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Résumé :
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Copper and zinc are essential trace metals for microorganisms however they are toxic at high concentrations. Metal speciation plays a fundamental role in the bioavailability of copper and zinc, as metals strongly complexed to organic ligands are not bioavailable. Under laboratory conditions, metal bioavailability appears to be generally controlled by the free metal ion concentration, following the free ion activity model (FIAM). Nevertheless, there is a lack of data comparing metal speciation and metal bioaccumulation under natural conditions. Indeed, the presence of large quantities of organic ligands, such as humic and fulvic acids, in natural water drastically complicates the determination of metal speciation. The goals of this work were to study how copper and zinc speciation in natural freshwaters influence the bioaccumulation of these metals in periphyton and to establish if the FIAM is applicable under field conditions. Periphyton, a natural algal biofilm which grows on submerged solid substrates and which plays a preponderant ecological role in natural surface waters, was used as model for an algal community in this study. In the Furtbach stream (Switzerland), total dissolved copper and zinc concentrations increased drastically during rain events due to the release of metals from contaminated sediments. Metal speciation and bioaccumulation in periphyton were measured over several days during rain events when variations of metal concentration occur. Dissolved metal in the stream increased to maxima of 118 nM Cu and 147 nM Zn. Increases in free copper and zinc ion concentrations and variations in labile metal concentrations in the water were also recorded. Periphyton was collected from glass slides colonized by periphytic communities and fixed to holders directly in the stream. Metal content in periphyton responded rapidly to metal concentration variations in the water. The bioaccumulation of zinc appeared to be controlled by the free metal ion, following the FIAM prediction, whereas a labile fraction of copper (fraction detected by voltammetry after exchanging with catechol) appeared to control the intracellular copper content in periphyton. These relationships were also investigated in a microcosm study performed with natural river water. This approach allows examining bioaccumulation in identical periphytic communities under controlled conditions but with different copper and zinc concentrations and with different metal speciation. This study supported the observations made in a variable environment, namely that zinc accumulation in periphyton follows the FIAM while labile copper species control copper accumulation in periphyton. This difference is explained by the high extent of copper complexation (>99.99% complexed) which implies that only a small amount of copper is free. The diffusion fluxes of free copper ions from the bulk to the algal cells were estimated and compared to uptake fluxes in periphyton. It showed that the diffusion is largely limiting at environmental copper concentrations. Under these conditions, weakly complexed copper controls the uptake of this essential trace metal by providing metal at the cell membrane. An important part of this work focused on the evaluation of techniques to determine metal speciation in natural water. An in situ technique, diffusive gradients in thin films (DGT), a laboratory technique, competitive ligand exchange followed by voltammetric measurement, and three speciation programs using models for the complexation by humic and fulvic substances, namely Model VI (WHAM), NICADonnan and SHM were compared. The DGT-labile copper and zinc measurements showed significant correlations with labile metal measured by voltammetry. These two techniques measured similar fractions of the total dissolved copper and zinc concentrations. The speciation programs were able to relatively accurately predict zinc speciation whereas the calculated free copper ion concentrations were largely overestimated because these models underestimate the effects of strong copperbinding organic ligands present in natural water. The results obtained in this study contribute to the comprehension of the concept of metal bioavailability in natural water systems and confirm the importance of metal speciation in estimating the impact of metals on aquatic organisms in natural freshwater. Metal speciation should thus be considered when defining relevant water quality criteria in the future.
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