## 2 Bioaccumulation of Hg in _Elodea nuttallii_  
### Uptake  
Primary producers, at the base of trophic webs, represent an important entry for Hg into food chains. It is thus essential to understand their Hg bioaccumulation to anticipate transfer and fate to higher trophic levels. As well as this, the bioaccumulation of IHg and MMHg is expected to determine responses to Hg exposure in primary producers. Even when Hg concentrations were very low in freshwaters, high bioaccumulation has been observed in macrophytes [7; 21; 22]. Macrophages collected in the field show concentrations ranging between 1-20-10\({}^{-2}\)\(\mathrm{\SIUnitSymbolMicro g}\)-1 THz \(\mathrm{\SIUnitSymbolMicro g}\)-1 THz \(\mathrm{\SIUnitSymbolMicro g}\)-dy weight (d\(\mathrm{\SIUnitSymbolMicro h}\)) [12], but show much higher concentrations in contaminated sites. Indeed, in Valdeazogues River (Spain), roots of _Typha domingensis_ reached 6 \(\mathrm{\SIUnitSymbolMicro g}\)-\(\mathrm{\SIUnitSymbolMicro h}\)\(\mathrm{\SIUnitSymbolMicro h}\)\  
accumulation and resulted in a similar biomagnification of MMHg in fish, e.g., up to 10 \(\mu\)g\(\cdot\)g\({}^{-1}\)dw and 95% of THg [7]. The MMHg bioaccumulation step from water to primary producers was shown to be the largest increase in MMHg concentrations in this system, reaching 10\({}^{5}\)-fold. We then performed a detailed study on Hg bioaccumulation in several macrophytes sampled in the Babeni Reservoir. Amongst the macrophytes, the submerged and rooted macrophyre _E.nuttallii_ showed among the higher IHg and MMHg bioaccumulation. More in detail, _E. nuttallii_ reached close to 2 \(\mu\)g\(\cdot\)g\({}^{-1}\)dw THg concentrations [21], similar to another submerged macrophyte _Potamogeton pectinatus_, and three times higher than THg measured in plankton (0.7 \(\mu\)g\(\cdot\)g\({}^{-1}\)dw THg) [7]. We subsequently studied Hg bioaccumulation in this plant in the field and in the laboratory in detail [9; 19; 21; 24]. First we performed a detailed measurement of Hg bioaccumulation and uptake in _E. nuttallii_ in the laboratory [21]. We investigated (i) accumulation of Hfg and MMHg in _E. nuttallii_ in Lake Geneva water (1 mg\(\cdot\)L\({}^{-1}\) DOC) spiked with increasing IHg and MMHg concentrations (3.5-3500 pM), (ii) to determine the source of Hg found in shoots and roots, as well as (iii) Hg fate in this representative aquatic plant. Tolerance to both forms of Hg was high, as evidenced by the lack of visible symptoms on leaves after 7 days of exposure for all concentrations. The accumulation of Hg was high in the microcosm with an enrichment factor (EF) of 10\({}^{3}\) for IHg and 10\({}^{4}\) for MMHg [21]. While EF decreased with increasing concentration of exposure for IHg, the EF increased for MMHg. IHg at 0.35 nM resulted in a higher bioaccumulation in leaves than stems (1.4 vs. 0.4 mgTHg\(\cdot\)kg\({}^{-1}\) dw), while 0.1 nM MMHg resulted in 0.6 and 0.3 mgTHg\(\cdot\)kg\({}^{-1}\) dw in stems and leaves, respectively. An uptake in the absence of DOM gave highly similar EF for IHg and MMHg, resulting in similar intracellular THg concentration in _E. nuttallii_, pointing to a role of DOM in the observed difference between the fate of IHg and MMHg observed in biota in the field [25]. Death, cold and Cu\({}^{+}\) co-exposure highly reduced accumulation in shoots [21]. Metabolism and copper transporters govern Hg uptake at the cellular level. A two-compartment exposure setting in the laboratory further revealed the predominant IHg basipetal transport, whereas MMHg acropetal transport was evidenced [21]. It has to be noted that in planta no measurable methylation or demethylation was evidenced. Data were confirmed with exposure to Babeni sediments for 2 months in controlled conditions that resulted in a 102.7% \(\pm\) 6.4% MMHg to THg proportion in shoots of _E. nuttalli_, evidencing the immobilization of MMHg from sediments to shoots [24].