By Jean Balchin 05/07/2017

It has long been established that corals in shallow waters glow because of fluorescent proteins that act as sunblock, protecting the endangered species from the sun’s intense rays.

As any kiwi can attest, too much sunlight is bad for humans. Excess sunlight is also detrimental to corals. Some shallow water corals produce fluorescent proteins to block excessive sunlight that could harm the zooxanthellae. However, this doesn’t explain why numerous corals in deep waters are also brightly fluorescent.

It turns out that corals in deep water are fluorescent for the exact opposite reason – to absorb the little light there is for the benefit of photosynthetic microorganisms that provide most of the coral’s energy needs.

Found within most corals are photosynthetic algae called zooxanthellae. These microscopic algae live within the coral’s tissues, and the two share a mutually beneficial relationship. The coral provides the tiny algae with a protected environment, carbon dioxide and nutrients, while the algae produce oxygen and help the coral to remove wastes. The zooxanthellae also generate glucose, glycerol and amino acids; the products of photosynthesis. These products are employed by the coral to create proteins, fats, and carbohydrates, and produce calcium carbonate.

Coral glowing brightly in the deep blue sea.

Why do these deep water corals glow? 

Sunlight is an incredibly important resource for these corals. However, corals may be found in water as deep as 165m, where little sunlight is available. What light that does filter through to these depths is almost all in the blue spectral range, the other colours having been reduced in intensity by the water column.

Researchers from the University of Southampton, working with scientists from the Interuniversity Institute for Marine Sciences of Eilat and the University of Haifa, Israel, set out to resolve why deep water corals glow. They analysed how well different light colours spread through dense layers of zooxanthellae, the microscopic algae vital to coral survival.

The researchers found that corals survive in deep water by making a special type of fluorescent protein that captures blue light and reemits it as orange-red light. This orange-red light has the potential to penetrate deeper into the coral’s tissue, and enhances the photosynthetic capacity of the zooxanthellae.

The team also simulated deep water light environments in their experimental coral aquarium and found that bright red fluorescent corals had a better chance of survival in the long run compared to their non-pigmented counterparts. This matched the distribution of corals in the Red Sea, where the team found that corals with these special red fluorescent pigments became more abundant in deep water.

Professor Jörg Wiedenmann, head of the Coral Reef Laboratory at the University of Southampton said: “This is an important step forward in understanding how the mysterious fluorescent pigments in corals work. Our finding help us to understand how the amazing diversity of coral colours structures the communities on coral reef.”

“Deep water habitats are discussed as potential refuges for corals from the increasingly degraded shallow water reefs. Our work shows that the ‘deep blue sea’ may not be the welcoming sanctuary our endangered coral reefs can retreat to without consequence. It becomes clear that corals need special features to adjust to life in these low-light depths for the benefit of their vital photosynthetic partners and not all shallow water corals might be able to use this escape route. It is of utmost importance we do our best to keep their homes in shallow water habitable”.

Applications for human health

These fluorescent proteins also have important applications for human health. Given that they brightly glow green, yellow and red under a blue light, these proteins enable scientists to make important discoveries in cancer and HIV research by lighting up living cells so they can be seen under a microscope.

Scientists are on the hunt for new fluorescent proteins, particularly those glowing at the far-red end of the light spectrum which are easier to detect than green fluorescent proteins in tissue made up of many cell layers.

“Coral reef survival is dangerously threatened by climate change and human activity. Reef organisms can provide us with potential life-saving tools and products, so we need to act now and protect this pharmaceutical treasure chest,” adds Professor Jörg Wiedenmann.

A team of scientists from the University of Southampton is displaying their research at this year’s Royal Society Summer Science Exhibition and publishing their work in Proceedings of the Royal Society B. Read more about this study here