Author's personal copy Subcellular distribution of Cd in the aquatic oligochaete Tubifex tubifex, implications for trophic availability and toxicity E. Steen Redeker * , K. van Campenhout, L. Bervoets, H. Reijnders, R. Blust Ecophysiology, Biochemistry and Toxicology Group, Department of Biology, University of Antwerp, Groenenborgerlaan 171, B-2020 Antwerp, Belgium Received 8 March 2006; received in revised form 17 September 2006; accepted 22 October 2006 The internal metal speciation in prey items is not clearly linked to the actual assimilation in predators. Abstract We studied the compartmentalization of cadmium and zinc in the oligochaete Tubifex tubifex. The subcellular distribution was followed over time and levels of metallothionein-like proteins were measured. The impact of the speciation on the trophic transfer was studied by calculating the assimilation efficiencies of metals from Tubificidae fed to carp. It was found that carp were able to assimilate 9.8% of the cadmium. The expected assimilated amount of cadmium, based on the subcellular fractions which are thought to be trophically available, is however 72%. The zinc assimilation results suggest that the debris fraction is at least partially available to predators. Differential centrifugation techniques provide information about the tissue compartmentalization in aquatic organisms but it is not straightforward to directly link internal speciation in prey items to the actual assimilation in the predator. The possible impact that the compartmentalization of cadmium in T. tubifex will have on the toxicity to the organism is also discussed. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Fractionation; Metal compartmentalization; Metallothionein; Detoxification; Metal assimilation efficiency 1. Introduction Organisms living in metal polluted areas are exposed to these contaminants through several sources. Metals are present in the dissolved phase, sediment and food and can be accumu- lated from each of these sources and cause toxicity (Hare, 1992). Metals, once taken up by the organism, cannot be trans- formed into less toxic metabolites but can be sequestered in less toxic forms. In the past, several concepts have been devel- oped in an attempt to adequately link the exposure of contam- inants to toxicity. Of these concepts, the biotic ligand model (BLM) and the critical tissue residue (CTR) concept start from the basic principle that adverse biological effects are determined by the concentration of the contaminant at the target site (biotic ligand) (Di Toro et al., 2001; Paquin et al., 2002) or in the target tissue (CTR) (Hickie et al., 1995; McCarty and Mackay, 1993). However, with these approaches the mechanisms with which organisms are able to sequester or detoxify metals are often ignored. A certain level of essential metal (e.g. Cu and Zn) is required for normal functioning. This pool, required for opti- mal metabolic functioning, however has an upper limit. It is thought that toxicity is related to an overflow of the metabol- ically reactive pool when metals are in excess of metabolic requirements (Rainbow, 2002; Vijver et al., 2004). Non-essen- tial metals are not needed for normal metabolism so this pool can be ignored and non-essential metals are considered to be in excess (although for some marine phytoplankton cadmium has been shown to have a promoting function in growth (Cullen et al., 1999; Price and Morel, 1990)). Excessive amounts of reactive metal are able to bind to and inactivate functional centres of enzymes, displace essential metals from other biomolecules and modify the active * Corresponding author. Tel.: þ32 (0)3 265 33 50; fax: þ32 (0)3 265 34 97. E-mail address: erik.steenredeker@ua.ac.be (E. Steen Redeker). 0269-7491/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.envpol.2006.10.031 Environmental Pollution 148 (2007) 166e175 www.elsevier.com/locate/envpol