A Native Earthworm Accumulates Extraordinarily High Concentrations of Lead

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The Challenge: As a rule, plants and animals contain lower concentrations of lead than are present in soils that support them. Lead does not biomagnify along trophic levels in ecosystems but instead remains relatively immobile in soil. The exposure of wildlife to soil lead depends mainly on the incidental ingestion of soil. The native earthworm, Eisenoides lonnbergi, is anomalous in its ability to concentrate lead from acidic soils.  This raises the question what makes this earthworm so different from other organisms that have been studied.  As a rule, plants and animals contain lower concentrations of lead than are present in soils that support them. Lead does not biomagnify along trophic levels in ecosystems but instead remains relatively immobile in soil. The exposure of wildlife to soil lead depends mainly on the incidental ingestion of soil. The native earthworm, Eisenoides lonnbergi, is anomalous in its ability to concentrate lead from acidic soils.  This raises the question what makes this earthworm so different from other organisms that have been studied.  

The Science: Eisenoides lonnbergi were collected from strongly to extremely acid soils at 16 sites on the Patuxent Wildlife Refuge in Maryland, USA.  Lead concentrations as high as 766 mg Pb/kg, dry weight, were detected in E. lonnbergi collected from soil containing only 17 mg/kg of lead.  Concentration factors (ratio of lead concentration in earthworms to lead concentration in soil, dry weights) at the sites were as low as 1.0 and as high as 83. The value of 83 is remarkable considering that the ratio is well below 1 for almost all other organisms.  Earthworm researchers have previously suggested that low soil calcium concentrations may lead to higher tissue lead concentrations in earthworms. The ability of Eisenoides lonnbergi to live in severely calcium-deficient soils probably explains the high concentration factors. The pH’s of the soils at the sites were low, from 4.0 to 5.5 and the soil calcium concentrations were as low as 49 mg/kg, well below the national average of 15,900 mg/kg.  

The Future: Although the findings do not contradict the generalization that earthworms and other soil organisms normally have low concentrations of lead compared to those in the soil, risk assessors should be aware of the possibility of unexpected hazards from lead in extremely acid soils.  It remains unclear whether other kinds of soil organisms in these extreme soils could accumulate lead.