Metal Bioavailability and Toxicity

We have addressed the topic of metal bioavailability and metal toxicity in environmental samples. Traditionally, metal availability is investigated using a chemical approach. Afterwards, the concept of Water Effect Ratio (WER) was proposed by the U.S. EPA and employed bioassays (e.g., fish and invertebrate tests) to assess metal bioavailability and toxicity. In the HMBC approach discussed in this review, we have made use of a bacterial assay that is specific for metal toxicity to achieve this goal. This is only a preliminary survey of the potential applications of the HMBC concept. Some preliminary results on the use of MetPLATE for the fractionation of HMBC to obtain information on the factor(s) that control metal bioavailability in environmental samples were also presented. Using MetPLATE eliminates or diminishes the confounding factor represented by the presence of organic toxicants in a given sample. Further work is needed to refine the fractionation scheme. 

Brezonik, P.L., King, S.O. and Mach, C.E. (1991) The influence of water chemistry on trace metal bioavailability and toxicity to aquatic organisms. In Metal Ecotoxicology. Concepts and Applications (eds Newman, M.C. and McIntosh, A.W.). Lewis Publications, Chelsea, MI, pp. 1-31. 

Elder JF, Collins JJ (1991) Fresh-water mollusks as indicators of bioavailability and toxicity of metals in surface-water systems. Rev Environ Contam Toxicol 122 37-79... 

In many crucial biological processes, such as oxygen transport, electron transport, intermediary metabolism, metals play an important part. Therefore, disorders of metal homeostasis, metal bioavailability or toxicity caused by metal excess, are responsible for a large number of human diseases. We have already mentioned disorders of iron metabolism (see Chapter 7) and of copper metabolism (see Chapter 14). The important role, particularly of redox metals such as copper and iron, and also of zinc, in neurodegenerative diseases, such as Parkinson s disease, Alzheimer s disease, etc. has also been discussed (see Chapter 18). We will not further discuss them here. 

It is generally accepted that free ionic forms of heavy metals are generally more toxic to biota than chelated or precipitated forms. Several factors control metal bioavailability and, thus, toxicity in environmental samples. These factors include pH, redox potential, alkalinity, hardness, adsorption to suspended solids, cations and anions, as well as interaction with organic compounds (Kong et al., 1995). 

Vaughan, D., Lumsdon, D.G. and Linehan, D.J. (1993) Influence of dissolved organic matter on the bioavailability and toxicity of metals in soils and aquatic systems. Chem. Ecol, 8, 185-201. 

An approach similar to that in soils can be applied to metal-contaminated sediments, where sulfides, measured as acid-volatile sulfides (AVS), have been demonstrated as being the predominant factor controlling metal mobility and toxicity in anaerobic sediments. The difference or ratio between SEM (simultaneous extracted metals) and AVS (SEM-AVS) is used to predict toxicity. In cases where SEM does not exceed the AVS, this approach has been shown to consistently predict the absence of toxicity (Allen et al. 1993 Ankley et al. 1996 DiToro, Hansen et al. 2001b). When SEM exceeds the AVS, toxicity is predicted, but the appearance and extent of toxicity may be determined by other binding phases (e.g., organic carbon) in the pore water. Luoma and Fisher (1997) stated that the association of metal bioavailability with AVS in sediments is not, however, straightforward in all cases and should be treated with caution. 

On the other hand, reactivity, bioavailability, and toxicity of Pt are not necessarily correlated with its total content, but depend also upon factors other than the soil properties and the metal-uptake kinetics of plants. Proteins are known to play... 

Lee JS, Lee JH. 2005. Influence of acid volatile sulfides and simultaneously extracted metals on the bioavailability and toxicity of a mixture of sediment-associated Cd, Ni, and Zn to polychaetes Neanthes arenaceodentata. Sci Total Environ 338 229-241. 

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