Comparisons of metal accumulation and excretion kinetics in earthworms (Eisenia fetida) exposed to contaminated field and laboratory soils
Introduction
A range of factors has been found to determine the levels of pollutants accumulated by soil invertebrates. These include: the characteristics of the chemical (Belfroid et al., 1993; Janssen and Hogervorst, 1993), the concentration present in the soil (Hopkin, 1993; van Brummelen et al., 1996), substrate properties (Ma, 1987; Perämäki et al., 1992), temperature (van Hattum et al., 1993; Spurgeon et al., 1997), size of the individual (van Hattum et al., 1991) and the physiology of the species (Janssen et al., 1991; Morgan and Morgan, 1991; Hopkin et al., 1993). For some chemicals such as persistent lipophilic organic compounds, accumulation can also be related to the trophic status of the exposed species (Walker and Livingstone, 1992). However, for metals, body burdens are probably linked more strongly to the physiology of the species than to trophic level (van Straalen and van Wensem, 1986; Laskowski and Maryanski, 1993).
Studies of steady-state metal burdens have indicated that some elements are present at increased concentrations in earthworms inhabiting contaminated soils (for references see Ireland, 1983; Morgan et al., 1993; Romijn et al., 1994; Spurgeon and Hopkin, 1996a). The capacity of earthworms to assimilate metals has led to this group being recommended for monitoring the spatial distribution and effects of pollutants in the field (Samiullah, 1990). However, despite the volume of data that details metal concentrations in earthworms collected from contaminated soils, relatively little is known of the accumulation and excretion kinetics of individual metals.
Studies of accumulation kinetics and the factors that influence uptake and loss have a key role in ecotoxicology, as they can be used to predict the physiological fate of a pollutant (Walker et al., 1996). An approach to studying uptake and excretion has been described by Moriarty and Walker (1987). This system, which is based on the use of compartment models can be applied to calculate pollutant fluxes within the tissues of the exposed organism. A number of studies has used kinetic models to describe the accumulation of xenobiotics by soil invertebrates. Belfroid et al. (1995)modelled the accumulation of halogenated aromatic hydrocarbons in Eisenia fetida, while Janssen et al. (1991)studied the uptake and loss of cadmium by four soil arthropod species. Both studies demonstrated that compartmental models can be used to describe uptake and excretion. Similar approaches have been used in this study to assess the accumulation and excretion of two essential (copper and zinc) and two non-essential (cadmium and lead) metals.
Section snippets
Materials and methods
The accumulation and excretion kinetics of the four metals were assessed in time series studies with the earthworm Eisenia fetida in two soils. For the first experiment, uptake and excretion were determined in worms exposed to contaminated field soils collected from sites located in the area around a primary zinc/lead/cadmium smelting works situated at Avonmouth in south-west England and an uncontaminated (control) soil collected from the campus of the University of Reading. Previous studies
Characteristics of field soils
Measurements of the three field-collected soils indicated similar pHs, with values ranging from 5.93–6.67. The % LOI was also found to be broadly similar for the three soils. The lowest value of 9.5% was found for soil collected from Site 1, while the highest values of 15% was for the Site 8 soil (Table 1). The similarities found in soil pH and % LOI indicate that these factors are unlikely to contribute to any difference found in metal kinetics in the three selected soils. Highest metal
Physiological fates of metals
Kinetic studies can be used to predict the physiological fates of pollutants in exposed organisms. In particular, analysis of excretion patterns gives useful information on probable detoxification mechanisms. Three potential pathways exist for the removal of chemicals from sensitive tissues; elements can be regulated by excretion from the body, bound within the matrix of inorganic granules or attached to proteins or other ligands (Tessier et al., 1994). If metals are detoxified primarily by
Acknowledgements
We would like to thank Mrs V. Rimmer for help with the chemical analysis and Mr P. Bowless for assistance with the statistics. The work was conducted at Reading University while DJS was holding a research grant from the Leverhulme Trust. DJS is currently supported by a NERC Advanced Fellowship.
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