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Task 2 - Laboratory experiments
Task 2c -Partner in charge: C. Cloquet (CRPG-Nancy, partner 2)
Isotopic fractionation during metal (Ni, Hg, Se) bio-accumulation by plants and lichens (C.
Cloquet, T. Sterckeman)
This sub-task aims to better understand interactions between bio-available metals and plants/lichens
during bio-accumulation. For that, we will: 1) grow typical sub-arctic plants (e.g. graminaceae) in contact
will an artificial solution spiked with metals of interest (e.g. Ni, Hg, Se) of known isotopic composition; 2)
put in contact epiphytic lichens (Evernia, Bryoria, Usea, ...) with similar solutions and leave for
equilibration. Results from this sub-task will be used for the interpretation of data obtained from tasks 3
and 4.
Metals accumulation in plants and lichens: Metals accumulating in plants may be essential or toxic ones.
Some essential metals like Zn or Se become toxic if their contents are higher than needed. Although
metals accumulate through roots (e.g. Perriguey et al. 2008; Redjala et al. 2009), atmospheric fall out may
also be an efficient way for bio-accumulation (e.g. Smolders 2001). In nature, trace metals often comes as
a group, depending on both their source and their geochemical affinities and behavior. A multiple
contamination may generate antagonist effects for bio-accumulation (Krznaric et al. 2009). Some plants
have hyper-accumulation strategy and stock metals as complexes with carboxylic acids like citrate and
malate (Montargès-Pelletier et al. 2008). During the bio-accumulation cycle, from soil to plants, even
within plants, metals adsorb, move from one ligand to another and incorporate proteins or vitamins (Lu et
al. 2008, 2009). These processes may lead to significant isotopic fractionation (e.g. Cloquet et al. 2008).
For example, Fe isotopic variations might be related to accumulation strategies (Guelke et von
Blanckenburg, 2007) and translocation processes, as evidence for Zn (Viers et al, 2007, Weiss et al. 2005,
Moynier et al. 2008).
Metals bio-accumulation in epiphytic lichens is exclusively through atmospheric depositions. As for
plants, metals are "inactivated" by oxalate and carboxylic groups (Sarret at al. 1998). Carignan et al.
(2002, 2009) and Cloquet et al. (2006b) argued that no or insignificant isotopic fractionation of Pb, Hg
and Zn should occur during bio-accumulation in lichens. However, Cloquet et al. (2009) showed that bioaccumulation
processes in lichens might be more complicated than expected.
Metals accumulation experiments and measurements: Plants: selected plants representative of the
studied sites (see section 3.3.3 Task 3) will be grown (1-3 months) in the laboratory in contact with trace
metals Ni, Hg, Se of known concentration and isotopic compositions. The solution will also comprise
usual nutrients (Ca, Mg, P, ...). Once the plants grown enough, they will be separated from the solution,
the roots will be rinsed rapidly with distilled water to remove spike solution in excess and they will be
freeze-dry. Metals in the remaining solution stock will be analyzed along with roots, stems and shoots of
the grown plants. Experiments will be conducted at the LSE in Nancy (collaborator of partner 2) and
analyses will be performed at CRPG-Nancy (partner 2). Metals concentrations will be determined in order
to verify the mass budget. Isotopic analyses will be done using an on-line gas separator (Se hydride and
Hg cold vapor generation) coupled to a MC-ICP-MS, as reported by Carignan and Wen (2007) and
Estrade et al. (2009). Concentration and isotopic mass balance will be verified.
Lichens: we have an on-going project (Cloquet-Carignan) on metal bio-accumulation in lichens.
Preliminary results suggest kinetic adsorption of Pb, Hg, Se and Zn on lichen Evernia sp. is very fast and
imply important diffusion of ions through the solution towards lichens. Work in task 2c proposes to
analyze the Se and Zn isotopic composition on our samples to check on any isotopic fractionation of these
metals during bio-accumulation. Such a bias between the composition of available metals and that of
accumulated ones would have important implication for data interpretations, in particular source tracing.
The isotopic composition of Hg is already in progress.
Risks: A small or insignificant isotopic fractionation during bio-accumulation in plants would
considerably reduce the constraints on accumulation processes. Contrarily, a significant isotopic
fractionation during accumulation in lichens would make difficult to track the composition of atmospheric
fall out metals in the ecosystems. Our preliminary results on metal bio-accumulation suggest minor
isotopic fractionation.