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The fluorescent orange color of this Caribbean Great Star Coral comes from symbiotic bacteria that live inside the sea creature. Michael Lesser, a zoology research professor at the University of New Hampshire, Durham, and colleagues have discovered that these cyanobacteria, which produce the fluorescent compound phycoerythrin, also play a vital role in fixing nitrogen for the coral's use [Science, 305, 997 (2004)].
Coral harbors numerous symbiotic organisms. For example, dinoflagellate algae live in the coral's gastrodermal cells, supplying the animal with carbohydrates. Scientists have known that bacteria exist inside some coral species, but their functions have been largely unknown. Cyanobacteria fix nitrogen on coral reefs and so have been thought to do the same inside living coral.
Now, Lesser's group shows that is indeed the case. The bacteria express nitrogenase, which converts the seawater's N2, which the coral can't use, into organic forms that it can use.
The group also made clear the source of the Great Star's daytime orange glow. They note that the fluorescence spectrum of phycoerythrin is similar to that of proteins responsible for much of the fluorescence seen in different coral species.
In addition to its symbiosis with the coral, the cyanobacteria also may have a relationship with the dinoflagellates, the latter supplying the bacteria with glycerol, for example.
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