First report on the micro-essential metal concentrations in bivalve shells from deep-sea hydrothermal vents

The hydrothermal vent bivalve Bathymodiolus azoricus is naturally exposed to elevated levels of trace elements (Fe, Cu and Zn) and is therefore a suitable model organism to study physiological adaptations to extreme environments. Whole shells and various shell compartments of B. azoricus from two geochemically different hydrothermal vents of the Mid- Atlantic Ridge were analysed regarding their micro-essential metal concentration, and compared with levels in their shore analogues from contaminated sites reported in the relevant literature. Concentrations of Cu and Zn in hydrothermal shells exceeded many-fold levels reported for polluted areas worldwide, and thus reveal a prominent potential of this organ to accumulate bivalent metals (average concentrations of metals in whole shells of mussels from the Lucky Strike vent site were above average 280 μg g− 1 for Fe, 80 μg g− 1 for Cu and 40 μg g− 1 for Zn). Moreover, shell metal concentrations reflected fluid composition. In spite of higher metal concentrations in the periostracum than in the nacre, the latter has a higher metal burden because of its greater mass as compared to the thin periostracum. Additionally, metals were shown to bind reversibly to the outer surface of the shell (up to 6% of both total Cu and Zn, and 30% of total Fe from shells were recovered in an acidic solution in which the shells were soaked for 12 hours). It is concluded that shells are good indicators of environmental levels of Fe, Cu and Zn at hydrothermal vents and thus may be considered markers of putative changes in metal exposure over the mussel's lifespan.