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Of all divalent metals, mercury (HgII) has the highest affinity for metallothioneins. HgII is considered to be enclosed in the α and β domains as tetrahedral α‐type Hg4Cys11‐12 and β‐type Hg3Cys9 clusters similar to CdII and ZnII. However, neither the four‐fold coordination of Hg nor the existence of Hg–Hg atomic pairs have ever been demonstrated, and the HgII partitioning among the two protein domains is unknown. Using high energy‐resolution XANES spectroscopy, MP2 geometry optimization, and biochemical analysis, evidence for the coexistence of two‐coordinate Hg‐thiolate complex and four‐coordinate Hg‐thiolate cluster with a metacinnabar‐type (β‐HgS) structure in the α domain of separate metallothionein molecules from blue mussel under in vivo exposure is provided. The findings suggest that the CXXC claw setting of thiolate donors, which only exists in the α domain, acts as a nucleation center for the polynuclear complex and that the five CXC motifs from this domain serve as the cluster‐forming motifs. Oligomerization is driven by metallophilic Hg⋅⋅⋅Hg interactions. Our results provide clues as to why Hg has higher affinity for the α than the β domain. More generally, this work provides a foundation for understanding how metallothioneins mediate mercury detoxification in the cell under in vivo conditions. Mercury binding to metallothionein under in vivo exposition is unknown. HR‐XANES spectroscopy identifies two mercury‐thiolate complexes in the α domains of separate MT molecules. One is mononuclear and modeled as a bis‐six‐membered ring chelate through two axial sulfur atoms and two equatorial peptide oxygens from a CXXC (single‐letter amino acid code, where X can be any amino acid) motif. The other complex is polynuclear with a metacinnabar‐type (β‐HgS) core bound to the CXC motifs. The CXXC motif acts as a nucleation center for the Hg clusters. [ABSTRACT FROM AUTHOR]

Rainbow vent field is one of the most metal-contaminated hydrothermal sites on the Mid-Atlantic Ridge near the Azores region. Two hydrothermal shrimp species dominate the fauna at the Rainbow site along with the mussel Bathymodiolus azoricus. Although the levels of essential and non-essential metals in these shrimps have been studied, the biological consequences of a metal-rich environment are still largely unknown. Therefore, the aim of this study was to determine the levels of metal-binding proteins – metallothioneins (MT) and the activities of antioxidant enzymes – superoxide dismutase, catalase, total glutathione peroxidase and selenium-dependent glutathione peroxidase in two hydrothermal vent shrimps ( Mirocaris fortunata and Rimicaris exoculata) collected from the Rainbow site and to compare them with two coastal shrimps ( Palaemon elegans and Palaemonetes varians) from a south Portugal lagoon (Ria Formosa) to evaluate their different adaptation strategies towards metals in their environment. Results show significant differences in MT levels and antioxidant enzymatic activities between vent and coastal shrimps and also between shrimp species collected from the same site. This suggests that biochemical responses in both vent and coastal shrimps are affected not only by the environmental characteristics but also by inter-specific differences. Nevertheless, these responses apparently confer successful adaptation for survival in a metal-extreme environment. [ABSTRACT FROM AUTHOR]

Abstract: Hydrothermal vents are extreme deep-sea habitats that, due to their singular features, still intrigue scientific communities. Swift growth rates and profuse biomass of biological communities can be observed, despite of their inherently unstable physical–chemical and toxic conditions, indicating that organisms inhabiting this environment must be well adapted to these inhospitable conditions. The caridean shrimp, Chorocaris chacei, Mirocaris fortunata and Rimicaris exoculata, together with bathymodiolid mussels, dominate the vent fauna along the Mid-Atlantic Ridge (MAR). Crustacean species are widely used as biological indicators of environmental alterations, since they play a key ecological role as planktivorous grazers, epibenthic scavengers or as prey species. The biological consequences of the hydrothermal metal-rich environment in shrimp species are still largely unknown. Therefore, the aim of this study was the determination of the metal levels (Ag, Cd, Cu, Fe, Mn and Zn), metallothioneins (MT) and lipid peroxidation (LPO) in shrimp species collected in Rainbow, Lucky Strike and Menez-Gwen vent sites, in order to evaluate their different adaptation strategies toward metals when compared with two common coastal shrimp species (Palaemon elegans and Palaemonetes varians) from a fairly unpolluted estuarine system in south Portugal (Ria Formosa). Results show significant differences in metal concentrations, MT levels and lipid peroxidation between vent and coastal shrimp and also between shrimp species from the same site. This indicates that biochemical responses in both vent and coastal shrimp are affected not only by the environmental characteristics but also by inter-specific differences. Nevertheless, these responses apparently grant a successful adaptation for the survival in a metal-extreme environment. [Copyright &y& Elsevier]

Blue mussel, a widely distributed filter‐feeder mollusk often used as a sentinel organism for marine pollutants, copes with metal toxicity by overexpressing metallothionein, represented as wicked enchantments on the frontispiece. Two mercury‐thiolate complexes were identified in metallothionein in response to in vivo Hg exposure using high energy‐resolution XANES spectroscopy. One is mononuclear and modeled as a bis‐six‐membered ring chelate through two axial sulfur atoms and two equatorial peptide oxygens from a CXXC motif. The other complex is polynuclear with a metacinnabar‐type (β‐HgS) core bound to the CXC motifs. We propose that the CXXC motif, only present in the α‐domain, acts as a nucleation center for the Hg clusters via metallophilic Hg‐Hg interactions. Results from this research provide a molecular understanding for the higher mercury affinity of the α than the β domain, and provide a foundation for understanding how metallothionein mediates the detoxification of toxic metals in the cell under in vivo conditions. For more details, see the Full Paper by A. Manceau et al. on page 997 ff. [ABSTRACT FROM AUTHOR]