Seaweed Wages Chemical Warfare Against Corals

An Acropora coral showing dead branches near the green seaweed Chlorodesmis. This seaweed chemically damages corals upon contact. CREDIT: D.B. Rasher View full size image

Seaweed and coral reefs may seem to be happy partners in our planet’s oceans, but an ongoing battle — complete with chemical warfare — appears to be brewing beneath the sea surface.

That's the conclusion of a new study that finds the seaweed (a type of algae) is actually killing corals and preventing them from returning to damaged areas to create new reefs.

Coral reefs around the world are being attacked from all sides, with climate change, overfishing, disease and excess nutrients from fertilizer runoff all taking their toll. The factors add up to a situation where seaweed has an easier job taking hold than coral.

"Corals are under an onslaught from lots of things — climate change, overfishing and others. They have declined due to all of these things, but they actually can recover in many parts of the world, as long as seaweed doesn't get in the way and create a layer of toxic stuff,” said Mark Hay, a marine ecologist at Georgia Tech in Atlanta who co-authored the new study, published Oct. 17 in the journal Proceedings of the National Academy of Sciences.

Attack of the killer seaweed

Typically, corals rule the underwater kingdom. But when overfishing takes away the fish that munch on algae and seaweed, big swaths of green and brown stuff can enter the picture and blanket the coral. Seaweed can cover close to 60 percent of the ocean bottom, the researchers said.

When the seaweed and corals come into contact, the algae attacks with special organic compounds known as terpenes, which probably originally gave the plants toxic protection against infectious microbes, or fish looking for a quick meal. But they're bad news for the reefs.

"These molecules cause coral bleaching.  We don't know the nitty-gritty of the mechanism behind this, but preliminary studies show that the coral being damaged by up-regulating genes responsible for fighting cytotoxins, reducing free radicals, and initiating cell repair,” said Douglas Rasher, a graduate student at Georgia Tech, the other study co-author. That means that the corals aren't able to fight off the attacking toxins. One way to fix this, Hay said, could be to give the coral small doses of the seaweed's toxins over time to make them build up a response, working like a vaccine to inoculate the coral.

By imbedding seaweed chemicals in gel pads on windowscreen and placing these on corals in the field, researchers identified seaweed molecules that damaged corals.
CREDIT: D.B. Rasher

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Better protection

To examine how the seaweed and the coral were interacting, Hay and Rasher worked in Fiji testing the competition between three species of coral and eight seaweed species. They found that most of the types of algae started to kill the coral within two weeks through direct contact, changing the coral's color and, in some cases, killing it quickly. The team also created extracts of the seaweed and applied it to the corals, with similar effects. There were some differences in the level of toxicity, though, with some seaweed being deadlier than the rest.

Using this information, people can create coral management plans, Rasher told OurAmazingPlanet. For example, Fiji has a unique system of marine-protected areas that are governed by local elders. If they can apply the science and eat fewer of the fish that eat the toxic seaweed, the corals have a better chance of surviving.

"While local managers can do little about the global reasons for coral decline, they can do something about local stresses, using data-driven management methods,” Rasher said.

This story was provided by OurAmazingPlanet, a sister site to LiveScience.