Metal uptake of two ecophysiologically different earthworms ( Eisenia fetida and Aporrectodea caliginosa) exposed to ultramafic soils

Abstract

Ultramafic soils are characterized by a number of disadvantageous living conditions, especially extremely high concentrations of different metals. To evaluate the bioavailability of these metals, two ecophysiologically different earthworm species, Aporrectodea caliginosa and Eisenia fetida, were exposed for at least 14 weeks to ultramafic soils collected in the Barberton Region, Mpumalanga, South Africa (25°33′S, 30°47′E), containing extremely high concentrations of chromium, manganese and nickel with a maximum of 2894 mg Cr/kg, 1994 mg Mn/kg and 13,926 mg Ni/kg soil dry wt. In addition, a sequential fractionation evaluating the different metal phases in the soils was conducted to determine the environmental availability of selected metals. For the mobile metal fraction, easily exchangeable metals were substituted by a 0.01 mol/L CaCl 2 solution. The mobilisable fraction, resembling easily remobilisable complexed and carbonated metal ions, was addressed by a DTPA extraction. Especially in the bodies of A. caliginosa, a high individual variability, for example between 3 and 264 mg/kg for Cr, 19 and 454 mg/kg for Mn, and 0 and 86 mg/kg for Ni (24 weeks exposure, Agnes Mine) was measured even in the same replicate. For that reason, an analysis of three different body sections of both earthworm species was performed. In E. fetida a dynamic uptake behavior with an initial increase followed by a decrease of metal body burden and thereafter fluctuating concentrations was observed (e.g. 12 mg Ni/kg after 2 weeks of exposure and 3 mg Ni/kg after 4 weeks; Kaapsehoop 1). That makes it difficult to relate metal concentrations as measured by the different fractionations to the concentrations in the two earthworm species. The analysis of the different body sections showed that these ecophysiologically different species differed strongly in the distribution of chromium, manganese and nickel in their bodies. A. caliginosa seems to concentrate these metals preferentially in the posterior section, which is probably linked to the phenomenon of autotomization. In contrast to that, no differences in the longitudinal distribution of the metals were observed in E. fetida. Apart from different elimination and/or uptake mechanisms, A. caliginosa might also be able to avoid elevated metal concentrations by going into quiescence. Although no relation between extractable fractions and uptake of metals was found, the fractionation revealed that a lower proportion of metals was extractable from ultramafic soils than from non-ultramafic soils.