Symbionts affect coral’s chemistry

The presence of symbiotic organisms in the tiny animals that build coral reefs changes the rates at which the animals take in minerals from the water, new experiments show. This finding may affect the results of many research projects that have used chemical analyses of coral remains to infer past sea-surface temperatures.

TWO-TONE. Algal symbionts in Astrangia poculata make the right lobe of this coral mass darker than the left lobe, which contain no symbionts. The lobes absorb dissolved minerals at different rates. T. Rioux

Coral’s hard parts are composed of aragonite, a form of calcium carbonate. The animal pulls the ions that combine into the aragonite from seawater. As it builds the reef, it also deposits varying amounts of strontium, a chemical relative of calcium that’s also in seawater. Scientists have attributed most of the variation in strontium deposition in coral to changes in water temperature, and therefore have used the calcium-strontium ratio in fossil and living coral as a thermometer for the water that bathed the corals as they grew.

But for some species of reef builders, that thermometer may be in error, says Anne L. Cohen of the Woods Hole (Mass.) Oceanographic Institution. Her analyses of Astrangia poculata, a coral species that grows in the waters off Woods Hole, show that up to 65 percent of the variation in strontium deposition when symbiotic algae are present can be attributed to the algae and not to changes in water temperature.

That’s troublesome because scientists often analyze bits of fossil coral to infer ancient sea-surface temperatures. Some of those techniques may now need to change so they take into account the effects that symbionts may have had on the samples’ chemical composition, says Cohen. For example, assaying only the layers of old coral that were deposited at night, when the algal symbionts wouldn’t have been active, might provide strontium-calcium ratios more likely to be influenced by water temperature alone. Cohen and her team report their findings in the April 12 Science.

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