Scientists Bring Light to Coral Bleaching Mystery

Coral reefs are the rainforests of the sea, hosting a wide variety of creatures that rely on the hard, coral structures for protection and food. As ocean waters warm, though, these reefs are in danger. Now, scientists have taken a closer look at the phenomenon of coral bleaching and have found out how it occurs at a molecular level.

Coral bleaching occurs when the symbiotic algae in corals either lose their identifying photosynthetic pigmentation that allows them to perform photosynthesis, or disappear from the corals entirely. Since corals rely on this algae to provide essential nutrients, bleaching can often kill vast swathes of coral reefs, leaving just white skeletons in its wake. That's why it's more important than ever to understand exactly how this phenomenon occurs.

Previous theories have suggested that bleaching occurs when heat damages a cellular organelle in the algae called the chloroplast. This chloroplast is responsible for photosynthesis and can produce toxic, highly reactive oxygen molecules during heat stress.

In order to better understand bleaching as see if this was the case, the researchers examined how the algae reacted in the dark. They found that, surprisingly, bleaching can still occur in warm, dark conditions. This means that the toxic oxygen molecules formed in heat-damaged chloroplasts during photosynthetic reactions during the light are likely not the major culprits that cause bleaching.

So what does cause bleaching? That's a good question. Scientists believe that bleaching in the dark could be an advantage to corals in some way.

"One theory that we are exploring is that under heat-stress conditions the coral eject the algal symbionts at night in order to avoid the production and accumulation of photosynthetically-derived toxic oxygen molecules during the day," said Arthur Grossman, one of the researchers, in a news release. "If such molecules were to accumulate, they would threaten the viability of both the alga and the host."

The researchers didn't just investigate bleaching, though. They also pointed out that some strategies aimed at protecting reefs may not necessarily work. For example, shading corals wouldn't halt bleaching, since bleaching still occurs in the dark.

The findings are published in the journal Current Biology.