A comparison of nickel toxicity to pre-exposed earthworms ( Eisenia fetida, oligochaeta) in two different test substrates

Abstract

Earthworms are soil living organisms of high ecological importance. For that reason, earthworms can be considered as feasible biological indicators for many pollutants in soils. Soils are extremely complex and dynamic systems influenced by a number of different abiotic and biological factors determining the effects of potentially toxic substances. To be able to evaluate the toxicity of a single substance to soil living organisms such as earthworms, soil-related influences and interactions need to be excluded. For that purpose, the use of defined, artificial soil free substrates has been widely established. In this study, a new artificial substrate based on the knowledge gained by the application of different methods, in particular the exposure in reconstituted water resembling natural ground water and an agar medium initially developed to void the guts of earthworms, is proposed. Eisenia fetida have been exposed for 96 h to a medium consisting of 1.5% normal melting agarose added to reconstituted ground water. For validation of the substrate, the effects of nickel toxicity, a substance known to be toxic to E. fetida, were evaluated. In addition, to investigate the possible establishment of resistance or adaptation, as well as cross-resistance, E. fetida individuals with a history of previous exposure (more than 10 generations) to nickel and manganese were exposed to different concentrations of nickel. Both media, one consisting of reconstituted ground water [Kiewiet, A.T., Ma, W.-C., 1991. Effect of pH and calcium on lead and cadmium uptake by earthworms in water. Ecotoxicology and Environmental Safety 21, 32–37] and the other of reconstituted ground water plus agar, seemed equally suited for the evaluation of intrinsic effects of nickel and manganese on E. fetida. With regard to the possible development of resistance, E. fetida did not show an increased tolerance towards nickel in spite of being exposed to elevated levels for more than 10 generations; to the contrary, worms with a history of exposure to nickel showed an increased sensitivity towards nickel. No indications of cross-resistance between manganese and nickel pre-exposed worms were detected.