OUR OCEANS ARE changing at an alarming rate – alongside localised pollution, reefs worldwide are becoming warmer and more acidic – which together threaten the survival of coral reefs. 2016 has seen the world’s third mass coral bleaching event. (Not sure what “coral bleaching” is? See the box below.) We have all seen images and videos from around the world showing fields of white coral skeletons, where only weeks before existed colourful, healthy coral reefs.


Warmer seawater is often the cause of coral bleaching; this year alone, it is estimated that over a third of Australia’s Northern Great Barrier Reef has been killed due to coral bleaching from the warmer-than-normal water associated with an El Niño event.

  • Above average seawater temperature is the main culprit causing coral bleaching but it can also occur from other stressors such as disease, high light, pollution and changes in salinity.
  • Corals can only recover from bleaching if “host tissue” remains intact on the bleached skeleton to reacquire zooxanthellae and rebuild energy reserves. This is only possible given enough time between repeat El Niño (or stress) events.

  • Corals that lose their host tissue are dead, and collapse of the reef framework follows. Corals can only repopulate impacted areas from neighbouring reefs that are unimpacted or through “reef restoration” practices.


    Adding to the challenge for coral to survive our changing oceans is the rising acidity of seawater. As oceans absorb carbon dioxide they become more acidic. This is a problem for marine organisms, like corals, that build a calcium carbonate skeleton. In more acidic water, the ability for corals to build teir skeleton becomes compromised: Quite simply they need to invest more energy to sustain their skeletons or otherwise lose their structural integrity.

  • Each year the oceans absorb 25 percent of all carbon dioxide that we emit.

  • The ocean pH is around 8.1 (which is actually slightly alkaline) and is predicted to drop to 7.8 (still slightly alkaline) by 2100.

  • The oceans are nearly 30 percent more acidic than they were at the start of the industrial revolution.

  • By the year 2100, it is predicted that the oceans could be 100 to 150 percent more acidic.


With so much pressure on the Earth’s fragile coral reefs, their survival is uncertain. Scientists around the world are exploring how corals may be able to survive in the future, within a changed climate. One way that researchers can study this is by laboratory experiments, where future ocean conditions are simulated in a controlled setting.

Another approach is to locate environments where corals are already, naturally, living in extreme conditions. Here, scientists can explore which organisms are able to survive and how. Such natural extreme systems are more common than you would think, and it seems that if we look, Nature may already have provided some of the answers we are after.


Corals are animals that live in harmony with microscopic algae (commonly called zooxanthallae) that act as solar cells for the coral, producing energy by absorbing sunlight. Unfortunately, this relationship can break down under stressful conditions, causing corals to expel their microalgae, the sign of stress we know as “coral bleaching”.

Corals typically survive within a narrow range of environmental conditions [e.g., light, temperature pH (acidity) and salinity], which are critical to sustain optimum growth. However, accelerating human impacts are changing the world’s oceans, requiring corals to survive under conditions that are suboptimal. Many coral reefs worldwide are therefore now in a state of change as they attempt to keep pace with environmental change and successfully function under new conditions.


Read the rest of this article in 2016 Issue 4 Volume 143 of Asian Diver magazine by subscribing here or check out all of our publications here.

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