Corals that glow with the flow.

Corals that glow with the flow

Like trees with rings, corals embody a record of past environments. Wide growth bands and dense skeletal colonies of corals mark times when the temperature, sea level and climate were to their liking. Last year, Peter Isdale of the Australian Institute of Marine Science in Queensland disocvered that Porites corals of the Great Barrier Reef house yet another clue to the past: bands that fluoresce yellow-green when the coral is exposed to ultraviolet light. In the May 30 NATURE, he and colleague Kevin Boto examine the origin of these bands.

The researchers knew that all regions of the corals contain fulvic acids, derived from organic matter most probably in seawater, that give off a blue background fluorescence. They found that the yellow-green bands also contain fulvic acids, but with a greater proportion of low-molecular-mass compounds.

These compounds, they suspect, originate on the land because the timing, width and intensity of the yellow-green bands match peaks in rainfall and coastal runoff and because only the corals situated within 20 kilometers of the shore have the yellow-green bands. Indeed, Boto and Isdale were able to induce yellow-green fluorescene in fast-growing corals by incubating them in a mixture of seawater and fulvic acid extracted from local soils.

The researchers conclude that the wide distribution of Porites corals combined with the ubiquity of fulvic compounds in rivers and runoff suggests that fluorescent banding is common in near-shore corals. This means that centuries of rainfall and runoff data are available in corals the world over.