Metallothionein (MT) response after chronic palladium exposure in the zebra mussel, Dreissena polymorpha

Abstract

The effects of different exposure concentrations of palladium (Pd) on relative metallothionein (MT) response and bioaccumulation were investigated in zebra mussels (Dreissena polymorpha). The mussels were exposed to 0.05, 5, 50, and 500 μg/L Pd²⁺ for 10 weeks under controlled temperature and fasting conditions. Relative MT contents were assessed by a modified Ag-saturation method, which allows to discriminate between MT bound to Pd (Pd-MT) and MT bound to unidentified metals (Ag-MT). Determination of metal contents resulted from atomic absorption spectrometry following a microwave digestion. For unexposed mussels and mussels exposed to 0.05 μg/L Pd no metal accumulation could be detected. All other exposure concentrations resulted in detectable Pd accumulation in mussels with final tissue concentrations of 96 μg/g (500 μg/L), 45 μg/g (50 μg/L), and 9 μg/g (5 μg/L). Compared with initial levels Pd-MT concentrations at the end of the exposure period were 600 (500 μg/L), 160 (50 μg/L), and 27 (5 μg/L) times higher. These results show that an increase in MTs in D. polymorpha already occurs at relatively low aqueous Pd concentrations indicating that there is the need for detoxification of Pd in the mussel. Furthermore, correlations between Ag-MT and Pd accumulation indicate that higher exposure concentrations are associated with adverse effects on the mussels. Thus, harmful effects of chronic Pd exposure of organisms even in lowest concentrations cannot be excluded in the environment.

Introduction

During the last years, increasing concentrations of the platinum group metals (PGM) platinum (Pt), palladium (Pd), and rhodium (Rh) used for automobile catalytic converters have been detected along heavily frequented roads as these metals are emitted with exhaust fumes (Hoppstock and Sures, 2004). PGM emitted from cars are at least partly soluble and are therefore mobile in the environment (e.g. Rauch and Morrison, 2000; Jarvis et al., 2001). In addition to their presence in terrestrial habitats, PGM also commonly occur in aquatic habitats (Rauch and Hemond, 2003; Rauch et al., 2004; Haus et al., 2007) and can be accumulated by aquatic organisms in the field (Zimmermann and Sures, 2004; Sures et al., 2005; Haus et al., 2007). Among the PGM Pd seems to be the most important metal in terms of its mobility, biological availability and its toxicological effects (Sures et al., 2006, Sures et al., 2001; Zimmermann and Sures, 2006). Experimental investigations revealed uptake of Pd by zebra mussels using soluble metal salts (Singer et al., 2005; Sures and Zimmermann, 2007), ground catalytic converter material (Zimmermann et al., 2005), and road dust (Zimmermann et al., 2002) as metal sources. Compared with other heavy metals, the biological availability of Pd from road dust to zebra mussels (Dreissena polymorpha) ranged between that of Cd (five times higher than Pd) and Pb (four times lower than Pd) when mussels were exposed under identical conditions to water containing road dust (Zimmermann et al., 2002).

As most laboratory exposure studies published so far were performed with relatively high aqueous Pd concentrations effects of exposure to lower Pd concentrations on aquatic organisms still remain unclear. It is already known that Pd accumulation in mussels after exposure to high aqueous Pd concentrations is associated with a general biological effect such as the induction of heat-shock proteins (Singer et al., 2005), but only little information is available on metal-specific response effects such as the induction of metal-binding proteins, particularly metallothioneins (MT). MT are low-molecular-weight cysteine-rich metal-binding proteins present in many aquatic animal species (Roesijadi, 1992), which bind a couple of metal ions, such as Zn²⁺, Cd²⁺, Hg²⁺, and Cu²⁺ (Wang et al., 1996). MTs are considered to play a central role in regulation of tissue concentrations of essential metals (e.g. Zn and Cu) and to be involved in detoxification of non-essential toxic metals such as Cd and Hg (Kägi, 1991; Roesijadi, 1992, Roesijadi, 1996). As levels of MTs in organisms vary depending on the element and the exposure concentration MTs have been regarded as potential specific biomarkers for metal pollution (Benson et al., 1990; Amiard et al., 2004). Field studies revealed significant increases in MT concentrations in response to metal bioaccumulation in D. polymorpha (De Lafontaine et al., 2000; Gagne et al., 2002).

Therefore, the aim of the present study was to investigate the MT-response of D. polymorpha exposed to Pd. Since environmental concentrations of Pd are still very low prolonged exposure of organisms to low Pd concentrations is of great importance. Accordingly, aqueous exposure concentrations of Pd ranged between 50 ng/L and 500 μg/L and organisms were exposed for 10 weeks. Application of the modified silver saturation method (Scheuhammer and Cherian, 1985) allowed discrimination between MTs which bound Pd (Pd-MT) or unidentified metals (Ag-MT).