Coral produces its own sunscreen.
June 23 2021
Colorful corals are in fact fighting to survive. Learn more about how bleaching affects corals and how their reaction to environmental stress acts as a natural sunscreen.
Coral reefs, like the Great Barrier Reef in Australia, are known for their magnificence, with bright shades of multiple colors that are certainly a natural pleasure to watch. They provide shelter and food to the multiple marine species that inhabit them such as anemones, fish, sea sponges or even turtles among others.
As essential as they are, coral reefs can be very fragile and easily affected by environmental pressures like sea temperatures.
Behind a colorful and vibrant display, lies a survival mechanism. In many reefs worldwide, “colorful bleaching” has become a reality due to the effect of ocean warming or UV damage.
It all starts with coral bleaching
Healthy corals have symbiotic algae (zooxanthellae) living within its tissues. This helps both of them thrive and gives them the external appearance we are used to seeing. When corals are healthy, most of the sunlight they get is absorbed by these algal symbionts. If corals are subject to environmental stress, they expel the algae living within them leaving their skeleton exposed to sunlight and turning completely white. This process is called coral bleaching. When this happens, they are not completely dead. Corals can recover from a bleaching event, but if this process is maintained over time (just a few weeks), the coral eventually dies.
What causes coral bleaching?
Corals are easily affected by environmental pressures. These are four main factors that can make coral lose its algae and bleach out.
1. Increased ocean temperature. Caused by global warming, this is the number one cause of coral bleaching.
2. Sunlight overexposure. In shallow waters, direct exposure to sun rays can also make coral tissues bleach.
3. Extremely low tides. Again, in shallow waters, air exposure during really low tides can have a bleaching effect in coral cells.
4. Pollution and runoff. Generated from storm water drainage, carrying contaminating substances that can bleach corals near shore.
An eye-catching survival tactic
According to research by scientists from the University of Southampton, some corals create a sunscreen that acts as their own natural protection. This sunblock is manifested as a colorful display of bright neon colors during mild warming incidents, and encourages the expelled algae to return in what has been named as an “optical feedback loop”.
Also according to the study, some corals have even created adaptations to anticipate bleaching by creating a shield around the coral and the algae protecting them from the negative effect of sunlight. UV and blue wavelengths are changed to low energy wavelengths (such as green or yellow) helping fluorescent corals to survive bleaching events. This would mean fluorescent corals have a better chance of survival over those without fluorescent pigments, which may lead to future evolutionary changes in coral species.
Climate change has decimated coral reefs worldwide over the last few years. From 2014 to 2017 a global bleaching event affected more than 75% of global reefs, with almost 30% suffering from mortality levels of stress. Although color bleaching may be able to save some coral, it’s not the ultimate solution and we still need to work on reducing global warming if we want to make a change and save the world’s oceans.
- Queensland Museum - Sunscreen for corals . QM website, https://www.qm.qld.gov.au/microsites/biodiscovery/05human-impact/sunscreen-for-corals.html, visited 09/07/21.
- NOAA - What is coral bleaching?. NOAA website, https://oceanservice.noaa.gov/facts/coral_bleach.html, updated 26/02/21.
- AMETSOC - State of the climate in 2017. American Meteorological Society website, https://www.ametsoc.net/sotc2017/StateoftheClimate2017_lowres.pdf, August 2018
- Current Biology - Optical Feedback Loop Involving Dinoflagellate Symbiont and Scleractinian Host Drives Colorful Coral Bleaching. Current Biology website, https://www.cell.com/current-biology/fulltext/S0960-9822(20)30571-6, 21/05/2020