Molecular Effects, Speciation, and Competition of Inorganic and Methyl Mercury in the Aquatic Plant Elodea nuttallii

Abstract

Mercury (Hg) remains hazardous in aquatic environments because of its biomagnification in food webs. Nonetheless, Hg uptake and impact in primary producers is still poorly understood. Here, we compared the cellular toxicity of inorganic and methyl Hg (IHg and MeHg, respectively) in the aquatic plant Elodea nuttallii. IHg and MeHg regulated contigs involved in similar categories (e.g., energy metabolism, development, transport, secondary metabolism), but MeHg regulated more contigs, supporting a higher molecular impact than IHg. At the organism level, MeHg induced antioxidants, while IHg decreased chlorophyll content. The uptake of Hg and expression of a subset of contigs was subsequently studied in complex media. Measured uptake pointed to a contrasted impact of cell walls and copper (Cu) on IHg and MeHg. Using a speciation modeling, differences in uptake were attributed to the differences in affinities of IHg and MeHg to organic matter in relation to Cu speciation. We also identified a distinct gene expression signature for IHg, MeHg, and Cu, further supporting different molecular toxicity of these trace elements. Our data provided fundamental knowledge on IHg and MeHg uptake in a key aquatic primary producer and confirmed the potential of transcriptomics to assess Hg exposure in environmentally realistic systems.

Comparative study of Cu uptake and early transcriptome responses in the green microalga Chlamydomonas reinhardtii and the macrophyte Elodea nuttallii

“…impact on photosynthesis, RedOx, growth and nutrition, although only 2 h exposure was performed. This confirms the potential of transcriptomics to reveal early-responses at environmental concentrations (Beauvais-Fluck et al, 2018b;Dranguet et al, 2017;Regier et al, 2013a). Not surprisingly this short exposure resulted in few physiological endpoints significantly different vs control (Table S4 and S5) (Jamers et al, 2013;Jiang et al, 2016).…”

Molecular Effects of Inorganic and Methyl Mercury in Aquatic Primary Producers: Comparing Impact to A Macrophyte and A Green Microalga in Controlled Conditions

“…Nevertheless, the transport across the plasma membrane is most likely the limiting step of Hg absorption in cells ( [1]). The presence of the cell wall in primary producers represents a supplemental barrier for Hg intracellular uptake ( [7]). However, the transport of Hg across membranes is poorly understood in aquatic primary producers.…”

“…However, in C. reinhardtii, at 1 and 10 nM IHg in absence of DOM, 16 ± 1% of IHg was intracellular, while 53 ± 12% of MeHg was intracellular at similar concentrations ( [36]). Similarly, in E. nuttallii exposed to 10 pM IHg and MeHg in absence of DOM, 33% of Hg was intracellular for IHg treatment, while 100% of Hg was found intracellularly for MeHg treatment ( [7]). Differences in binding affinity of IHg and MeHg to DOM and cell walls could result in the higher bioaccumulation of MeHg than IHg in primary producers in natural waters, as well as consequently in a higher toxicity of MeHg in natural waters ( [7]).…”

“…Typical environmental concentrations measured in rivers and lakes range between 0.003 and 30 nM THg ( [5]). Both IHg and MeHg are mainly bound to dissolved organic matter (DOM) ( [1,6,7]) in particular to thiol (RSH) groups. For the fraction of Hg not bound with DOM, water chemistry (e.g., pH and chloride concentration) controls its distribution among different dissolved chemical species and complexes ( [8,9]).…”

“…In freshwaters, predominant species in the dissolved portion of Hg are hydroxo-(e.g., Hg(OH) + , CH 3 HgOH) and chloro-(e.g., HgCl 2 , HgCl + , CH 3 HgCl) complexes ( [8]). It has been shown that IHg and MeHg show distinct affinities to DOM and ions in water ( [1,6,7]). Nevertheless, currently the measurement and the modelling of speciation of Hg in natural waters and linking Hg speciation with uptake in biota both remain challenging.…”