How some corals recover from 'bleaching'?

A sharp rise in sea temperature can inflict potentially catastrophic "bleaching" on corals, but research published Wednesday identified factors that may render some reefs more resilient than others.
Five conditions can determine whether or not a reef is doomed after bleaching -- episodes that threaten a valuable source of biodiversity, tourism and fishing, scientists reported in the journal Nature.
"Water depth, the physical structure of the reef before disturbance, nutrient levels, the amount of grazing by fish and survival of juvenile corals could help predict reef recovery," said Nicholas Graham at Australia's James Cook University, who headed the probe.
Bleaching occurs when reef symbiosis -- the mutually beneficial relationship between two organisms that inhabit corals -- is disrupted by a surge in ocean warming, although there can also be other causes.
One of the worst episodes of mass bleaching, which affected reefs in 60 tropical countries, took place in 1998, a year of an exceptionally strong El Nino weather pattern.
Corals depend on single-cell algae called dinoflagellates that live in vast colonies on their surface.
The dinoflagellates feed on nitrogen, phosphorus and other nutrients provided by the coral, and use light to transform this food into energy.
The photosynthesis also releases energy into the tissues of the coral, enabling it to build the calcium skeleton which houses the dinoflagellates.
When corals come under stress, such as from significantly warmer seas, they expel the dinoflagellates. The corals turn visibly pale, as the algae have the pigments which give the skeletons their distinctive colour.
The reefs are not dead are this point.
But they become more susceptible to disease and will die if they fail to regain their plankton friends.
Graham's team draw their conclusions after scrutinising 17 years of data from the Seychelles, before and after the 1998 bleaching, which hit more than 90 percent of the country's coral cover.
Twelve of the country's 21 reefs recovered, but nine did not, with the differences between providing the researchers with clues.
The five factors identified could help inform policymakers struggling to manage coral reefs in the face of global warming as well as sedimentary deposit, over-fishing, and sewage and fertiliser runoff, the experts hope.
Last year, the UN's Intergovernmental Panel on Climate Change (IPCC), in its fifth assessment report, described coral reefs as "the most vulnerable marine ecosystem" on Earth.
Reefs have "little scope for adaptation" to warmer seas and acidification, a result of the ocean's absorption of carbon dioxide from the atmosphere, it said.
But John Pandolfi, a biologist at the University of Queensland, said there was a glimmer of hope.
The new findings "fit with experimental work suggesting that corals can quickly adapt to environmental change," Pandolfi said in a commentary also carried by Nature.
"Put simply, many reef corals just might be able to survive current rates of global environmental change."