Significance of metal-induced proteins in metal bioaccumulation in marine bivalves and pollution assessment

Metal contamination is one of the most ubiquitous, persistent and complex environmental issues. Successful management of economic and ecological risks from metal contamination depends on our knowledge of the key factors controlling metal bioaccumulation and toxicity as well as the robust diagnostic tools in pollution risk assessment. In this study, the possible roles of metallothionein-like proteins (MTLP) in Cd bioaccumulation and their use as biomarkers were firstly investigated in the marine scallops. Dissolved Cd exposure significantly increased the de novo synthesis and the standing stock concentration of MTLP, but there was no dose-dependent pattern of MTLP induction observed in the dietary-Cd exposed scallops. The result suggested that MTLP was probably unsuitable as biomarkers for Cd pollution monitoring. MTLP induction played little role in the Cd uptake, but MTLP induction significantly reduced the Cd elimination rate by recycling and remobilizing the metal from other intracellular pools. Significance of MTLP induction was further investigated in the Zn-exposed oysters. Facilitated Cd and Cu accumulation following solely Zn exposure was consistently observed in all oysters, as a consequence of the enhanced sequestration of Cd and Cu in the Zn-induced MTLP and granules. Then, proteomics technologies were employed to spot novel proteins associated with metal exposure. Laboratory Cd exposure resulted in limited proteome change in the four marine bivalves, and protein expression signature (PES) successfully distinguished the clams and the mussels with different body Cd levels. A field study further demonstrated that PES is a good diagnostic tool for contamination and toxicity of metals in real metal-contaminated ecosystems. Overall, the study provides new insights into the roles of metal-induced proteins in metal bioaccumulation, and points to a novel diagnostic tool in environmental monitoring programs.