## 3 Responses to Hg Exposure at Different Levels of Biological Organization in _Elodea nuttallii_  
### Physiological Responses  
Among the effects of Hg toxicity at the individual level, growth is a widely used endpoint [22]. For example, the growth of cells in the presence of IHg or MMHg in _Thalassisina weissflogii_ was compared and showed a reduction of 50% at \(5\times 10^{-10}\) M IHg and MMHg [29]. In addition, scientists observed different toxicity pathways at the cell level: growth rate was reduced by MMHg, while strong cell damage, but no effect on division rate, resulted from IHg exposure [29]. Exposure for 7d to \(3.5\times 10^{-10}\) M IHg reduced growth of roots and increased cell wall lignification in _E. nuttallii_, whereas exposure for 7d to \(1\times 10^{-10}\) M MMHg had no effect on those endpoints [19] (Table 1). Under similar conditions, an inhibition of root growth was observed for 1 nM MMHg [21]. When uptake, Hg compounds bind to DNA, proteins and various enzymes that are primordial for cell normal physiology. Hg also increases the production of reactive oxygen species (ROS) that might result in oxidative stress, and is evidenced by the alteration in the activity of enzymes involved in ROS regulation, such as catalases, class III peroxididases (POD), lipoxygenases, or superoxide dismutases (SOD) [30; 31]. SOD, POD and catalase activities were induced by 5 \(\upmu\)M IHg 7d in _Lemnu minor_[22] and excessive ROS generation was evidenced in _Chlamydomonas reinhardtii_ exposed to \(1\times 10^{-7}\) M IHg or \(1\times 10^{-9}\) M MMHg [32] or for 96 h to \(2\times 10^{-6}\) M IHg [33], and resulting in lipid peroxidation after 24 h exposure to \(5\times 10^{-6}\) M IHg [34]. In _E. nuttallii_, after 24 h exposure to \(4\times 10^{-10}\) M and \(4\times 10^{-5}\) M IHg, SOD and POD activities showed a 1.3\(\times\) and 1.3\(\times\) increase and a 2.1\(\times\) and 4.6\(\times\) decrease, respectively [35] (Table 1). In addition, numerous studies in plants and algae evidenced the impact of Hg (mostly \(10^{-6}\) M IHg) on photosynthesis, notably the chlorophyll breakdown and the reduction in photosynthesis efficiency [11; 22; 31; 36; 37]. Hg toxicity results from its binding to various  
Figure 1: Distribution (%) of THz at the subcellular level in _E. nuttallii_ exposed 2 h to IHg or MMHg in water without DOM [25]. groups in proteins, e.g., sulfhydryl (-SH), phosphate and active groups of ADP or ATP, as well as essential major cations' substitution [36; 38]. The impact of IHg in all kinds of phytoplankton and macrophyte species on the photosynthesis and the generation of an oxidative stress were widely reported. However, MMHg toxicity in aquatic primary producers has been globally less studied than IHg. The data reveal similar targets for MMHg and IHg (e.g., oxidative stress, photosynthesis), but when compared in strictly identical experimental conditions, MMHg seems to impact distinct toxicity pathways than IHg [25; 39]]. Nevertheless, physiological responses are rarely significant at environmental concentrations [21]. Indeed, many of the studies described above were obtained in controlled laboratory experiments at concentrations \(10^{3}\) to \(10^{6}\) higher than Hg concentrations generally found in natural waters. Recently, a study in _C. reinhardtii_ exposed for 2 h to \(10^{-8}\) and \(10^{-11}\) M MMHg showed an increase in photosynthesis efficiency. This paradoxical effect observed for lower range concentration is called hormesis and results from the overcompensation of a moderate stress [37; 40]. A distinct impact of IHg and MMHg was also reported on photosynthesis in _T. weissflogii_, a marine diatom. Exposure for 72 h to 2-11\(\cdot 10^{-7}\) M IHg blocked the electron chain of photosynthesis and increased chlorophyll fluorescence lifetime. In identical experimental conditions, exposure to up to 2.8\(\cdot 10^{-6}\) M MMHg had no effect [29]. Similarly, chlorophyll content was reduced by IHg, while a significant antioxidant response was induced by MMHg in _E. nuttallii_ (Table 1) [25].