Bleaching hell what's going to happen to the Great Barrier Reef?

Coral bleaching is an increasing threat to coral reefs due to the increasing frequency and severity of bleaching events. Environmental stressors such as increase in temperature inflict stress on the coral individuals and their photosynthetic microalgal symbionts, known as zooxanthellae. Stress response involves expulsion or damage to the coral’s symbiont zooxanthellae, which can ultimately lead to death for the coral if they are exposed to stress for a prolonged period.

Coral are able to acclimatise by altering the ecospecies of their associated symbionts to a more thermal tolerant ecospecies, however the extent of this adaptation remains uncertain. Great Barrier Reef has been subjected to extensive coral loss over the past three decades and with predicted increase in sea surface temperatures in the future, knowing the recovery ability of these corals is vital for conservation of these ecologically important reefs.

Ainsworth et al. 2015 conducted a study to test the physiological response of the coral species Acropora aspera in response to elevated temperatures. Coral branches were collected from Heron Island and exposed to one of three thermal pre stress regimes, which replicated three observed SST trajectories –

 1 – “Protective trajectory’ – this involved a gradual increase in temperature over 4 days up to 32°C (below bleaching threshold of 34°C). Decrease in temperature to 26°C. Following this there was another gradual increase in temperature up to 34°C over a 7 day period.

 2 – “Repetitive “ – Followed the same regime as the protective trajectory however the first increase in temperature was to 34°C (thermal bleaching threshold).

 3 – “Single” – coral were not exposed to any initial pre stress they were only exposed to increase in temperature up to 34°C

Subsamples of coral were flash frozen after the 7 days and sent for qPCR. Samples were then given a 7 day recovery period before being exposed to another gradual heat stress up to 34°C. Samples were flash frozen and sent for qPCR and Symbiodinium cell density count. Temperature affects were applied for the interannual and ENSO event variability shown as well as different potential predicted future CO₂ emissions also being taken into account.

The study found that only the coral subjected to sublethal heat stress acquired thermotolerance after exposure by altering the coral symbiodinium community to a more thermotolerant ecospecies. Protective trajectory showed significant increase in expression of stress related genes compared to single and repetitive regimes and protective trajectory coral also showed a significant decrease in bleaching and symbiodinium loss. Variation in SST trajectories showed that increase of 2°C increases single stress events and decreases protective stress events from 77% to 22%, which would result in more severe bleaching events. CO₂ predictions showed that if emissions remained as they are coral cover would be less than 5% by 2100, however decrease in emissions showed protective trajectory would have no significant long term effect on coral cover whereas single and repetitive events would reduce coral cover to 5%.

This study was important in understanding the response of corals to different intensities of thermal stress and also providing thermotolerance limits of the GBR and the implications if the sublethal temperature were to be exceeded. More work could be done to extend this study by looking at the recovery of numerous different coral species as this only focused on one –Acropora aspera so it would be interesting to see how other coral species recovered. Testing whether the duration of the protective trajectory pre stress affects the coral’s ability of acquiring thermotolerant symbionts would also be interesting to see.

Ainsworth, Tracy D., et al. "Climate change disables coral bleaching protection on the Great Barrier Reef." Science 352.6283 (2016): 338-342.

http://science.sciencemag.org/content/352/6283/338