Corals' "Internal Communication" Process Critical to Maintaining Healthy Reefs

Disruptions causing decline of coral reefs around the world.


Corals, it appears, have a genetic complexity that rivals that of humans, have sophisticated systems of biological communication that are being stressed by global change, and are only able to survive based on proper function of an intricate symbiotic relationship with algae that live within their bodies, say researchers in a paper published in this week's issue of the journal Science.

Disruptions in these biological and communication systems are the underlying cause of the coral bleaching and collapse of coral reef ecosystems around the world.

"We've known for some time the general functioning of corals and the problems they are facing from climate change," said Virginia Weis, a zoologist at Oregon State University.

"But until just recently, much less has been known about their fundamental biology, genome structure and internal communication. Only when we really understand how their physiology works will we know if they can adapt to climate changes, or ways we might help."

Reef-building corals are facing severe environmental threats, said Clayton Cook, program director in the National Science Foundation (NSF)'s Division of Integrative and Organismal Systems, which funded the research. "The most evident are bleaching events linked to higher ocean temperatures, and the effects of ocean acidification on reef-building.

"It's critical to understand the cellular and molecular bases of how corals respond to these threats. This research demonstrates the importance of such basic processes as immune responses to these globally important questions."

Corals are tiny animals, polyps that exist as genetically identical individuals, and can defend themselves and kill plankton for food. In the process they also secrete calcium carbonate that becomes the basis for an external skeleton on which they sit.

These calcified deposits can grow to enormous sizes over long periods of time and form coral reefs--one of the world's most productive ecosystems, which can harbor more than 4,000 species of fish and many other marine life forms.

But corals are not really self-sufficient. Within their bodies they harbor highly productive algae--a form of marine plant life--that can "fix" carbon, using the energy of the sun to conduct photosynthesis and produce sugars.

"Some of the algae that live within corals are amazingly productive, and in some cases give 95 percent of the sugars they produce to the coral to use for energy," Weis said. "In return the algae gain nitrogen, a limiting nutrient in the ocean, by feeding off the waste from the coral. It's a finely developed symbiotic relationship."

What scientists are learning, however, is that this relationship is also based on a delicate communication process from the algae to the coral, telling it that the algae belong there and that everything is fine. Otherwise the corals would treat the algae as a parasite or invader and attempt to kill it.

"We now believe that what's happening when the water warms or something else stresses the coral is that the communication from the algae to the coral breaks down, and the all-is-well message doesn't get through," said Weis.

"The algae essentially comes out of hiding and faces an immune response from the coral."

This internal communication process, Weis said, is not unlike some of the biological processes found in humans and other animals. One of the revelations in recent research, she said, is the enormous complexity of coral biology, and its similarity to that of other life forms.

A gene that controls skeletal development in humans, for instance, is the identical gene in corals that helps them develop an external skeleton--conserved in different species over hundreds of millions of years since they parted from a common ancestor on separate evolutionary paths.

There's still much to learn about this process, researchers say, and tremendous variation in it. There are 1,000 species of coral and perhaps thousands of species of algae all mixing and matching in a symbiotic dance.

That variation provides at least some hope, scientists believe, that combinations will be found that can better adapt to changing conditions of ocean temperature, acidity and other threats.