Rise in ocean temperature due to global warming triggered coral bleaching, the frequency of which is increasing, which caused the mass death of corals worldwide.

The findings of a team of Australian scientists from the ARC Centre of Excellence for Coral Reef Studies and James Cook University showed that a complex cascade of molecular signals can lead to the self-inflicted death of corals and their symbiotic algae as the sea water begins to warm.

Several major bleaching events happened in the last 30 years, the most recent is the 2010 across the Indian Ocean and Coral Triangle.

Australia's Great Barrier Reef, which had eight coral bleachingevents since 1980, suffered its worst in 2002 when 55% of the total reef area was affected.

The researchers found the cascade begins at ocean temperatures as much as 3 degrees lower than those normally associated with coral bleaching.This process culminates in "apoptosis" or programmed cell-death, a situation where living organisms deliberately destroy their weakened or infected cells, to protect the organism as a whole.

"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," said Dr Tracy Ainsworth, lead author of the study.

"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," Ainsworth added.

However, the scientists also found molecular signals both promoting and discouraging programmed cell-death in the corals, thus, they theorized that corals respond to the stresses caused by warming sea water by killing off some of the cells while strengthening others for them to be able to recover after the water conditions have returned to normal.

Scientists said this explains why some corals are able to recover from a bleaching event.

"It is far too early to speculate, but understanding the recovery process for any living organism is always a big help, as human medicine has constantly demonstrated,"Dr Ainsworth said.

"The next step in our research will be to see how we can use this new insight into the processes of coral bleaching to understand their recovery mechanisms. We also need to know more about how this process works at lower temperatures, or under varying temperatures," he added.

Published in the Scientific Reports, the team's paper "Defining the tipping point. A complex cellular life/death balance in corals in response to stress" is by Ainsworth TD, Wasmund, K, Ukani L, Seneca F, Yellowlees D, Miller D, and Leggat W.