Scleractinian corals make their own exoskeletons by producing aragonitic calcium carbonate through a process known as bioprecipitation.

Examining the coral structure very closely using Scanning Electron Microscopy (SEM), we can see the needle-shaped aragonite crystals produced by the bioprecipitation process.



Although the calcium ion dominates the calcium sites of the calcium carbonate, other elements can substitute into the lattice at trace levels. The concentrations of these elements are of interest to us. Other researchers have shown that corals have the ability to concentrate specific ions in their lattice relative the concentration in ocean water. This ability has been called the vital effect. Our interest lies in determining if different genera of corals, under a variety of living conditions, have consistent compositional variances in their exoskeleton.

For this work, we have chosen the tropical marine environment surrounding the island of Moorea in the French Polynesian Islands. Our collecting activities have been focused on the lagoon, reef crest and fore-reef. Our first expedition, in July 2011, provided corals from 15 genera. Using both SEM/EDS for gross structural analysis and Atomic Absorption Spectroscopic Analysis of digests of the samples, we are determining the metal concentrations in the corals.


The fundamental question we are investigating is: "do corals alter their chemical composition to adjust their mineralogy for a selective advantage in their environment?" This question is predicated on the fact that different elements can alter the solubility, the hardness, the modulus and impact resistance of minerals. For this reason, it may be that corals can improve their ability to survive under environmental pressures by altering their composition relative to pure calcium carbonate.