Cellular toxicity pathways of inorganic and methyl mercury in the green microalga Chlamydomonas reinhardtii

Abstract

Contamination by mercury (Hg) is a worldwide concern because of Hg toxicity and biomagnification in aquatic food webs. Nevertheless, bioavailability and cellular toxicity pathways of inorganic (IHg) and methyl-Hg (MeHg) remain poorly understood. We analyzed the uptake, transcriptomic, and physiological responses in the microalga Chlamydomonas reinhardtii exposed to IHg or MeHg. Bioavailability of MeHg was up to 27× higher than for IHg. Genes involved in cell processes, energy metabolism and transport were dysregulated by both Hg species. Physiological analysis revealed an impact on photosynthesis and reduction-oxidation reaction metabolism. Nevertheless, MeHg dysregulated a larger number of genes and with a stronger fold-change than IHg at equivalent intracellular concentration. Analysis of the perturbations of the cell's functions helped to derive a detailed mechanistic understanding of differences in cellular handling of IHg and MeHg resulting in MeHg having a stronger impact. This knowledge is central for the prediction of impact of toxicants on organisms.

Figure 1

Intracellular total Hg concentrations ([THg]_intra, mean ± sd, n = 3) in C. reinhardtii exposed to IHg or MeHg, as a function of initial IHg or MeHg exposure concentration expressed as THg ([THg]_water). Values indicate ratio of [THg]_intra to [THg]_intra+ads, lines show linear regressions of log₁₀ values (A). Number of significantly up- and down-regulated genes (EdgeR FDR < 0.1%) as a function of [THg]_intra (B). Venn diagrams showing the number of up- (red, upper numbers) and down- (green, lower numbers) significantly regulated genes in C. reinhardtii after 2 h exposure to IHg (C) or MeHg (D) (EdgeR, FDR < 0.1%).

Figure 2

Number of genes significantly dysregulated in C. reinhardtii after 2 h exposure to increasing concentrations of IHg or MeHg (MapMan) in the categories involved in gene expression (A), cell processes (B), energy metabolism (C) and transport (D).

Figure 3

Gene expression regulation of 19 metal (A) and 8 amino acid (B) transporters significantly dysregulated in C. reinhardtii exposed 2 h to IHg or MeHg (ZIP = zinc-, iron-regulated transporter family; Zn = zinc transporter precursors; HM-ATPase = heavy metal ATPase; NRAMP = natural resistance-associated macrophage protein metal ion transporter family protein; Co = cobalt ion transmembrane transporter; Ni = high-affinity nickel-transport family protein; n. def. = no definition) (Phytozome V9.0).

Figure 4

Schematic representation of cellular toxicity pathways and tolerance responses, as derived from uptake, transcriptome and physiological effects in C. reinhardtii exposed 2 h to IHg and MeHg (AA: amino acid transporter; ABC: ABC transporter; C: chloroplast; CCM: carbon concentrating mechanism; Cys: cysteine; L: lipid bodies; ER: endoplastic reticulum; G: golgi; Glu: glutamate; M: mitochondria; Met: methionine; N: nucleus; P: pyrenoid; S: S transporter; St: starch; V: vacuole; Zn: metal transporter).