The effects of mercuric chloride on survival, growth, reproduction, burrowing speed and glutathione concentrations in the earthworm species Eisenia fetida Savigny

Abstract

Abstract

The common compost earthworm Eisenia fetida was used for toxicity testing of mercury, in a well-characterized agricultural soil obtained from a local field (Ås, Norway). Groups of five or ten clitellate earthworms were exposed to various concentrations of mercury in soil, added in the form of mercuric chloride (HgCl2) in distilled water. The effects on survival, burrowing behaviour, reproduction, growth, and glutathione levels were recorded. Very low mercury concentration (0.22 mg/kg soil) significantly stimulated cocoon production after two weeks, but higher concentrations reduced it after four weeks (EC50 for reproduction was approximately 7.4 mg Hg/kg soil). Protein and glutathione (non-protein thiols) were analyzed by standard spectrophotometric methods. Earthworms exposed to mercury (22 to 740 mg Hg/kg soil for four weeks) experienced a threefold increase in glutathione levels from background levels of approximately 0.62 µmol/g earthworm wet weight. Pre-exposure to mercury (22 mg/kg soil for one week) strongly increased survival in a subsequent standard test carried out one week after the pre-treatment. Differences in cocoon production and growth between pre-exposed and non pre-exposed worms were smaller, though statistically significant. Glutathione levels were similar in preexposed and non pre-exposed earthworms, suggesting that increased glutathione levels did not cause the increased survival in pre-exposed animals. Internal concentrations of mercury were determined in both pre-exposed and non pre-exposed earthworms, and followed a one site binding hyperbola. This reached a maximum around 740 mg Hg/kg soil, corresponding to the internal concentrations 139 + 9 and 165 + 16 µg Hg/g earthworm wet weight in two independent experiments. The shape of the accumulation curves resembled the shape of the curves for glutathione concentrations plotted against mercury concentrations in soil, suggesting that glutathione levels in earthworms are closely related to internal concentrations of mercury. Therefore, glutathione levels may be a useful biomarker for mercury exposure.