Imaging differential mercury and selenium species bioaccumulation by glass eels using isotopic tracers and laser ablation

International audienc

Identification of sources and bioaccumulation pathways of MeHg in subantarctic penguins: a stable isotopic investigation

International audienceSeabirds are widely used as bioindicators of mercury (Hg) contamination in marine ecosystems and the investigation of their foraging strategies is of key importance to better understand methylmercury (MeHg) exposure pathways and environmental sources within the different ecosystems. Here we report stable isotopic composition for both Hg mass-dependent (e.g. δ202Hg) and mass-independent (e.g. Δ199Hg) fractionation (proxies of Hg sources and transformations), carbon (δ13C, proxy of foraging habitat) and nitrogen (δ15N, proxy of trophic position) in blood of four species of sympatric penguins breeding at the subantarctic Crozet Islands (Southern Indian Ocean). Penguins have species-specific foraging strategies, from coastal to oceanic waters and from benthic to pelagic dives, and feed on different prey. A progressive increase to heavier Hg isotopic composition (δ202Hg and Δ199Hg, respectively) was observed from benthic (1.45 ± 0.12 and 1.41 ± 0.06‰) to epipelagic (1.93 ± 0.18 and 1.77 ± 0.13‰) penguins, indicating a benthic-pelagic gradient of MeHg sources close to Crozet Islands. The relative variations of MeHg concentration, δ202Hg and Δ199Hg with pelagic penguins feeding in Polar Front circumpolar waters (1.66 ± 0.11 and 1.54 ± 0.06‰) support that different MeHg sources occur at large scales in Southern Ocean deep waters

Identical Hg isotope mass dependent fractionation signature during methylation by sulfate-reducing bacteria in sulfate and sulfate-free environment

International audienceInorganic mercury (iHg) methylation in aquatic environments is the first step leading to monomethylmercury (MMHg) bioaccumulation in food webs and might play a role in the Hg isotopic composition measured in sediments and organisms. Methylation by sulfate reducing bacteria (SRB) under sulfate-reducing conditions is probably one of the most important sources of MMHg in natural aquatic environments, but its influence on natural Hg isotopic composition remains to be ascertained. In this context, the methylating SRB Desulfovibrio dechloracetivorans (strain BerOc1) was incubated under sulfate reducing and fumarate respiration conditions (SR and FR, respectively) to determine Hg species specific (MMHg and IHg) isotopic composition associated with methylation and demethylation kinetics. Our results clearly establish Hg isotope mass-dependent fractionation (MDF) during biotic methylation (-1.20 to +0.58‰ for δ202Hg), but insignificant mass-independent fractionation (MIF) (-0.12 to +0.15‰ for Δ201Hg). During the 24h of the time-course experiments Hg isotopic composition in the produced MMHg becomes significantly lighter than the residual IHg after 1.5h and shows similar δ202Hg values under both FR and SR conditions at the end of the experiments. This suggests a unique pathway responsible for the MDF of Hg isotopes during methylation by this strain regardless the metabolism of the cells. After 9 h of experiment, significant simultaneous demethylation is occurring in the culture and demethylates preferentially the lighter Hg isotopes of MMHg. Therefore, depending on their methylation/demethylation capacities, SRB communities in natural sulfate reducing conditions likely have a significant and specific influence on the Hg isotope composition of MMHg (MDF) in sediments and aquatic organisms

Transformation, localization, and biomolecular binding of Hg species at subcellular level in methylating and nonmethylating sulfate-reducing bacteria

International audienceMicrobial activity is recognized to play an important role on Hg methylation in aquatic ecosystems. However, the mechanism at the cellular level is still poorly understood. In this work subcellular partitioning and transformation of Hg species in two strains: Desulfovibrio sp. BerOc1 and Desulfovibrio desulfuricans G200 (which exhibit different Hg methylation potential) are studied as an approach to the elucidation of Hg methylation/demethylation processes. The incubation with isotopically labeled Hg species (199Hgi and Me201Hg) not only allows the determination of methylation and demethylation rates simultaneously, but also the comparison of the localization of the originally added and resulting species of such metabolic processes. A dissimilar Hg species distribution was observed. In general terms, monomethylmercury (MeHg) is preferentially localized in the extracellular fraction; meanwhile inorganic mercury (Hgi) is associated to the cells. The investigation of Hg binding biomolecules on the cytoplasmatic and extracellular fractions (size exclusion chromatography coupled to ICP-MS) revealed noticeable differences in the pattern corresponding to the Hg methylating and nonmethylating strains

Hemoglobin as a major binding protein for methylmercury in white-sided dolphin liver

International audienceAs methylmercury (MeHg) can be bioaccumulated and biomagnified in the trophic web, its toxicity for marine mammals is of major concern. Mercury speciation in marine biota has been widely studied, mainly focused on the discrimination and quantification of inorganic Hg and MeHg. Less attention has been paid to the interactions of Hg with biomolecules and the characterization of its specific binding, which play a key role in metabolic pathways controlling its uptake, transformation, and toxicity. In the studied white-sided dolphin (Lagenorhynchus acutus) liver homogenate (QC04LH4) sample, approximately 60 % of the total MeHg was found in the water soluble fraction, specifically associated with high molecular weight biomolecules. The identity of the involved proteins was investigated (after tryptic digestion of the fraction) by μRPLC with parallel detection by ICP-MS and ESI-MS/MS. Molecular mass spectrometry experiments were carried out at high resolution (100000) to ensure accurate protein identification and determination of the MeHg binding sites. Cysteine residue on the dolphin hemoglobin β chain was found to be the main MeHg binding site, suggesting that hemoglobin is a major MeHg binding protein in this marine mammal and could be a potential carrier of this MeHg from blood to liver prior to its degradation in this organ. In parallel, a significant proportion of selenium was found to be present as selenoneine and a potential role for this compound in Hg detoxification is discussed

Identification of mercury and other metals complexes with metallothioneins in dolphin liver by hydrophilic interaction liquid chromatography with the parallel detection by ICP MS and electrospray hybrid linear/orbital trap MS/MS

International audienceA novel analytical procedure for the identification of metal (Hg, Cd, Cu, Zn) complexes with individual metallothionein (MT) isoforms in biological tissues by electrospray MS/MS was developed. The sample preparation was reduced to three rapid steps: the two-fold dilution of the sample cytosol with acetonitrile, the recovery of the supernatant containing MT-complexes by centrifugation and its concentration under nitrogen flow. The replacement of reversed phase HPLC by hydrophilic interaction LC (HILIC) allowed the preservation of the unstable and low abundant metallothionein zinc-mercury mixed complexes (MT-Zn6Hg). The MT complexes eluted were detected by ICP MS and identified in terms of molecular mass by electrospray high resolution (100000) MS. The identification was completed by on line demetallation and the determination of the molecular mass of the apoform, followed by amino acid sequencing in the top-down mode using high energy collision fragmentation (HCD). The method was applied to the identification of MT complexes in a white-sided dolphin (Lagenorhynchus acutus) liver homogenate. The Zn complex of the N-acetylated MT2 isoform was found to be predominant, the presence of mixed complexes with Cd, Cu and, for the first time ever, Hg, was demonstrated. The latter finding has the potential to shed new light on the mercury detoxification mechanism in marine organisms