Grottoli said that when the temperature of the waters around a reef exceeds that upper limit and stays there for more than two weeks or so, it triggers a bleaching event. Once that happens, the symbiotic algae and the brown or green photosynthetic pigments inside are lost. The result is a "bleached" white coral.
"In most cases, corals get 100 percent of their daily metabolic energy needs from the algae. Once they are gone, the coral polyp is left with only two alternatives: Draw energy from stored fats within its body, or eat organic matter and plankton in the surrounding water," she said.
But what has puzzled Grottoli and other researchers is why in some bleaching events, some corals quickly died off while others close by were able to recover. To answer that, she returned to Hawaii Institute of Marine Biology where she has been studying corals for the past 13 years.
There, she and her collaborators focused on two types of common coral that thrived on the local reefs, Montipora capitata, or "rice" coral, and Porites compressa, "finger" coral. They collected samples of both types and placed them in sets of tanks supplied with natural seawater. Water from the reef was filtered to remove any plankton and flowed through the tanks in the same way it did through their natural environment. In one set of tanks, the water was heated, mimicking the rising temperatures leading to a bleaching event.
After a month, fragments of the coral were gathered from all of the tanks and put through a series of tests measuring energy reserves, photosynthetic rates and growth rates of the coral. The results showed that both Porites and Montipora used up their internal energy reserves. However, after a month of recovery on the reef (where plankton is naturally available) Porites continued to use up its reserves whil
Source:Ohio State University