A warmer ocean: coral bleaching
Want to keep
University of Southampton online course,
Exploring Our Ocean
Causes of coral bleachingThe key trigger of coral bleaching is heat stress. If temperatures exceed the long-term average of annual peak value of a certain reef region by only one degree for a few days to weeks, corals will start to bleach. At the molecular level, the heat stress causes damage to the photosynthetic apparatus of the symbionts. It is thought that the symbionts start to release harmful oxygen derivatives into the host cells under these conditions so that the previously beneficial partners become toxic for the host. As a consequence, the symbiotic relationship breaks down and the symbionts are be lost. Due to the involvement of the light-dependent symbiont photosynthesis in the process, exposure to sunlight can aggravate the bleaching response.Our research at the University of Southampton has shown that the disturbance of the natural balance of the dissolved nitrogen and phosphorus nutrients in the water can render corals more susceptible to heat-induced bleaching. In fact, when corals are deprived of phosphate, the bleaching may occur at ambient temperatures. However, raising nutrient concentrations in the water will not necessarily render corals more tolerant to heat stress as numerous side-effects of elevated nutrient levels in the water will likely accelerate the degradation of reefs.
Can reefs adapt to higher seawater temperatures?The temperature at which corals and their symbionts become stressed depends on environment to which they are adapted in long-term evolutionary processes. The most heat tolerant coral symbiont of the world, the species Symbiodinium thermophilum has been discovered during our research in the Persian / Arabian Gulf. This species can stand summer temperatures of up to 35˚C on a regular basis, temperatures that cause bleaching and coral mortality elsewhere on the planet. Microscopic image of the world’s most heat tolerant coral symbiont species, Symbiodinium thermophilum from the Persian/Arabian Gulf. Photo by J. Wiedenmann & C. D’Angelo © University of Southampton 2019.However, corals associated with this algal symbiont will bleach when the temperatures exceed the usual summer maximum. We found that potentially thermotolerant symbiont species exist in small numbers in reefs that are currently dominated by heat sensitive symbionts and they may help corals to adjust to higher temperatures. However, even if this adaptation works, the most diverse reefs are still likely to lose 90% of their coral species that form the basis of many ecosystem services. Corals hosting the extreme temperature tolerant symbiont species Symbiodinium thermophilum in Saadiyat Reef (UAE) before (left) and after bleaching (right) during the hot summer of 2012. Photo by J. Wiedenmann & C. D’Angelo © University of Southampton 2019.
How can we save coral reefs?At the moment, the most important solution is to halt climate change by establishing carbon neutral societies. At the regional level, a managed and sustainable use of reefs that ensures best-possible water quality and prevents overfishing can help corals to resist temperature stress.
Your taskRead these suggestions from the National Oceanic and Atmospheric Administration and the International Coral Reef Society for ideas that all of us can do to help to save coral reefs.Please comment what you plan to do below!
Exploring Our Ocean
Our purpose is to transform access to education.
We offer a diverse selection of courses from leading universities and cultural institutions from around the world. These are delivered one step at a time, and are accessible on mobile, tablet and desktop, so you can fit learning around your life.
We believe learning should be an enjoyable, social experience, so our courses offer the opportunity to discuss what you’re learning with others as you go, helping you make fresh discoveries and form new ideas.
You can unlock new opportunities with unlimited access to hundreds of online short courses for a year by subscribing to our Unlimited package. Build your knowledge with top universities and organisations.