title: A structural equation model of soil metal bioavailability to earthworms: confronting causal theory and observations using a laboratory exposure to field-contaminated soils.
abstract: Structural equation models (SEM) are increasingly used in ecology as multivariate analysis that can represent theoretical variables and address complex sets of hypotheses. Here we demonstrate the interest of SEM in ecotoxicology, more precisely to test the three-step concept of metal bioavailability to earthworms. The SEM modeled the three-step causal chain between environmental availability, environmental bioavailability and toxicological bioavailability. In the model, each step is an unmeasured (latent) variable reflected by several observed variables. In an exposure experiment designed specifically to test this SEM for Cd, Pb and Zn, Aporrectodea caliginosa was exposed to 31 agricultural field-contaminated soils. Chemical and biological measurements used included CaC12-extractable metal concentrations in soils, free ion concentration in soil solution as predicted by a geochemical model, dissolved metal concentration as predicted by a semi-mechanistic model, internal metal concentrations in total earthworms and in subcellular fractions, and several biomarkers. The observations verified the causal definition of Cd and Pb bioavailability in the SEM, but not for Zn. Several indicators consistently reflected the hypothetical causal definition and could thus be pertinent measurements of Cd and Pb bioavailability to earthworm in field-contaminated soils. SEM highlights that the metals present in the soil solution and easily extractable are not the main source of available metals for earthworms. This study further highlights SEM as a powerful tool that can handle natural ecosystem complexity, thus participating to the paradigm change in ecotoxicology from a bottom-up to a top-down approach.
authors.affiliation: UMR ECOSYS, INRA, AgroParisTech, Université Paris-Saclay, 78026 Versailles, France. Electronic address: lea.beaumelle@gmail.com.
authors.forename: Léa
authors.initials: L
authors.lastname: Beaumelle
authors.affiliation: INRA, Montpellier SupAgro, UMR 759 LEPSE, 34060 Montpellier, France.
authors.forename: Denis
authors.initials: D
authors.lastname: Vile
authors.affiliation: UMR ECOSYS, INRA, AgroParisTech, Université Paris-Saclay, 78026 Versailles, France. Electronic address: isabelle.lamy@versailles.inra.fr.
authors.forename: Isabelle
authors.initials: I
authors.lastname: Lamy
authors.affiliation: LGCgE-Lille 1, Ecologie Numérique et Ecotoxicologie, Université de Lille, 59650 Villeneuve d'Ascq, France.
authors.forename: Franck
authors.initials: F
authors.lastname: Vandenbulcke
authors.affiliation: Laboratoire Chrono-Environnement, UMR 6249 University of Franche-Comté/CNRS Usc INRA, 16 route de Gray, 25030 Besançon Cedex, France.
authors.forename: Frédéric
authors.initials: F
authors.lastname: Gimbert
authors.affiliation: UMR ECOSYS, INRA, AgroParisTech, Université Paris-Saclay, 78026 Versailles, France.
authors.forename: Mickaël
authors.initials: M
authors.lastname: Hedde
mesh_terms: D000818:Animals; D001682:Biological Availability; D002104:Cadmium / Q000378:metabolism*; D004784:Environmental Monitoring; D007854:Lead / Q000378:metabolism*; D008954:Models, Biological; D009835:Oligochaeta / Q000378:metabolism*; D012989:Soil Pollutants / Q000378:metabolism*; D015032:Zinc / Q000378:metabolism*
chemical_list: D012989:Soil Pollutants; D002104:Cadmium; D007854:Lead; D015032:Zinc
keywords: Aporrectodea caliginosa; Cadmium; Lead; Low dose; Structural equation model; Zinc