Assistant Professor, Faculty of Science (Earth Sciences, Mineralogy)

MOTAI Satoko

Reef-building corals are animals that live in shallow sea waters of the tropics and subtropics. They grow toward the sea surface while building an exoskeleton of calcium carbonate (CaCO₃). Two main polymorphs of calcium carbonate are known: calcite and aragonite that differ in their atomic arrangement. Calcite is  thermodynamically more stable phase under the temperature and pressure conditions in which reef-building corals live. However, reef-building corals form their skeletons with aragonite, which is thermodynamically more stable phase under high-pressure phase. How do reef-building corals form aragonite skeletons?

The calcification space where reef-building corals form their skeleton is very small (<1 µm), making direct observation of skeletal formation difficult. We have been studying skeletal growth on observations of the skeletons as product using mineralogical perspective. Recently, we grew reef-building corals in a special environment and examined whether their skeletons were aragonite or calcite. Previous studies have noted the Mg²⁺ concentration in seawater is related to aragonite formation, but we found that zooxanthellae symbiotic with reef-building corals might be also involved.

Many aspects of skeletal formation by reef-building corals are still unknown, but we will continue our efforts to elucidate that process based on mineralogy.