What's going on?Many corals harbour microscopic, single-celled algae (called zooxanthellae) inside their bodies. The zooxanthellae undergo photosynthesis to produce food from sunlight. The food produced is shared with the coral, which in return provides the zooxanthellae with shelter and minerals.
A study in 2000 suggests that in shallow water corals, flourescence acts like a sunscreen to prevent harmful UV light from damaging the zooxanthellae.
How does it work? It's a bit technical, and this is the simple version:
“During fluorescence, the electrons inside the coral’s colour pigment molecules become excited. They jump higher and emit energy. The blue colours come from high energy emissions, then as the energy decreases the colours change to greens, then yellows and reds. Basically, the coral pigment cells break up solar radiation into the same energy wavelengths seen in the colours of the rainbow and by absorbing higher energy - UV and blue light - and emitting it in green to red fluorescent colours, they transform damaging light into less energetically damaging wavelengths.”
Corals may actually fluoresce continually under sunlight but the sunlight is so strong you can’t see it.
The study also found that corals that contained the highest density of fluorescent pigments tended to survive coral bleaching!
Another finding was that 70-90% of shallow-water corals have fluorescent colour pigments, this proportion falling to about 50% in deeper waters.
In deeper waters, flourescence have a different purpose. Because light is scarce in deep water, the corals' fluorescent proteins absorb UV light and reemit it in the blue and green spectrums. This amplifies the light available for photosynthesis.
In deep-water corals, the pigments are packed among or below the zooxanthellae, acting like tiny mirrors, reflecting some of the light back to the zooxanthellae.
But in shallow-water corals, the pigments are stacked above the zooxanthellae, shielding them from the sun.
A ray of hope for coral reefs
ABC Science 15 Dec 00;
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.
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?
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.
"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.
Coral cover
Rachel Nowak, New Scientist 13 Dec 00;
Fluorescent pigments in the skin of corals act as sunscreens that could help some reefs survive the worst ravages of global warming, say researchers from the University of Sydney.
The pigments were known to enhance visible light levels in gloomy conditions but this is the first evidence of a blocking action when the light is bright.
"It seems that corals don't just have to sit there and take it when it comes to bleaching. It's great," says David Barnes of the Australian Institute of Marine Science in Townsville.
Gavin Greenoak, a director of the melanoma and skin cancer research institute at the University of Sydney, told New Scientist: "These coral sunscreens are certainly interesting with respect to their absorption profiles, especially in the UVA region. They could be potential sunscreens for humans."
Corals depend on the photosynthetic powers of microscopic algae that live within them to provide them with carbohydrates. But light is scarce in deep water so the corals contain fluorescent proteins that absorb light in the ultraviolet A to violet spectrum and reemit it in the blue and green spectrums. This amplifies the light available for photosynthesis.
But Anya Salih of the University of Sydney and her colleagues also found fluorescent pigments in 97 percent of shallow-water corals, even though those corals were actually at risk of being damaged by too much sun. That suggested that the pigments were also acting as a sunscreen.
The team created cross-section images of individual polyps using confocal laser scanning microscopy. These revealed how the same pigment could do two very different jobs.
In deep-water corals, the pigments are packed among or below the microalgae in the polyp's endoderm. "They act like broken mirrors, reflecting some of the light back to the microalgae," says Salih.
But in shallow-water corals, the pigments are stacked in the polyp's ectoderm, above the algae and shielding them from the sun.
When corals are exposed to slight increases in heat or light over the usual summer maximums, they often spit out their microalgae and become "bleached". This has raised fears that global warming will destroy coral communities like the Great Barrier Reef, where substantial bleaching has recently occurred.
However, Salih found that coral that contained the highest density of fluorescent pigments tended to survive.
"Maybe not all coral are destined to die. Maybe some will survive and repopulate the reef," says Salih. But she cautions that it is unclear how long even heavily pigmented coral can survive extreme conditions. She also notes that even if some coral survives, diversity will still be lost.
Colourful key to coral survival
Heidi Gibson www.undersea.com
While speculation continues over the impact of global climate change on the world’s tropical reefs, Dr Anya Salih, marine biologist with Australia’s University of Sydney, is working to identify the capability of the Great Barrier Reef to adapt to the warming of the world’s oceans.
“When corals are stressed by changes like increased water temperature or exposure to sunlight, they can lose their colours or ‘bleach’,” says Anya. “Bleaching, is a sign that the coral is unwell. It may recover but if the bleaching is too severe it will die.”
Over the last 20 years, a rise in mass coral bleaching throughout the world has been linked to the increased frequency of El Nino Southern Oscillation - a phenomenon that, in turn, has been linked to global warming.
“As a result, the world’s mean ocean temperature has increased by about 0.5 degrees,” says Anya, “and it is predicted that it will warm by another one to three degrees worldwide by the middle of this century.
“For corals, a change as little as one to three degrees can lead to bleaching so we are greatly concerned about their survival.”
When stressed, a coral will expel the zooxanthellae - the microscopic algae or tiny plants that live in its tissue - causing the loss of colour known as “bleaching”. Zooxanthellae use photosynthesis to transform sunlight into energy and help corals to survive by creating an extra source of carbohydrates or energy.
“We had already noticed that some corals didn’t bleach and some did so only partially,” said Anya, “[and] during night dives, I had also observed that some coral colours had glow-in-the-dark or fluorescent properties.”
“I wondered if there could be a relationship between the two phenomena.”
To investigate, Anya and her team joined the crew of Undersea Explorer - a combined scientific research and adventure dive vessel based in Port Douglas, Queensland - for an expedition to the remote outer regions of the Great Barrier Reef.
Anya estimates that about 70 to 90 per cent of corals in shallow waters have the fluorescent colour pigments with the number decreasing to about 50 per cent at lower depths.
“These coral’s fluoresce continually under sunlight but the sunlight is so strong you can’t see it,” she says.
“We believe that the fluorescent pigments in some coral tissues act like a kind of sunscreen by dissipating excessive sunlight as fluorescence before it damages the zooxanthellae.”
By placing special light filters on their underwater torches at night, Anya and her team identified a range of fluorescent coral shades from intense blues to brilliant reds, and most significantly, they found that fluorescent corals did seem to survive bleaching better.
Back in the lab, Anya explains: “During fluorescence, the electrons inside the coral’s colour pigment molecules become excited. They jump higher and emit energy. The blue colours come from high energy emissions, then as the energy decreases the colours change to greens, then yellows and reds. Basically, the coral pigment cells break up solar radiation into the same energy wavelengths seen in the colours of the rainbow and by absorbing higher energy - UV and blue light - and emitting it in green to red fluorescent colours, they transform damaging light into less energetically damaging wavelengths.”
In the long term, Anya hopes their research will be used to help protect coral from the damaging effects of continued global warming.
Related links
Fluorescent pigments in corals are photoprotective
Anya Salih1, Anthony Larkum, Guy Cox, Michael Kühl and Ove Hoegh-Guldberg
Abstract from Nature 408, 850-853 (14 December 2000)
