Swimming through a tropical coral sea, you can't fail to be fascinated by the scintillating colours of the gently waving coral polyps. Supported by chalky skeletons, the delicate polyps filter nutritious zooplankton from the surrounding waters, to supplement the nutrition supplied by the coral's symbiotic dinoflagellate lodgers. Christine Ferrier-Pagès is keen to understand how the resourceful creatures lay down their internal skeleton and the factors that influence the skeleton's development. Working with Fanny Houlbrèque, she began investigating the effect that feeding has on the coral's skeletal development(p. 1461).
First Houlbrèque had to venture out into the Mediterranean twice-weekly to gather zooplankton to keep her corals satisfied. Back home in Denis Allemand's lab in Monaco, Houlbrèque began working with small coral colonies, which she divided into two groups: those she fed, and those that relied on nutrition supplied by the dinoflagellates alone. Supplementing the fed coral's diet with lab-reared copepods, Houlbrèque waited to see whether the fed corals faired better than their starved siblings, and was surprised after eight weeks to see that the fed coral's colour was much richer than the corals that had gone without. Feeding had allowed the corals to increase the density of dinoflagellate symbionts in the coral's tissues,giving them a rich colour. And when Houlbrèque investigated the coral's skeleton development, she found that the fed colonies produced more of the coral's organic matrix base, as well as laying down more calcium carbonate than the starved corals.
Clearly the corals had benefited from their enhanced diet, but so had their dinoflagellate lodgers. Had the corals' diet increased skeletal development directly, or were the fed coral benefiting from the nutrition provided by the enlarged dinoflagellate population? The team weren't sure, but they decided to test how well the fed corals' skeletons faired when their dinoflagellate companions could no longer photosynthesise; they switched off the light.
Houlbrèque monitored both the rates of organic matrix formation, and skeletal calcification in corals in the dark, when they couldn't benefit from the dinoflagellate's nutritional input. Monitoring the coral's uptake of aspartic acid into the matrix base Houlbrèque realised that the fed corals laid down far more organic matrix than the corals that were starving;feeding was having a direct effect on organic matrix synthesis. And when she monitored the fed coral's calcification rate with radiolabelled calcium, it was also higher than the starved coral's. Although feeding had a direct effect on both processes, Houlbrèque realised that `the increase in the rates of calcification in fed corals might be induced by stimulation of the organic matrix'.