## 1 Introduction  
Anthropogenic activities cause environmental contamination [1; 2], affecting biota and human populations via contaminated food [3; 4]. In this context, mercury (Hg) is a past as well as a current priority for legislators, because of its widespread occurrence and high toxicity. Freshwater Hg contamination results from atmospheric Hg depositions as rainfall or dry settling, polluted soils runoffs, effluents and emissions of industrial and gold-mining activity, as well as reemission from historical contaminations [5; 6]. Anthropogenic activities release mainly (i) the highly volatile Hg\({}^{0}\) that is transported in the atmosphere and (ii) inorganic Hg (Hg\({}^{\Pi}\), Hfg). IHg is the most abundant form in the freshwater environment. In surface waters, divalent Hg\({}^{\Pi}\) can be bio-converted to methylated Hg (CH\({}_{3}\)Hg\({}^{+}\), MMHg), generally representing \(>\) 1% of total Hg (THg = MMHg + Hfg) [7; 8]. In rivers and lakes, measured concentrations typically range between 0.003-30\(\cdot\)10\({}^{-9}\) M THz [9]. In natural waters, IHg and MMHg are both mainly bound to thiol groups of dissolved organic matter (DOM) [5; 9; 10]. Water chemistry (e.g., chloride concentration and pH) governs the abundance of the minor Hg fraction not bound with DOM among the different complexes and dissolved chemical species [11]. The data revealed that Hfg and MMHg show different affinities to ions and DOM [5; 9; 10]. Nonetheless, in natural waters the analysis and the modeling of Hg speciation remain challenging. Consequently, uptake in biota is difficult to link to classical water chemistry analysis. However, data repeatedly show IHg and MMHg bioaccumulation in aquatic biota. In addition, MMHg is biomagnified along the trophic chain, reaching very high and toxic body burdens in fish and threatening humans' health through fish consumption, including when low concentrations of Hfg and MMHg are found in primary producers [3; 7]. Primary producers are central for Hg uptake and transfer to higher trophic levels [7; 11; 12]. In numerous littoral and shallow freshwater ecosystems (e.g., lakes, marshes, ponds and rivers), macrophytes are dominant in terms of primary production, biomass and have major ecological roles as a source of food for animals as well as in nutrient cycling [13; 14]. Understanding macrophytes' interactions with Hg and the factors controlling IHg and MMHg bioavailability to them in more detail will enhance our capacity to anticipate Hg impact and fate in aquatic ecosystems. Moreover, macrophytes could be used for biomonitoring and risk assessment of ecosystems and for ecotoxicology. However, current standardized ecotoxicology tests fail to measure the impact of Hg on primary producers in chronic exposure that involve realistic environmental concentrations [12]. In ecotoxicology, a current research priority is identifying the mechanisms of the disturbance of contaminants at a sublethal level to normal biological performance and to use this knowledge to develop new early-warning biomarkers relevant in the context of chronic exposure. High-throughput omics have revolutionized the understanding of molecular responses to sublethal stresses [15]. Early, specific and robust biomarkers directly linking responses at the level of the molecule and the cell to the effects at the level of the whole organism, population and community are envisioned in line with the adverse outcome pathway theory [10; 16; 17; 18]. In recent years, we identified the macrophyte _Elodea nuttallii_ as an interesting model to study Hg bioaccumulation and develop innovative biomarkers of exposure [9; 19; 20; 21]. _E. nuttallii_ is a rooted macrophyte that, as an aggressive invader in temperate climates, is often found in contaminated environments in all temperate regions of the world. Here, the paper aims to review the recent knowledge gained on Hg bioaccumulation and effects in this species. In more detail, we focused on Hg bioaccumulation and responses to Hg exposure in _E.