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International audience; Large-scale studies are needed to identify the drivers of total mercury (THg) and monomethyl-mercury (MeHg) concentrations in aquatic ecosystems. Studies attempting to link dissolved organic matter (DOM) to levels of THg or MeHg are few and geographically constrained. Additionally, stream and river systems have been understudied as compared to lakes. Hence, the aim of this study was to examine the influence of DOM concentration and composition, morphological descriptors, land uses and water chemistry on THg and MeHg concentrations and the percentage of THg as MeHg (%MeHg) in 29 streams across Europe spanning from 41°N to 64°N. THg concentrations (0.06e2.78 ng L-1) were highest in streams characterized by DOM with a high terrestrial soil signature and low nutrient content. MeHg concentrations (7.8 e159 pg L-1) varied non-systematically across systems. Relationships between DOM bulk characteristics and THg and MeHg suggest that while soil derived DOM inputs control THg concentrations, autochthonous DOM (aquatically produced) and the availability of electron acceptors for Hg methylatingmicroorganisms (e.g. sulfate) drive %MeHg and potentially MeHg concentration. Overall, these results highlight the large spatial variability in THg and MeHg concentrations at the European scale, and underscore the importance of DOM composition on mercury cycling in fluvial systems.

International audience; As the methylation of inorganic mercury to neurotoxic methylmercury has been attributed to the activity of anaerobic bacteria, the formation of methylmercury in the oxic water column of marine ecosystems has puzzled scientists over the past years. Here we show for the first time that methylmercury can be produced in particles sinking through oxygenated water column of lakes. Total mercury and methylmercury concentrations were measured in the settling particles and in surface sediments of the largest freshwater lake in Western Europe (Lake Geneva). While total mercury concentration differences between sediments and settling particles were not significant, methylmercury concentrations were about ten-fold greater in settling particles. Methylmercury demethylation rate constants (kd) were of similar magnitude in both compartments. In contrast, mercury methylation rate constants (km) were one order of magnitude greater in settling particles. The net potential for methylmercury formation, assessed by the ratio between the two rate constants (km kd -1), was therefore up to ten fold greater in settling particles, denoting that in situ transformations likely contributed to the high methylmercury concentration found in settling particles. Mercury methylation was inhibited (∼80%) in settling particles amended with molybdate, demonstrating the prominent role of biological sulfate-reduction in the process.

The present study demonstrates that species-specific isotope tracing is an useful tool to precisely measure Hg accumulation and transformations capabilities of living organisms at concentrations naturally encountered in the environment. To that end, a phytoplanktonic green alga Chlamydomonas reinhardtii Dangeard (Chlamydomonadales, Chlorophyceae) was exposed to mixtures of 199 -isotopically enriched inorganic mercury ( 199 IHg) and of 201 -isotopically enriched monomethylmercury ( 201 CH 3 Hg) at a concentration range between less than 1 pM to 4 nM. Additionally, one exposure concentration of both mercury species was also studied separately to evaluate possible interactive effects. No difference in the intracellular contents was observed for algae exposed to 199 IHg and 201 CH 3 Hg alone or in their mixture, suggesting similar accumulation capacity for both species at the studied concentrations. Demethylation of 201 CH 3 Hg was observed at the highest exposure concentrations, whereas no methylation was detected.

We examined mercury (Hg) biogeochemistry and biomagnification in the Babeni Reservoir,a system strongly affected by the release of Hg from a chlor-alkali plant. Total mercury(THg) concentrations in river water reached 88 ng L 1 but decreased rapidly in the reservoir(to 9 ng L ). In contrast, monomethylmercury (MMHg) concentrations increased from the1upstream part of the reservoir to the central part (0.7 ng L ), suggesting high methylation1within the reservoir. Moreover, vertical water column profiles of THg and MMHg indicatedthat Hg methylation mainly occurred deep in the water column and at the sediment ewaterinterface. The discharge of Hg from a chlor-alkali plant in Valcea region caused the highestMMHg concentrations ever found in non-piscivorous fish worldwide. MMHg concentra-tions and bioconcentration factors (BCF) of plankton and macrophytes revealed that thehighest biomagnification of MMHg takes place in primary producers.a 2013 Elsevier Ltd. All rights reserved.* Corresponding author. Current address: Limnology/Department of Ecology and Genetics, Uppsala University, Norbyva¨gen 18D, 75236Uppsala, Sweden.E-mail addresses: andrea.garcia@gmail.com, andrea.garcia@ebc.uu.se (A.G. Bravo).

While purified bacterial cells and DNA – the signature of life – from soil and sediment matrices have been extensively studied in a wide range of environments and in different microbial ecosystems, the paucity of data on DNA extraction from contaminated sediments emphasizes the need for further research on the isolation and quantification of bacterial cells and DNA in sediments. Consequently, the Nycondez gradient centrifugation method was applied to extract bacterial cells from contaminated and uncontaminated sediments. Quantitative estimates of recovered bacterial cells were obtained from direct counts performed using DAPI (4′,6′-diamino-2-phenylindole hypochloride) staining couples with fluorescence microscopy and indirect counts (colony-forming units). The estimation was improved by using an efficient method of comparing sediment types composed of quantifying bacterial densities in three steps: S1 the initial freshwater sediments; S2 the first supernatant recovered after mixing the sediments with sodium hexametaphosphate solution followed by centrifugation; and S3 the extracted cells. Total and extracellular DNA were extracted and quantified in each of the three steps. Additional analysis of faecal indicator bacteria (FIB) including E. coli and Enterococcus (ENT) was also performed in each step. The results display considerable variability in the quantity of bacteria cells depending on sediment type, ranging from 1.2 × 105 to 6.2 × 109 cell g−1 dry sediments. The treatment with sodium hexametaphosphate solution (2%) leads to the desorption of bacterial populations which were firmly adsorbed on contaminated sediment surfaces resulting in more than 90% of the FIB being recovered. The Nycondez density gradient centrifugation method makes it possible to extract bacterial cells from freshwater sediments without extracellular DNA so it is ideal for metagenomic analysis of bacteria.