The stunning colours of coral attract many divers to the world’s reefs but, for us coral scientists, one mystery has always remained. Swimming over a reef, you can frequently spot brightly coloured coral sat next to differently coloured or colourless individuals of the same species. Why such variation in the same environment?
We now have the answer. Our research at the Coral Reef Laboratory at the University of Southampton lets coral colours appear in a new light: as sunscreening pigments that help explain how corals adapt to environmental stress. Our findings are published in the journal Molecular Ecology.
The underlying ecological concept may not be restricted to pigments in corals but might help to explain how species respond to changes in environmental conditions. Answers to these questions are urgently required in times of global change and alarming species extinction rates.
The brownish appearances of many corals under daylight is due to photosynthetic pigments from the microscopically small plants that live in symbiotic partnership within them. But most of the green, red and purple-blue hues are caused by a family of Nobel prize-winning protein pigments.
Some coral pigments glow green or red under ultra-violet or blue light, a physical phenomenon called fluorescence. During this process, light of a distinct colour is taken up by certain dye particles and re-emitted with a different, more red-shifted colour. The same process is responsible for the neon colours of marker pens or high-visibility clothing.
In shallow waters the pink, purple and blue coral colours are most striking. Beyond depths of around seven metres these colours tend to become dull since their brilliance depends on the reflection of red light. Down there, where blue light dominates, the green and red fluorescence of some corals makes them stand out from the bluish-grey background.
Fluorescence can be best observed with the help of blue light torches and special filter masks under low light conditions. Using this equipment, the glowing corals make a night dive a psychedelic adventure.
Why corals are colourful
Some corals increase the production of colourful protein pigments when they are exposed to more intense sunlight. Humans get a sun tan – corals become more colourful.
We found the pink and purple proteins act as sunscreens for the corals by removing substantial light components that might otherwise become harmful to the algae hosted in their tissue. Corals rely on these light-dependent miniature plants, the so-called zooxanthellae, since they provide a substantial amount of food.
We have also explained why some corals accumulate exceptionally high amounts of colourful pigments in growing areas such as branch tips or near wounds. These areas contain essentially no symbiotic algae, so much of the light is reflected by the white coral skeleton instead of being used by the algae.
The resulting increased light intensities in the new parts of the coral represent a potential danger for the algal cells that need to colonise these areas. Hence, it seems the corals use a clever trick to help their symbionts. The higher light intensity switches on the genes that are responsible for the production of the sun-screening pigments.
Our results suggest that this shading effect could help the algae to enter the new tissue and establish the necessary symbiotic association. Once the population of symbiotic algae is fully established, the light levels in the tissue decrease as the algae use most of the incident light for photosynthesis. As a consequence, the genes of the chromoproteins are switched off again, allowing the coral to save the energy required for their production.
Increased growth is associated with wound healing and neutralising potentially dangerous organisms by overgrowing them. The growth-related increase in pigmentation can also explain the bright colours of corals in areas where the animals have been damaged or are struggling with other organisms settling on their surface or in their skeleton.
Despite these recent advances in understanding the functions of coral pigments, we still didn’t know why corals of the same species could display such different colours, even when sat next to each other.
It raises challenging questions. If the production of the pigments is triggered by the light intensity, then why don’t all individuals have the same colours when they are exposed to the same light environment? And if these pigments help survival by acting as a sunscreen, then why aren’t corals in shallow waters always colourful?
Our most recent publication explains the genetic framework that results in the dramatic differences in coral individuals. We found that instead of using a single gene to control the production of sunscreening pigments, corals use multiple copies of the same gene.
These genes do indeed respond to light, but not all of them, thus it is the number of these active genes that is important – and this varies between individual corals of the same species. Depending on how many genes are active, the individual coral will become more or less colourful, even despite being exposed to the same light conditions.
However, the enhanced protection offered by the sunscreening pigments costs the corals a lot of energy that might be diverted away from growth or reproduction. Therefore, being brightly coloured might not be a good investment for corals settling in more shady parts of the reefs.
This genetic variation ensures some individuals within a coral population are well protected and are likely to survive better in stressful environments. Others are less protected but can instead invest their energy in processes that could help them to succeed in habitats with less light stress. These are probably the driving forces that keep multiple colour variants in the game for survival.
The resulting colour polymorphism makes it easier for coral species to inhabit more ecological niches in a reef. Humans can support the efforts of the corals by sheltering them from other forms of stress that they might not be able to deal with by themselves: heated waters, pollution, nutrient enrichment, sedimentation, overfishing, to name only some.