Effect of long-term exposure to cadmium on the earthworm Eisenia fetida (Oligochaeta) : possible resistance

[摘要] ENGLISH ABSTRACT: Environmental cadmium is the result of emissions from non-ferrous smeltersand mining activities, as well as disposal of Cd-containing household waste Cd-containingsewage sludge and phosphate fertilisers. In polluted soil, the Cd associateswith the biological fraction and therefore is relatively available for uptake by soildwelling organismsAnimals living in contaminated soil are subjected to selection pressures causedby the pollutant and can either acclimate (the pressure is overcome by naturalindividual resistance), or develop a tolerance/adaptation (genetic increase in the meanresistance of the population). Numerous studies on soil invertebrates show that somespecies are capable of developing resistance to heavy metal pollution. For earthwormshowever, such data is non-existent.The aim of this study was to determine whether a population of Eisenia fetidathat had been exposed in the laboratory to sublethal concentrations of Cd for fouryears, had developed resistance to Cd.This was done by subjecting pre-exposed and control worms to variousexperiments and comparing the results. The first parameter under investigation wasbiomass change of the different groups in uncontaminated, moderately contaminated,and heavily contaminated substrates. During these experiments, the pre-exposedworms displayed signs of Cd-dependency by performing worse in the uncontaminatedsubstrate than in the moderately contaminated substrate. This was also reflected whencocoon production was monitored, the control worms showing a linear decrease incocoon production as the concentration of CdS04 in the substrate rose, while the pre-exposedworms had the highest cocoon production in the moderately contaminatedsubstrate. Physiologically, the osmolality- as well as the cell percentage of the coelomicfluid for pre-exposed and control worms, exposed to high concentrations of CdS04was measured. Although no differences between the two groups were observed whenlooking at the osmolality, a possible resistance was observed for the pre-exposedworms when looking at the differences in cell percentage.Total body accumulation did not differ between the pre-exposed and controlworms after exposure to Cd. The pre-exposed worms did however show a markedincrease in the nephridial accumulation of Cd as shown by PIXE analyses of theorgans, when compared to the control groups.This study has shown that there is definite evidence of an increased resistanceto Cd in the form of CdS04 in the pre-exposed worms when compared to unexposedcontrol worms. Whether this resistance is the result of acclimation or a geneticadaptation could however not be determined by this study and further research in thatdirection is recommended.

[发布机构] Stellenbosch University