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The biological production of monomethylmercury (MeHg) in soils and sediments is an important factor controlling mercury (Hg) accumulation in aquatic and terrestrial food webs. In this study we examined the fractionation of Hg stable isotopes during Hg methylation in nongrowing cultures of the anaerobic bacteria Geobacter sulfurreducens PCA and Desulfovibrio desulfuricans ND132. Both organisms showed mass-dependent, but no mass-independent fractionation of Hg stable isotopes during Hg methylation. Despite differences in methylation rates, the two bacteria had similar Hg fractionation factors (αr/p = 1.0009 and 1.0011, respectively). Unexpectedly, δ(202)Hg values of MeHg for both organisms were 0.4‰ higher than the value of initial inorganic Hg after about 35% of inorganic Hg had been methylated. These results indicate that a (202)Hg-enriched pool of inorganic Hg was preferentially utilized as a substrate for methylation by these organisms, but that multiple intra- and/or extracellular pools supplied inorganic Hg for biological methylation. Understanding the controls of the Hg stable isotopic composition of microbially produced MeHg is important to identifying bioavailable Hg in natural systems and the interpretation of Hg stable isotopes in aquatic food webs.

Sources of methylmercury (MeHg) in adult Pacific bluefin tuna (Thunnus orientalis, PBT) from the western North Pacific Ocean (WPO) were examined using mercury stable isotopes. Significant increases in δ202Hg and Δ199Hg values with PBT size and age, along with those of potential prey, indicate a shift in the source of MeHg accumulated by PBT as they age. Among adults from the WPO, this shift likely involves greater accumulation of MeHg from epipelagic prey in the Kuroshio extension in large vs. small and medium-sized PBT. For all adults, little MeHg is accumulated in the spawning grounds near Taiwan. Significantly lower Δ199Hg/Δ201Hg ratios in adult PBT and their prey from the WPO than from the central and eastern North Pacific indicate different sources or transformations of MeHg prior to accumulation in the WPO food web than further east. Our results show that MeHg sources to oceanic food webs vary across the North Pacific Ocean and regionally within the WPO. [ABSTRACT FROM AUTHOR]

• legacy mercury in upper Berry's Creek has a δ202Hg value of −0.29‰. • organic matter content was used to identify a low δ202Hg pool of mercury that dilutes legacy contaminant mercury. • upper Berry's Creek contributes 21%–82% of mercury to the Hackensack River estuary. • legacy mercury may affect marsh habitats far from sites of primary contamination. Spatial redistribution of legacy mercury (Hg) contamination in the Hackensack River estuary (New Jersey, USA) was evaluated using mercury stable isotopes. Total Hg varied from 0.06 to 3.8 μg g−1 in sediment from the tidal Hackensack River and from 15 to 154 μg g−1 near historically contaminated sites in upper Berry's Creek, a tributary of the Hackensack River. δ202Hg values for total Hg from Berry's Creek and Hackensack River estuaries varied over a fairly narrow range (−0.44‰ to −0.21‰), but were highest for sediment from upper Berry's Creek. Isotope mixing plots show that residual legacy mercury from upper Berry's Creek is partially diluted by a low concentration and low δ202Hg pool of mercury associated with low organic matter content sediments similar to those in Newark Bay. Based on an isotope mixing model, we estimate that upper Berry's Creek contributes 21%–82% of the mercury in sediments in the Hackensack River estuary and its tidal tributaries, including upstream marsh habitats far from the primary source. Our results show that mercury stable isotopes can be used to track the redistribution of mercury in tidal ecosystems and highlight the potentially large areas which may be affected by legacy mercury contamination in estuaries. [ABSTRACT FROM AUTHOR]

• Long-term volumetric Hg TSP concentration was decreasing in Beijing. • Mass-based concentrations of Hg TSP were not reduced during emission control. • Industrial emission control has limited change in Hg isotope ratios of TSP. We investigated the effect of the temporary ban of local industrial activities during the Asia-Pacific Economic Cooperation (APEC) summit (4th–14th Nov 2014) in Beijing, China on total suspended particulate mercury (Hg TSP) concentrations and isotope compositions. We measured Hg concentrations and isotope ratios in 33 TSP samples from central Beijing, including 21 samples collected from Jun 2012 to Apr 2014, and 12 samples collected from 14th Oct–19th Nov 2014. Volumetric concentrations of both TSP and Hg TSP during the APEC summit were a factor of 2 lower than during the pre-APEC period, indicating substantial reductions in total particulate matter and Hg TSP as a result of emissions controls. However, mass-normalized concentrations and mercury isotope ratios of Hg TSP did not vary significantly between samples collected before, during, or after the APEC summit. These results show that local emissions are important sources of particle bound mercury (PBM) in Beijing and that their control can be used to immediately lower the volumetric concentration of Hg TSP. They also indicate that a similarly complex mixture of sources contributed to PBM in Beijing before and during emissions controls were put in place and that PBM concentrations in Beijing are primarily controlled by emissions and secondarily by photoreduction. [ABSTRACT FROM AUTHOR]

Abstract: Mercury (Hg) is a trace metal with potentially serious public health consequences, especially when its neurotoxic form, monomethylmercury (MMHg), accumulates in aquatic and terrestrial food chains. Given the variation in Hg stable isotope abundances (up to 6‰ in δ202/198Hg and 11‰ in Δ201/198Hg) in natural environmental samples and evidence for Hg isotopic fractionation during microbial (e.g., reduction of Hg(II), degradation and formation of MMHg) and abiotic processes (e.g., photo-degradation of MMHg and volatilization, evaporation and diffusion of Hg(0)), Hg isotope biogeochemistry is currently being developed as a tool to study sources, sinks, and transformations of Hg in the environment. The use of Hg stable isotopes to identify sources of MMHg in aquatic and terrestrial food chains depends on the knowledge of reasonably precise fractionation factors for individual Hg transformations that influence speciation and accumulation of Hg. Microbial transformations are critical to the formation of MMHg and Hg(0) and to the degradation of MMHg. This article a) highlights the importance of experimental determination of microbial fractionation factors for the development of Hg stable isotope systematics because of the limitations of theoretical calculations; b) provides a summary of the current understanding of microbial stable isotope fractionation of Hg, especially during kinetically controlled processes; and c) demonstrates the various factors likely to affect cell-level fractionation during microbial transformations through use of an iterative finite-step model. We also identify future directions, conditions and controls that could help rigorously advance the development of mass-dependent Hg isotope systematics at the enzymatic, cellular and ecosystem level. [Copyright &y& Elsevier]

Estuarine sediments support the production of monomethylmercury (MeHg) which accumulates in aquatic organisms. While natural variation in mercury stable isotope ratios can potentially be used to track sources and transformations of mercury in the environment, the isotopic signature of MeHg in sediments has not been measured directly. The isotopic composition of MeHg has been studied in laboratory experiments and fish using tandem gas chromatography-multicollector inductively coupled plasma mass spectrometry (MC–ICP-MS) systems; however, the precision and sensitivity of this method may be too low for the analysis of many environmental samples including sediments in which MeHg constitutes 1% or less of the total mercury. In this study, we developed an offline separation method for the precise measurement of the Hg isotopic composition of MeHg in estuarine sediments. Separation of MeHg from inorganic species was accomplished by distillation and chemical ethylation-GC, and was followed by gold amalgam trapping to collect and preconcentrate pyrolyzed MeHg, which was then released into an oxidizing solution. MeHg standards processed in this way were collected with an average yield of 97.5%. External precision for all replicate isotope analyses of MeHg process standards was ± 0.14‰ (2 SD, n = 8) for δ 202 Hg and no detectable fractionation of Hg stable isotopes occurred during the separation. δ 202 Hg values for MeHg separated from estuarine sediments using our approach varied from − 0.41 to + 0.41‰ and were generally higher, and spatially and temporally more variable, than those for total Hg (− 0.21 to − 0.48‰). [ABSTRACT FROM AUTHOR]