Effects of soil chemistry of zinc and zinc oxide NP toxicity to the earthworm E. fetida

Abstract

Metal and metal oxide nanoparticles present a series of challenges for terrestrial ecotoxicology. Chemical transformations of the nanoparticle may modify the exposure of organisms to nanoparticles. Additionally, the chemistry of the soil itself may influence exposure, for example by promoting particle aggregation or dissolution. Within the TINE project, we aim to develop models to take account of these processes, based on sublethal exposures of zinc oxide (ZnO) and silver (Ag) particles to the earthworm Eisenia fetida across a range of soil types. The exposure model will account for (i) particle dissolution and consequent toxicity of the ionic metal across soil types; (ii) variation in the toxicity of undissolved nanoparticles across soil types. Initial results will be shown from earthworm exposures to ZnO nanoparticles, non-nano ZnO and ionic Zn, in a set of soils created in the laboratory by liming of an acidic heathland soil from the southern UK. Initial results show that (i) exposure to all three types of Zn result in effects on earthworm reproduction; (ii) the toxicity consistently increases with decreasing soil pH; (iii) ZnO, both nano and non-nano, is consistently less toxic on a Zn basis than ionic Zn; (iv) the toxicity in ZnO exposures cannot be solely attributed to ionic Zn formed by particle dissolution; (v) accumulation of Zn by the worms is higher in particle exposures than in ionic exposures. Taken together, these findings suggest that direct nanoparticle toxicity to earthworms does occur in soil exposures. The differences in uptake and toxicity between ZnO particles and ionic Zn suggest that (i) particles are directly taken up by earthworms; (ii) uptake does not trigger zinc elimination processes in the worm, hence the relatively high accumulation of Zn in particle exposures; (iii) on a Zn basis, particles are less toxic to worms than ionic Zn once within the tissues. We will discuss these findings in the context of developing the concepts behind the earthworm-nanoparticle exposure model