Metal Compartmentation and Speciation in a Soil Sentinel: The Earthworm, Dendrodrilus rubidus

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• 作者：Janet Cotter-Howells ; John M. Charnock ; Carole Winters ; Peter Kille ; John C. Fry ; and A. John Morgan
• 刊名：Environmental Science & Technology
• 出版年：2005
• 出版时间：October 1, 2005
• 年：2005
• 卷：39
• 期：19
• 页码：7731 - 7740
• 全文大小：1611K
• 年卷期：v.39,no.19(October 1, 2005)
• ISSN：1520-5851

文摘

Earthworms are well-studied organisms in ecotoxicologybecause of their keystone ecological status and metal-accumulating capacity. However, the direct estimation ofthe bioreactive fractions of accumulated metal burdensremains technically elusive. In this study we exploited twophysical techniques, electron probe X-ray microanalysis(EPXMA) and X-ray absorption spectroscopy (XAS), to improveunderstanding of the subcellular spatial distributions,ligand affinities, and coordination chemistries of Cd, Pband Zn in a field population of the epigeic earthworm,Dendrodrilus rubidus. EPXMA and XAS analyses wereperformed on cryopreparations to maintain compositionalfidelity; EPXMA data were analyzed by multivariatestatistics. XAS provided whole-worm insights; EPXMAprovided in situ, subcellular data from the major metal-sequestering tissue, the chloragog. Both techniques showedthat Cd is coordinated with S; the measured Cd-S bonddistance in XAS suggests a metallothionein-type ligand. Themean Cd:S molar ratio (EPXMA) of 0.36 is higher thanthe ratio of 0.29 estimated from published biochemical data.EPXMA and XAS data also found that Ca, Pb, and Znare predominantly bound to one or more O-donating, probablyphosphate-rich, ligands. X-ray distribution maps (EPXMA)of the hepatocyte-resembling chloragocytes revealedthat the O-seeking (Ca, Pb, Zn) metals and S-seeking Cdbioaccumulate in distinct organelles. Extended X-rayabsorption fine structure showed that the Pb complex isnot biogenic pyromorphite, although X-ray absorption nearedge structure did not eliminate the possibility. XASprovided no evidence of Pb spillage from the "sequestrationcompartment" within D. rubidus. However, the correspondence of Pb with Ca and P in EPXMA is not as strongas that of Zn. This is indicative either of spillover or of asecond, hitherto unidentified, sequestered-Pb pool. Byexploiting the complimentary techniques of EPXMA andXAS, we are closer to describing the mechanistic link betweenequilibrated body burdens and biomarker responses inearthworms.