The sunscreen of the sea

Bleached coral, with healthy coral in the background. Image credit: Wikipedia, CC BY 3.0.

Coral reefs are one of the most bountiful – and beautiful – ecosystems on the planet. These diverse, underwater rainforests host at least a quarter of all marine life, despite only occupying a tiny fraction of the ocean floor, and provide stability and filtration to our oceans and shores. However, in recent years more and more of the Earth’s precious coral has undergone bleaching – the loss of algae crucial to the coral’s survival – turning once vibrant reefs into bone-white graveyards. These bleaching events are hard-hitting, ever-more frequent, and often fatal to the coral. And yet, miraculously, some coral is fighting back, in one last – extremely colourful – stand against the changing oceans. 

Bleaching is a phenomenon that, while not new, has become more and more frequent and widespread to an unprecedented degree in recent decades. In 2017, National Geographic found that “three-fourths of the world’s reef systems…have experienced severe bleaching events”.  The process of bleaching can be triggered by a number of factors. Coral is incredibly sensitive to disturbances in its surroundings and, as one might predict, in these tumultuous times of global climate change, our oceans are experiencing an awful lot of disturbance. Change in water temperature, increase in solar irradiance, increased sedimentation from silt runoff, changes in salinity, elevated sea levels, pollutants, ocean acidification… any one of these triggers can cause bleaching, and, in many parts of the world, coral is experiencing most of them at once.

These triggers cause a breakdown in the delicate symbiosis shared by coral polyps and the microscopic algae that live within them. These algae are single-celled dinoflagellates (a form of plankton) called zooxanthellae, and they provide the coral with around 90% of its nutrient needs through photosynthesis, while the coral provides them with the carbon dioxide and ammonium that they require for this process. They also give the coral the vibrant colours it is famous for, so when the symbiosis between the two organisms is broken down and the polyps expel the algae from their tissues, all that remains is the coral’s pale calcium carbonate exoskeleton – the coral has been “bleached”. The coral is still alive, but vulnerable, and without its food source. Unless there is a rapid return to its optimal conditions, the coral will die. Or so we thought. 

While this remains true in the majority of cases, there are some rather colourful examples to the contrary. Some bleached corals – notably, ones that are likely to have survived milder disturbances and stresses in the past – undergo a vibrant transformation and put on hugely colourful displays. Researchers at the University of Southampton have revealed that these displays are a last-ditch effort by the corals in their fight to survive. The corals have lost their symbionts, meaning that high levels of excess light are trapped within the coral tissue, reflected by its white exoskeleton. For most corals, this decreases further still the chance that the zooxanthellae will return, as this is very stressful for them. However, in coral cells where some function remains, corals are able to use these increased levels of light to produce their own photoprotective pigments. These colourful pigments act as a layer of sunscreen, optimizing the coral for the return of its symbionts, which, if they are able to return, can begin to use the light for photosynthesis, and the relationship can – eventually –  revert to normal once again. 

It is crucial to note that these corals are more of an exception than a rule. This is an encouraging discovery, and implies that, at the very least, some patches of coral reef have a chance of recovery. But if climate change continues undeterred, coral reefs will not survive to see the end of this century. The only true hope for reefs is the global reduction of greenhouse gases and sustained improvement in water quality. Without this, there is no future for the coral reef, and the thousands of species that make their home there. 

Written by Heather Jones and edited by Ailie McWhinnie.

Heather’s thoughts… If you’re interested in learning more about coral reefs and what life has been like for them in recent years, I’d really recommend checking out Chasing Coral on Netflix! It’s from the same director as Chasing Ice – which I would also really recommend if you haven’t seen it yet. Both are really poignant and honest takes on how climate change is ravaging the world. Plus, it’s for science, so you don’t have to feel bad about taking another Netflix day… 

Find me on… Instagram @heatherfranj and LinkedIn @Heather Jones

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