Application of Pb isotopes to track the sources and routes of metal uptake in the earthworm Eisenia fetida

Abstract

The aim of this work is to determine the important routes of metal uptake in earthworms to enable a better understanding of the primary source of metal uptake in the environment. Earthworms can take up chemicals from pore water and soil both by ingestion and through contact with their skin. However, it is unclear which pathway is the most important for metal uptake. An experiment was designed in which both soil chemistry and foods were artificially manipulated, producing different pools of soil lead (Pb) with different isotope compositions at a range of Pb concentrations. Earthworms (Eiseniafetida) were exposed to different lead concentrations through the addition of 500 mg/kg lead oxide (Pb₃O₄)  to soil and 500 mg/kg lead nitrate to food (manure), with distinctly different isotopic compositions. Earthworms were also exposed to combinations of soil only and soil plus food in order to quantify the proportions of Pb taken up from each component. After acid digestion of the earthworm tissues, the Pb isotope composition of the accumulated lead in the earthworms was measured using a Thermo-fisher, iCAPQ, ICP-MS for ²⁰⁸Pb/²⁰⁶Pb and ²⁰⁷Pb/²⁰⁶Pb ratios measured relative to NIST SRM 981, allowing us to determine the pathway of lead uptake. Mixing calculations have been used to deconvolute the lead isotope signatures and identify the amount of lead taken up by the earthworms from the different soil pools. Differences in bioaccumulation factors and the relative amounts of lead accumulated from different pools changes as a function of concentration in the different pools. Earthworms were shown to uptake lead from bothsoil and food sources through ingestion route. Our findings suggest that a major pathway of lead uptake in earthworm species is heavily influenced by their ecology.

Keywords

• Earthworms, metals, uptake, lead, isotopic

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