Simultaneous determination of mercury methylation and demethylation capacities of various sulfate-reducing bacteria using species-specific isotopic tracers

International audienceThe use of species-specific isotopic tracers for inorganic and methyl mercury has allowed the simultaneous determination of the methylation and demethylation potentials of pure culture of isolated sulfate-reducing (SR) bacterial strains using low Hg species concentration levels (7 μg/L 199Hg(II), 1 μg/L Me 201Hg). A major advantage of the method reported here is that it can be used to follow simultaneously both the degradation of the species added but also the formation of their degradation products and thus the determination during the same incubation of the specific methylation/demethylation yields and rate constants. Methylation/demethylation capacities and extents have been found to differ between the tested strains and the tested conditions. The methylating/demethylating capacities of bacteria appear to be strain specific. All the methylating strains were found to demethylate methylmercury (MeHg). The active mechanism responsible for Hg methylation appears directly dependent on the bacterial activity but is not dependent on the metabolism used by the tested bacteria (sulfate reduction, fermentation, or nitrate respiration). The results provide confirmation that SR strains contribute to MeHg demethylation under anoxic conditions, leading to Hg(II) as the end product, consistent with the oxidative degradation pathway. Kinetic experiments have allowed specific transformation rate constants to be addressed for the two reversible processes and the reactivity of each isotopic tracer to be compared. The differential reactivity highlighted the different steps involved in the two apparent processes (i.e., uptake plus internal transformation of mercury species). Methylation appears as the slowest process, mainly controlled by the assimilation of Hg(II), whereas demethylation is faster and not dependent on the MeHg concentration

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

Miroirs multicouches apériodiques à large bande passante (2–10 keV) pour les diagnostics d’imagerie X à haute résolution spatiale

Les miroirs multicouches interférentiels, constitués d’un empilement alternant des couches minces de matériaux lourds et légers, permettent, par un phénomène d’addition en phase des rayons réfléchis à chaque interface, d’obtenir des pouvoirs réflecteurs élevés dans le domaine des rayons X. Malheureusement les largeurs spectrales accessibles avec des multicouches périodiques sont peu étendues, de l’ordre de quelques centaines d’eV aux énergies de photons de quelques keV et sont incompatibles avec le fonctionnement large bande envisagé pour certains microscopes X que nous développons. Les supermiroirs ou en anglais « Depth – graded X ray mirrors »  sont une alternative intéressante pour accroître la bande passante du miroir par « superposition»  de la contribution des différentes périodes présentes dans l’empilement. Cet article précise les résultats expérimentaux qui ont permis la mise au point d’un supermiroir de qualité exceptionnelle travaillant entre 2 et 10 keV sous une incidence rasante de 0,7°

Investigations into the differential reactivity of endogenous and exogenous mercury species in coastal sediments

International audienceStable isotopic tracer methodologies now allow the evaluation of the reactivity of the endogenous (ambient) and exogenous (added) Hg to further predict the potential effect of Hg inputs in ecosystems. The differential reactivity of endogenous and exogenous Hg was compared in superficial sediments collected in a coastal lagoon (Arcachon Bay) and in an estuary (Adour River) from the Bay of Biscay (SW France). All Hg species (gaseous, aqueous, and solid fraction) and ancillary data were measured during time course slurry experiments under variable redox conditions. The average endogenous methylation yield was higher in the estuarine (1. 2 %) than in the lagoonal sediment (0. 5 %), although both methylation and demethylation rates were higher in the lagoonal sediment in relation with a higher sulfate-reducing activity. Demethylation was overall more consistent than methylation in both sediments. The endogenous and exogenous Hg behaviors were always correlated but the exogenous inorganic Hg (IHg) partitioning into water was 2. 0-4. 3 times higher than the endogenous one. Its methylation was just slightly higher (1. 4) in the estuarine sediment while the difference in the lagoonal sediment was much larger (3. 6). The relative endogenous and exogenous methylation yields were not correlated to IHg partitioning, demonstrating that the bioavailable species distributions were different for the two IHg pools. In both sediments, the exogenous IHg partitioning equaled the endogenous one within a week, while its higher methylation lasted for months. Such results provide an original assessment approach to compare coastal sediment response to Hg inputs