## 3 Responses to Hg Exposure at Different Levels of Biological Organization in _Elodea nuttallii_
### Physiological Responses
As such, in _E. nuttallii_, a direct impact of IHg on the integrity of chloroplast membranes was hypothesized, while MMHg would impact the metabolism of cytoplasmic organelles. A more recent study in _E. nuttallii_ confirmed that 0.1 nM MMHg for 24 h did not impact chlorophyll content (Table 1) [19]. To summarize, IHg toxic action appears to target the photosynthetic components with, for example, the chelation of IHg with chloroplast proteins and the Mg ion replacement in the heme of chlorophyll [41]. MMHg toxicity would rather target the cellular components, increasing the production of ROS. Investigating mechanisms at a low level of biological organization increases the understanding of IHg and MMHg toxicity at the molecular level. Short exposure lengths reveal early toxicity responses that are thought to be more specific to each stress than a longer exposure [40]. As such, omics appear to be particularly promising for new ecotoxicology endpoints, because their sensitivity makes them suitable for environmental realism, including low concentrations and short (transient) exposures [42]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Treatment** & **Method** & **Main Results** & **Physiology** & **Reference** \\ \hline (Field) 1.2-10\({}^{-11}\) M THz 2 h & T: RNAseq & Regulation of 8700 contigs: gene regulation, anti-ROS response, energy metabolism, hormone metabolism, transport, stress and secondary metabolism & none measured & [10] \\ \hline
4-10\({}^{-10}\) M Hig 24 h & \multirow{4}{*}{T: RNAseq} & Up-regulation 79 contigs: down-regulation 48 contigs: no pathway enriched} & \multirow{4}{*}{[35]} \\ \cline{3-3}
4-10\({}^{-7}\) M Hig 24 h & & Up-regulation 4472 contigs: expression of genes, oxidation of fatty acid} & POD \(\downarrow\) & \\  & & Down-regulation 2273 contigs: biosynthesis of chlorophyll, chloroplasts, photolysis & SOD \(\uparrow\) & \\  & & photosynthesis & & \\ \hline
0.04, 4 and 40-10\({}^{-7}\) M Hig 24 h & \multirow{4}{*}{T: RNAseq} & Up-regulation correlated to dose, e.g., HSP70, environment interactions} & \multirow{4}{*}{none measured} & \multirow{4}{*}{[20]} \\ \cline{3-3}
1-10\({}^{-11}\) M to 1-10\({}^{-8}\) M Hig and & & Up-regulation: \(\geq\)1677 contigs for Hig \(\geq\) 18557 contigs for MMHg. Similar GO & chlorophyll \(\downarrow\) for Hfg & \multirow{4}{*}{[25]} \\ MMHg 2 h & & categories: development, energy metabolism, secondary metabolism, transport for MMHg & & \\ \hline
1 and 10-10\({}^{-9}\) M MMHg 2 h & \multirow{4}{*}{T: RNAseq} & Regulation of 4389 and 16.853 contigs: amino acid, sugar and secondary metabolism (e.g., flavonoids) at both concentrations. At 10\({}^{8}\) M: homeostasis of metals, membrane integrity, photosynthesis, anti-oxidative enzymes and water transport} & POD \(\uparrow\) & [37] \\ \hline
3.5-10\({}^{-10}\) M Hig 24 h & \multirow{4}{*}{P: 2D-DICE} & Up-regulation 4 proteins, down-regulation 18 proteins: photosynthesis (e.g., complex of light harvesting), stress/defense (e.g., POD), biosynthesis of lignin & \multirow{4}{*}{[19]} \\  & & (e.g., phenyl/coumarin benzylic either reductase), carbon fixation, glycolysis, cytoskeleton organization (e.g., actin) & & \\ \cline{3-3}
1-10\({}^{-10}\) M MMHg 24 h & \multirow{4}{*}{no effect} & no effect & \\ \cline{3-3}
3.5-10\({}^{-10}\) M Hig 24 h & & In shoots, no significantly regulated metabolites. In cytosol, 4 regulated metabolites & \multirow{4}{*}{[43]} \\ \cline{3-3}
1-10\({}^{-10}\) M MMHg 24 h & \multirow{4}{*}{M: LC-MS and GC-MS} & In shoots, 15 significantly regulated metabolites: biosynthesis of aminoacyl-tRNA, metabolism of glycine, serine, threonine, nitrogen, arginine, proline and cyananomino acid. In cytosol, 3 regulated metabolites. & & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Responses to Hg revealed by metabolomic (M), proteomic (P) or transcriptomic (T) in _E. nuttullii_.