Chemical State Analysis of the Periostracum in the Corbicula Clam Hatched in Lake Biwa

The periostracum of corbicula clams is a unique outermost organic layer secreted from the inner surface of the outer mantle. It records the environmental history of corbicula clams which inhabited Lake Biwa. Two types of corbicula clams, yellowish and blackish shells, were studied with X-ray fluorescence element mapping and S, Fe and Mn K-edge X-ray absorption fine structure (XAFS) spectroscopy. It was revealed that the blackish shells grow in the more reducing environment than the yellowish ones. Main constituent of the periostracum is protein-based material, such as cysteine and methionine. The blackish shells contain much higher amount of Fe and Mn impurities. Fe was accumulated as FeOOH along the growth lines. Sulfur exists exclusively as organic molecules with the functional groups of thiol (-SH), which is oxidized to form disulfide (-SS-) by exposure to the oxidizing environment.