Multi-Element Profiling Analyses of Symbiotic Zooxanthellae and Soft Tissues in a Giant Clam (Tridacna crocea) Living in the Coral Reefs and Their Intake Process of Zn and Cd

Abstract

Abstract Approximately 20 major-to-ultratrace elements in each organ (mantle, muscle, liver, kidney) of a giant clam, Tridacna crocea, and 16 elements in symbiotic zooxanthellae were determined by ICP-AES, ICP-MS, and CHN coder. The biogeochemical properties of T. crocea were then investigated using multi-element profiling analyses. The values for most elements were the highest in the kidney among all organs. However, the concentrations of Mn, Co, Ni, Zn, Se, Ag, Cd, and Pb in the mantle were higher than those in the muscle and liver. Therefore, these results indicate that most trace elements taken in by T. crocea through zooxanthellae may finally accumulate in the kidney. Moreover, 14 trace elements in zooxanthellae were present in the concentration range from 181 µg g−1 for Zn to 0.58 µg g−1 for U based on dry weight after isolating zooxanthellae from the mantle. The concentrations of some trace metals in zooxanthellae were relatively higher than those in other organs except for the kidney. These results suggest that zooxanthellae could play an important role in extracting trace metals from seawater and providing them to the soft tissues in giant clams. The concentrations of Mn, Co, Ni, Zn, As, Se, Ag, and Cd in zooxanthellae were almost equal to or higher than those reported in literature. In particular, Mn, Co, Zn, and Cd were present in concentrations higher than those in whole soft tissues of T. crocea by factors of at least 105. In addition, we investigated the intake route of Zn and Cd from seawater for T. crocea using enriched stable isotopes of Zn and Cd. These results suggest that zooxanthellae could play an important role for the intake of trace metals such as Zn and Cd from seawater and for providing them to soft tissues in the giant clam.