“It’s an incredible find” says Anya Salih of the University of Western Sydney, “at a relatively shallow depth of 15m, an amazing community of highly fluorescent corals.” Fluorescent corals are thrilling to see, their incandescent hues invisible in daylight; it is at night where they are at their most spectacular. The mysterious miniscule proteins that provide corals with such ghostly radiance, are helping to save the world’s beleaguered coral reefs and are causing something of a revolution in the research of some devastating human diseases.
The site in Lord Howe Island Marine Park in Australia, found on a reef at the base of a sheer cliff is, says Salih, extraordinary. “Most commonly the highly red florescent corals are found deeper or inside reef overhangs and even then are never so abundant”. Salih and her team were surprised when they found underwater buttresses and caverns densely packed with hundreds of corals. And all of the corals they found were pigmented by the most intense blue, green and red fluorescence. Salih, an expert in coral fluorescence, says it was such a surprise find because these types of coral communities usually occur on deeper reefs in other parts of the world. The startling effect of fluorescent corals, like those Lord Howe Island, are invisible to us without the use of blue light and yellow filters used to eliminate the non-fluorescent light rendering the coral florescence at its most vivid.
Fluorescent Proteins In Nature
Coral reefs are made of formations we call corals. These impressive constructions are built by often-enormous colonies of organisms known as coral polyps. It is the combined efforts of these tiny creatures that create the coral skeletons, we know and love. Coral polyps are the result of a very special relationship between an animal (cnidarians) and plant (an algae called zooxanthellae) that has no parallel in nature. There are about 1 million zooxanthellae per centimetre of coral surface, and they are the powerhouse of the coral providing energy and nutrition for growth. All corals are coloured with their algae providing the brownish hues, but some coral have green, yellow or red hues, visible in daylight. These colours are due to fluorescent proteins; some of these, mainly in the blue and cyan spectra, are invisible in daylight. And these florescent proteins become highly visible when irradiated by certain wavelengths in the dark. Its light is a ghostly radiance that scientists are still largely baffled about; coral fluorescence.
Coral fluorescence is the absorption of light, say blue light, at one wavelength by a fluorescent protein, and then the emission at another wavelength, red or green light for example. In simple terms, fluorescence is giving off one form of light only after it receives another form of light. And it is a quite different phenomenon from bioluminescence. In florescence, energy comes from the light you shine on the animal; bioluminescence in contrast, comes from a chemical energy inside the animal.
The ethereal glow of corals is not unique; such startling displays have been documented in other marine animals; some jellyfish, anemones, hydroids and in some sponges. It has even been found in invertebrates such as shrimps, crawling sea stars, polychaete worms and nudibranchs. In fact, many organic substances with chlorophyll will give off red fluorescence. Nevertheless, corals provide the most spectacular and mysterious display. And they are unique in that the ability to fluoresce is encoded in a single gene, and it is this that has made them so valuable to scientists
Such peculiar fluorescence is not a new discovery. Some 45 years ago, a jellyfish living in the waters of the Pacific Northwest was first found to have a green fluorescent protein in it. Scientists found that it played a secondary role in the bioluminescence emitted occasionally by the jellyfish and this unusual protein became known as GFP. Its discovery marked the advent of a major breakthrough in biomedical research and led to a Nobel Prize in 2008 for the scientists who discovered it. And the fluorescent colours found in corals are produced by proteins related to this original GFP.
The Green Fluorescent Protein Revolution
Scientists were able to clone the GFP by the 1990s and then attach it to cells and proteins in the human body. Since then scientists have been on the hunt for more colours. Although scientists were able to manipulate jellyfish DNA to get yellow, dark blue or cyan, until relatively recently they could not engineer a red fluorescent protein. Red is critical, says Salih because it penetrates much deeper into living tissues than other proteins. But in 1999, scientists in Russia found a red GFP like protein in a common coral-like organisms, a zoanthid, sitting in a laboratory tank. The hunt was then on to find other fluorescent proteins and scientists studying corals on the Great Barrier Reef came across by far the highest number.
Scientists can now isolate the unique proteins in florescent corals, attach them to a dividing cancer cell or a virus and watch it as it spreads and divides. The process is then illuminated with a special light to enhance the fluorescence in the same way a diver would shine a blue light on corals underwater to see them fluoresce. It would be impossible to see such cellular activity without these remarkable proteins.
Glowing Proteins May Protect Corals
It is clear these colours are not provided for human entertainment so what of other reef inhabitants that may be daily exposed to such light shows? Scientists now know that fish have adapted to the different wavelengths of light in the marine environment caused by the unique filtering effects of water. In a recent study, damsel fish were found to have ultra violet vision. But there appears no evidence, as yet, that fish are preferentially tuned to wavelengths of fluorescence in corals, or that they have any particular different interaction between corals with fluorescence and non fluorescent corals.
Aside from providing an extravagant display to divers, it is still unclear what exactly the biological functions of fluorescent proteins in corals are. Research by Salih and others reveal that the fluorescent proteins main function appears to be light optimization. Having fluorescence may help corals in producing food by providing light in deeper and darker water to allow their algae to grow, since algae need light. They also suggest that the fluorescent proteins may protect coral against harmful sunrays acting much like a sunscreen. Salih and her team found that the fluorescent corals found at Lord Howe Island reefs were much less bleached than the non-fluorescent types occupying the same reefs. She says that this lends further support to the photo protection hypothesis.
Coral bleaching is a serious concern around the world as it increases in intensity and frequency, and these proteins may be critical in protection against heat stress in a warming ocean.
Lord Howe Island, known for its unique marine life and pristine conditions experienced a sudden mass bleaching event caused by warming of seawater in early 2010. “Lord Howe Island has the most significant southern most coral reefs, so when we start to see signs of extensive coral bleaching, we know that the climate is definitely changing” said Peter Harrison of Southern Cross University. Unusually calm conditions and high temperatures were to blame for the extensive bleaching. Salih is now monitoring and comparing the recovery of fluorescent versus non-fluorescent corals at Lord Howe Island. The results, she says, will help scientists find out how fluorescence affects the long-term survival of corals after a mass bleaching event.
Salih concludes though that under normal environmental conditions fluorescent proteins are photo protective, they can be overwhelmed by the extreme heat and light stress that causes mass coral bleaching. Luckily, for Salih and her team, a large number of corals were protected at Lord Howe Island and they are now being tested for new types of fluorescent labels. The GFP and its related fluorescent proteins are one of the most used proteins in biomedical research; they are a highly visible biological marker.
Shedding Light on Human Diseases
Using special laser microscopes scientists can use them to follow individual cells and molecules to see what actually happens in both healthy and cancerous cells. These proteins are also being used to monitor and study AIDS and Alzheimer’s disease as well as cancer. And they are now further being used to decipher the wiring in the brain to make, what scientists at Harvard University in the US, are calling a “brainbow”. The resulting kaleidoscope of colours from using GFPs is helping to light up neurones in the brain, shedding light on a number of other human diseases such as; autism, bipolar disorder as well as learning disabilities. The original GFP, and these newer coral proteins, have been spliced and manipulated, to create other colours of red, yellow and blue and their variants, creating around 90 such hues.
Researchers have investigated dozens of coral species in the hunt for fluorescent proteins. Fluorescing coral is now known to be common around the world and the Great Barrier Reef in Australia, not surprisingly, has the greatest number. On a study of shallow reefs on the GBR, scientists found that up to 97% of corals contained fluorescence. Fluorescence is further being used to monitor the impact of events such as climate change on the reef environment.
Glowing corals continue to baffle and intrigue science but one thing we know is that nature does not give up her secrets easily. Their study and use in medicine and conservation may take many years to develop to their full potential and we need to make sure they are around long enough to give up some of their mystery. “We need to protect marine ecosystems for environmental and social reasons,” says Salih “but also, importantly, for science”.
Sarah Curran-Ragan - Science journalist and marine biologist, Sarah Curran Ragan, keeps you informed about our Oceans.
