A ray of hope for coral reefs

Australian researchers have shown how some reef-building corals might protect themselves against the double threat of global warming and ozone depletion.

Publishing in this week's issue of Nature, Dr Anya Salih and colleagues at the University of Sydney have found that certain varieties of corals use fluorescence to take the sting out of intense UV light, which otherwise acts together with warmer water temperatures to cause coral bleaching.

Acropora nobilis: Corals depend on microscopic algae which live in symbiosis inside the coral tissues, providing a steady supply of sugars for the host coral. While the algae require light, too much of it shuts photosynthesis down - a process called photoinhibition. When this happens, the coral expels its damaged symbiont and dies.

It had been observed for some time that corals fluoresced green when blue light was shone on them - but no one knew why. Although these corals had a compound similar to GFP (green fluorescent protein) found in luminescent jellyfish, they did not glow in the dark like the jellyfish do. So what was the function of the fluorescent pigment?

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Salih and team postulated that the fluorescence played a role in protecting algae from damaging UV radiation and so set out to test the hypothesis.

Pocillopora damicornis "Our results show that in well-lit environments these fluorescent pigments act as 'sunscreens', protecting coral symbionts from photoinhibition by transforming excess light to wavelengths which are not absorbed by the algae and therefore will not damage them," say the researchers.

The research team transplanted corals in the laboratory and observed them under different light intensities and water temperatures. They found that the algae in fluorescent corals were able to continue photosynthesising while those in non-fluorescent corals shut down photosynthesis.

Their laboratory findings were confirmed by field work that showed that in massive bleaching events, fluorescent corals were damaged less than their non-fluorescent neighbours.

"This has important implications in the light of current climatic changes," say the researhcers. "Many species have fluorescent morphs - will these become predominant as temperatures and ultraviolet radiation increase? If so, they may offer the reef environment a partial buffer against global climate change."

"We do not suggest that we should be complacent about global warming or ozone depletion, but our studies may provide a ray of hope for the reef ecosystem."

The researchers are currently funded by an Australian Research Council SPIRT Fellowship (Stragetic Partnerships in Industry, Research and Technology) award.