Friday, July 18, 2008

Symbionts starve reef corals

Some corals may carry within them the seeds of their own destruction.

Reef-building corals are understood to be a combined biological creature, made up of coral animals that contain plants known as dinoflagellates. The coral can feed off the waste products of its symbiotic tenant, while the plant gains a range of benefits, including protection, from the coral.

(Image:Disease spreading across a coral. Credit: Michael Stat, HIMB/SOEST.)

But researchers have found that not all the symbionts—the plants contained within reef corals—are created equal.

“Symbioses are widespread in nature and occur along a continuum from parasitism to mutualism,” write Michael Stat, Emily Morris and Ruth Gates, of the University of Hawai'i's Hawai'i Institute of Marine Biology in the School of Ocean and Earth Science and Technology.

Their report, Functional diversity in coral-dinoflagellate symbiosis, was published in the Proceedings of the National Academy of Sciences.

In essence, they found that some of the single-celled plants that live within corals are excellent tenants and great partners.

But others are not, and can in fact are so dangerous to their hosts that they can be used to identify corals that will soon be in trouble from disease. It appears that by not providing nearly as much food to the corals as other such plants, they may weaken the corals and make them susceptible to diseases that healthy corals can avoid.

“The relationship between these dinoflagellates and corals has long been considered mutually beneficial, with the dinoflagellates supplying the coral with food via photosynthesis in return for recycled nutrients and shelter. Over the last 20 years it has been made clear that there are many different types of dinoflagellates in corals and that the unions or symbiosis between a given coral and their dinoflagellates can be very specific,” Stat said in a press release.

In work on the coral reefs of the Northwestern Hawaiian Islands at French Frigate Shoals, the researchers found that healthy corals often had a different variety, or clade, of dinoflagellate than diseased corals.

When they conducted laboratory tests, they found that the type found on diseased corals produced far fewer nutrients than the ones found in healthy corals.

“We have discovered that a group of diseased corals in the Northwestern Hawaiian Islands associate with a type of endosymbiotic algae that has never been found in Hawaiian corals before. Our analyses suggest that these endosymbiotic algae are not providing the coral with nutrition and that the corals may be starving, making them more susceptible to disease,” Gates said.

“This work shows for the first time that different types of coral dinoflagellates are not equally beneficial, and that there is a link between the type of dinoflagellate and coral disease,” Stat said.

The work suggests that reef researchers may be able to recognize a reef's susceptibility to disease by looking at the microscopic plantlife in its corals.

“Just as we have tests for human diseases such as cancer and tuberculosis, we now have the ability to screen corals for disease susceptibility. This discovery is a key finding that will contribute to the conservation and protection of ecologically important corals in Hawaii and elsewhere, Gates said.

© 2008 Jan W. TenBruggencate