Corals Give Their Life Partners The Green Light

The glow corals emit under UV light to attract their symbionts. Shunichi Takahashi

Corals and photosynthetic dinoflagellates are one of nature’s great symbiotic success stories, combining to create the dazzling reefs that host the greatest concentration of the world’s species. Yet one important aspect of their relationship has always been a mystery – how do they find each other? The answer may save dying reefs.

Although corals get some energy from catching plankton, most of what they need to live is supplied by members of the genus Symbiodinium, which also provide the gorgeous colors that make coral reefs such a joy to human eyes. When stressed, corals expel their dinoflagellates, but they die after too long without nourishment.

For all the research on what causes corals and dinoflagellates to part ways, little consideration has been given to how they come together in the first place. How do the single-celled organisms drifting across a reef find the coral that will become their home until heat stress do them part? Professor Jun Minagawa of Japan’s National Institute for Basic Biology investigated a theory that the corals' glow lights the way home.

Like jellyfish and a range of other invertebrates, corals produce a protein that glows green after exposure to shorter wavelength light. With great flair for language, biologists named this "green fluorescent protein" (GFP), and turned it into an invaluable tool for genetic research.

Minagawa collected free-living Symbiodinium and exposed them to different colors of light. In Proceedings of the National Academy of Sciences, he reports that Symbiodinium are attracted to green light and repelled by blue and violet wavelengths. When the containers were illuminated with blue light, causing the living coral to glow green and the dead ones to shine blue, 10 times as many Symbiodinium were found in the vicinity of the living coral than around the dead ones.

Convincing as this is, Minagawa conducted a further check to make sure some other factor was not distorting the results. A resin disk coated with fluorescent dye that released light with a peak wavelength just 1 nanometer higher than the GFP was also found to attract Symbiodinium. Again this was only when the dye’s fluorescence was stimulated with short wavelength light.

The attraction to green wavelengths allows Synbiodinium to find corals on the reef, and the repulsion from blue light prevents them being distracted by sunlight (which contains both green and blue light), although the authors think it initially evolved for other reasons.

The paper also suggests GFP may be necessary for corals that have expelled their Synbiodinium to recruit more, if the source of their stress is removed before they die. Hopefully, this knowledge can be applied for reef restoration.

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