareous skeleton of the coral breaks through the transparent tissue, leading to fading and death because this tissue is no longer protected.

Why some corals emit a multicoloured glow instead of white remained a mystery. Scientists at the university’s Coral Reef Laboratory conducted a series of experiments at their aquarium facility and discovered that the corals produce their own sunscreen layer, designed to encourage protective algae to return.

“Our study shows that coral fading involves a self-regulating mechanism, a so-called optical feedback loop in which both symbiosis partners are involved. In healthy corals, most sunlight is absorbed by photosynthetic pigments of algal symbionts. When corals lose symbionts, the excess light travels back and forth inside the animal tissue – reflected by the white coral skeleton,” said Prof. Jorg Wiedenmann, head of the Coral Reef laboratory

It turns out that it is this increased level of internal light that is very stressful for the symbionts and may delay or even prevent their return once environmental conditions have returned to normal. However, if the coral cells are still able to perform at least some of their normal functions, despite the environmental stress that caused the fading, the increased levels of internal light will increase the production of colourful, photo-protective pigments.

The resulting layer of sun protection will then promote the return of the symbionts. As the recovering algal population begins to reabsorb light for its photosynthesis, light levels within the coral will drop and the coral cells will reduce the production of coloured pigments to their normal levels.

Scientists believe that corals undergoing this process will experience mild or short-lived disturbances created by a warming ocean. Their recent reports suggest that this phenomenon occurred in parts of Australia’s Great Barrier Reef during the recent mass fade in March and April. This has increased the chances of the corals there recovering.

In concluding their study, however, the researchers stress that only significant global greenhouse gas reductions and sustained regional improvements in water quality can save coral reefs for generations to come.

^{Source: cell.com}