Impacts of metallic trace elements on an earthworm community in an urban wasteland: Emphasis on the bioaccumulation and genetic characteristics in Lumbricus castaneus.

Metallic trace elements (MTEs) soil pollution has become a worldwide concern, particularly regarding its impact on earthworms. Earthworms, which constitute the dominant taxon of soil macrofauna in temperate regions and are crucial ecosystem engineers, are in direct contact with MTEs. The impacts of MTE exposure on earthworms, however, vary by species, with some able to cope with high levels of contamination. We combined different approaches to study the effects of MTEs at different levels of biological organisation of an earthworm community, in a contaminated urban wasteland. Our work is based on field collection of soil and earthworm samples, with a total of 891 adult earthworms from 8 species collected, over 87 quadrats across the study plot. We found that MTE concentrations are highly structured at the plot scale and that some elements, such as Pb, Zn, and Cu, are highly correlated. Comparing species assemblage to MTE concentrations, we found that the juvenile and adult abundances, and community composition, were significantly affected by pollution. Along the pollution gradient, as species richness decreased, Lumbricus castaneus became more dominant. We thus investigated the physiological response of this species to a set of specific elements (Pb, Zn, Cu, and Cd) and studied the impacts of MTE concentrations at the plot scale on its population genetic. These analyses revealed that L. castaneus is able to bioaccumulate high quantities of Cd and Zn, but not of Cu and Pb. The population genetic analysis, based on the genotyping of 175 individuals using 8 microsatellite markers, provided no evidence of the role of the heterogeneity in MTE concentrations as a barrier to gene flow. The multidisciplinary approach we used enabled us to reveal the comparatively high tolerance of L. castaneus to MTE concentrations, suggesting that this is a promising model to study the molecular bases of MTE tolerance.