Coral bleaching is suicide writ large and corals are able to sense their impending doom before pulling the trigger, according to a paper published in a recent issue of the online journal Scientific Reports.
Coral bleaching is an ecologically devastating event triggered by rising water temperatures, in which corals expel the symbiotic algae (zooxanthellae) from their cells and die. A process known as apoptosis (programmed cell death) has been implied to be involved in coral bleaching, but the exact mechanism that leads to this cellular dissociation has remained a mystery until now.
By subjecting fragments of the staghorn coral Acropora aspera to increasing temperatures over eight days, Tracy Ainsworth and her colleagues from the ARC Centre of Excellence for Coral Reef Studies and James Cook University monitored the expression of coral homologues of six key regulators of apoptosis.
They found the expression of all six genes to significantly change prior to bleaching, with the cascade beginning at ocean temperatures as much as 3° lower than those normally associated with coral bleaching.
According to lead author Tracy Ainsworth: "Our results suggest that the control of apoptosis is highly complex in the coral-algae symbiosis and that apoptotic cell death cascades potentially play key roles in tipping the cellular life or death balance during environmental stress prior to the onset of coral bleaching."
"It is also clear that this chain reaction responds significantly to subtle, daily changes in the environment and to sea temperatures which were generally thought till now to have little impact on the function of coral and its symbiotic algae."
Interestingly, both genes that promote and discourage apoptosis were switched on prior to coral bleaching. This has led the authors to hypothesise that the corals deliberately kill off some cells while fortifying others to facilitate recovery once the water temperatures return to normal.
For more information, see the paper: Ainsworth, TD, K Wasmund, L Ukani, F Seneca, D Yellowlees, D Miller and W Leggat (2011) Defining the tipping point. A complex cellular life/death balance in corals in response to stress. Scientific Reports 1, 160 doi:10.1038/srep00160.
Why not take out a subscription to Practical Fishkeeping magazine? See our latest subscription offer.