HONG KONG, March 20 (Reuters) - Certain types of algae can help corals withstand higher sea temperatures and prevent them from bleaching, scientists in Australia have found.

Coral reefs are vulnerable to climate change and without rapid genetic adaptation, they will not survive projected sea temperature increases over the next 50 years, experts say.

But in an article published in latest issue of the Proceedings of the Royal Society B: Biological Sciences, the researchers said they may have found an answer to why some corals continue to thrive in warmer waters when others die.

The answer appears to lie in a heat-tolerant single-celled algae which lives in coral tissue, said Ray Berkelmans at the Australian Institute of Marine Science.

In the study, the researchers tagged and analysed some 480 coral colonies in the Keppel Islands of the Great Barrier Reef and found that some 94 percent of them contained a heat-sensitive strain of the algae, named C2.

But after a natural bleaching event in 2006, those corals that managed to survive were dominated instead by the heat-tolerant algae strain, called type D.

"The hypothesis is that C2 was dominant in the tissues, but present in background levels that are sometimes hard to detect were the D-type," Berkelmans explained.

"With the dominant algae being expelled (because of warmer temperatures), the more heat-tolerant algae had the chance to reoccupy the space. And as the coral recovers, the previously low-density algae became more dominant."

Some algae produce toxic compounds in warmer waters and corals start expelling them to try to survive. But very often, corals die before they are able to get rid of all the bad algae.

Looking ahead, Berkelmans said his team would continue to study corals that managed to survive bleaching.

"Is it because they have background levels of type D algae? And if so, we have to protect these a little bit more so they can repopulate at great speed," he suggested. (Reporting by Tan Ee Lyn; Editing by Alex Richardson)

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