Abstract
Mercury (Hg) is assumed to be predominantly methylated by microorganisms in the environment. However, the mechanisms and extent of abiotic methylation are poorly appreciated. The understanding of the mechanisms leading to abiotic methylation and demethylation in the aquatic environment is of special concern since methylmercury (MeHg) biomagnifies in the food web. Bioaccumulating organisms have also been found to preserve specific Hg isotopic signatures that provide direct insight into aquatic Hg transformations. In this study we investigated the influence of chloride on the magnitude of Hg isotope fractionation during abiotic methylation of inorganic Hg (Hg(II)) using methylcobalamin as methyl donor compound. Coupling of gas chromatography with multi-collector inductively coupled plasma mass spectrometry has allowed to determine simultaneously isotopic ratios of inorganic and methyl-Hg species. Kinetic experiments demonstrated that the presence of chloride not only slowed the chemical alkylation of Hg(II) by methylcobalamin, but also decreased the extent of the methylation, which it is especially significant under visible light conditions due to the enhancement of MeHg photodecomposition. Abiotic methylation of Hg(II) by methylcobalamin in the presence of chloride caused significant Hg mass-dependent isotope fractionation (MDF) for both Hg(II) substrate (δ202Hg(II) from −0.74‰ to 2.48‰) and produced MeHg (δ202MeHg from −1.44‰ to 0.38‰) both under dark and visible light conditions. The value of this MDF under such saline conditions was higher than that previously reported (δ202MeHg from −0.73‰ to 0.09‰) in the absence of chloride and appeared mainly related to inorganic Hg speciation in solution, which is predominantly mercuric chloro-complexes (i.e. HgCl42−). Different isotopic signatures were observed for the different Hg species at the same time of reaction for either dark or visible light (450–650nm wavelengths) conditions. However, no significant mass-independent fractionation (MIF) was induced under any conditions within the analytical uncertainties (−0.17±0.31<∆201Hg<0.17±0.28‰), suggesting that photo-induced demethylation does not always involve MIF. These results also suggest that methylation by methylcobalamin can be an experimental model to study Hg isotope fractionation extent during elementary reaction of methyl transfer in biotic systems.
Highlights
Methylmercury (MeHg) is the most hazardous compound among mercury (Hg) species due to its ability to bioaccumulate in aquatic biota, reaching higher levels than recommended values in top predators of both marine and freshwater environments
The presence of chloride levels (0.5 M) equivalent to seawater content has been already demonstrated to influence the methylation of Hg by methylating organisms such as sulphate reducing bacteria (Compeau and Bartha, 1984; Compeau and Bartha, 1987), as Hg– chloride complexes, similar to Hg–sulfide complexes, control the bioavailability and uptake of Hg by bacteria (Benoit et al, 1999; Ullrich et al, 2001)
The role of chloride for abiotic methylation of Hg by MeCo has been previously reported (Celo et al, 2006; Chen et al, 2007; Musante, 2008), despite of the fact that authors disagree with the ability of MeCo to methylate Hg at higher chloride levels
Summary
Methylmercury (MeHg) is the most hazardous compound among mercury (Hg) species due to its ability to bioaccumulate in aquatic biota, reaching higher levels than recommended values in top predators of both marine and freshwater environments. Understanding bioaccumulation of MeHg in aquatic food chains requires differentiation between biotic and abiotic pathways that lead to its production and degradation Microorganisms such as sulphate-reducing or iron-reducing bacteria are known to be widely involved both in the methylation of inorganic Hg (Hg(II)) and demethylation of MeHg (Compeau and Bartha, 1984; Compeau and Bartha, 1985) in aquatic ecosystems. Chemical abiotic methylation, including methylation by compounds released to the environment by biotic processes, is supposed to be a relevant process which can occur in the water column with the presence of methyl donor compounds such as methylcobalamin, methyl iodide or methyltin compounds (Thayer, 1989; Celo et al, 2006) In this context, methylcobalamin (MeCo), a naturally occurring coenzyme of vitamin B12, plays an important role in the environment since it can be considered as one of the main potential methyl donors. It is responsible for the biological methylation of mercury leading to highly toxic compounds such as MeHg (Schneider and Stroinski, 1987; Thayer, 1989; Choi and Bartha, 1993)
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