Over the years, our environment became polluted by various hazardous substances, such as heavy metals. Contamination by heavy metals may have severe effects on every level of biological organisation, i.e. from cell to ecosystem. An important problem that frequently occurs in ecotoxicological metal studies is that a clear relationship between exposure, accumulation and effects of heavy metals in an organism is often not found under natural conditions. There are two possible explanations. First, in many papers the absolute concentration of heavy metals in the soil and in the food are quantified while a large fraction of these metals are not bioavailable for the organism. Second, acclimatisation and/or adaptation to heavy metal contamination can result in a reduced uptake and accumulation of the bioavailable fraction of heavy metals and an altered sensitivity or tolerance. However, it is still unclear how and to what extent organisms can adapt and/or acclimatise to heavy metal contamination. Possible ways are: (1) a reduced absorption of heavy metals, (2) an increased excretion of heavy metals, (3) a different organ and tissue distribution and (3) a more efficient induction of detoxifying mechanisms like metallothionein (MT).
The effect of heavy metal pollution on aquatic organisms has already received considerable attention, both in a laboratory and a field context. Field studies on terrestrial mammals are however very rare. The aim of this study is to get a better understanding of the effects of heavy metal exposure on the accumulation of heavy metals in the Wood mouse (Apodemus sylvaticus, L.). In second instance, we will investigate the potential role of some key physiological adaptations to heavy metal exposure in this study species.
For this purpose, mice will be trapped in five different populations within a pollution gradient. Mice in three populations have been exposed to high levels of pollutants, mainly heavy metals, over at least 50 generations of mice. The mice in the two other populations have only been exposed to very low levels of contaminants and serve as control.
In first instance we will study the relationship between total concentrations of heavy metals in soil and food of the Wood mouse, soil bioavailability, and the accumulation and compartmentalisation of heavy metals in the mice. Secondly, we will measure MT levels in different organs and try to relate this to heavy metal exposure. Thirdly, we will try to relate animal condition to individual exposure and accumulation of heavy metals. We will not only focus on the inter-population differences in exposure, but we will also take intra-population differences into account. This part of the project will reveal whether there is a straightforward relationship between exposure to pollutants, tissue concentrations, MT levels and animal condition. Furthermore, we will get an indication whether MT's may serve as a valuable biomarker for heavy metal exposure in terrestrial mammals like the wood mouse.
In a second and more experimental part of the project, we will study some potential adaptations to heavy metal exposure. We will work with mice trapped in the most and the less exposed populations. In first instance, we will compare heavy metal absorption and elimination kinetics, and tissue distribution of heavy metals among the two populations of wood mice. Heavy metal absorption efficiency and elimination rate will be measured during a period of 10 days by whole-body counting of orally administered radioactive labelled Cd and Zn. Afterwards, mice will be sacrified and dissected in order to measure the remaining radioactivity in several tissues. In this way, we will be able to compare tissue allocation of heavy metals among the two selected populations.
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