Sammendrag
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.