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Bacteria 'light bulbs' inside coral

Underwater photograph of M. cavernosa exhibiting orange daytime flourescence throughout the colony. The colony is approximately 0.6 m high. Image Science.
Click here for the full picture.

Many coral glow, not in the dark, but in the blue light of the ocean. For example, some varieties of the Caribbean Great Star coral, Montastraea cavernosa, give off a sunny orange color.

The orange light comes from pigments that fluoresce, meaning they absorb light in one color (blue) and emit it in a different color (orange). Researchers have now discovered that one of the fluorescent pigments of the Great Star coral doesn't actually belong to the coral, as most of these pigments do. Instead it belongs to bacteria that live inside the coral.

These bacteria offer more than just orange mood lighting, reports the research team, led by Michael Lesser of the University of New Hampshire. They offer nitrogen.

Corals need nitrogen to grow, but most of the nitrogen in the seawater comes in a form that the corals can't use -- like having a can of soup but no can opener. The new study suggests that in exchange for having a place to stay, the bacteria convert the nitrogen in the water into a form that helps the coral.

These bacteria are called "cyanobacteria" or "blue green algae." Corals like the Great Star coral also have a type of algae as tenants, the zooxanthellae. These algae provide the coral with other food items like carbohydrates.

The coral and its partners seem to make up a cooperative system, with each organism offering the other some benefit. The type of nitrogen that the cyanobacteria produce may help the zooxanthellae as well as the coral, according to Lesser. The zooxanthellae may return the favor by giving the cyanobacteria an energy source for their activities.

Why the cyanobacteria glow orange, however, is still a mystery.

The researchers' findings appear in the 13 August 2004 issue of the journal Science.


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