A Hierarchic Method for Studying the Distribution of Phenanthrene in Eisenia fetida

Abstract

The distribution of heavy metals in earthworms has been widely studied, highlighting the importance of the fate of these metals. However, little information is available on the distribution of hydrophobic organic contaminants (HOCs) within earthworms. The aim of this study was to propose a hierarchic method to study the distribution of phenanthrene (PHE), a typical HOC, in Eisenia fetida at several levels: sub-organism (pre-clitellum, clitellum and post-clitellum), tissue (body wall, gut and body fluid) and subcellular (intracellular and extracellular fractions). Earthworms were incubated in the soils amended with low (LC, 10 mg kg−1) and high concentrations (HC, 50 mg kg−1) of PHE and sampled at different time intervals. At the sub-organism level, the distribution of PHE was homogeneous among the sub-organism fractions in the LC treatment but heterogeneous in the HC treatment and gradually reached the following form of post-clitellum ≈ clitellum > pre-clitellum. The uptake and elimination kinetics of PHE in the sub-organism were well described by a one-compartment model. At the tissue level, the concentration of PHE followed the order of gut > body fluid > body wall; while at the subcellular level, the concentration of PHE in the extracellular fraction was 1.23 to 4.68 times higher than that in the intracellular fraction. Therefore, the simple circulatory system of earthworms may account for the PHE distribution at the sub-organism level. Partition pathways (passive diffusion) of PHE between the body wall, body fluid and gut as well as the processes of PHE entrance into the inner cellular compartment via passive diffusion were experimentally supported.