Munson Kathleen M.

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Munson
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Kathleen M.
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Transformations of mercury in the marine water column

2014-02 , Munson, Kathleen M.

Methylation of mercury (Hg) in the marine water column has been hypothesized to serve as the primary source of the bioaccumulating chemical species monomethylmercury (MMHg) to marine food webs. Despite decades of research describing mercury methylation in anoxic sediments by anaerobic bacteria, mechanistic studies of water column methylation are severely limited. These essential studies have faced analytical challenges associated with quantifying femtomolar concentrations of the methylated Hg species dimethylmercury (DMHg) and MMHg in marine systems. In addition, the complex biogeochemical cycling of Hg in natural systems require consideration of gaseous, dissolved, and particulate species of Hg in order to probe potential controls on its ultimate transfer into marine food webs. The presented work provides a comprehensive study of Hg chemical speciation and transformations in Tropical Pacific waters. We developed an analytical method for MMHg determination from seawater that has the potential to ease measurements of MMHg distributions, as well as mechanistic studies of Hg species transformations. We used this method, in addition to previously established methods, to measure dissolved and particulate Hg species distributions and fluxes along a transect of the Pacific Ocean. Over significant gradients in oxygen utilization and primary productivity, we observed a region of methylated Hg species focused in the Equatorial Pacific that appeared spatially separated from higher concentrations in North Pacific Intermediate Waters. From the first full water column depth profiles of this region, we also observed the intrusion of elevated Hg into deep waters of the Equatorial and South Pacific Ocean. In addition we observed substantial potential rates of mercury methylation in subsurface and low oxygen waters along the Pacific transect as well as the Sargasso Sea using Hg isotope tracers. We observed dynamic production and decomposition of methylated Hg in low productivity waters, despite low ambient methylated Hg concentrations. From the addition of bulk organic matter as well as individual compounds important for methylation in anaerobic bacteria, we observe no simple limitation of Hg methylation in marine waters but highly dynamic conversion of Hg between methylated and inorganic species.

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Article

Dynamic mercury methylation and demethylation in oligotrophic marine water

2018-11-02 , Munson, Kathleen M. , Lamborg, Carl H. , Boiteau, Rene M. , Saito, Mak A.

Mercury bioaccumulation in open-ocean food webs depends on the net rate of inorganic mercury conversion to monomethylmercury in the water column. We measured significant methylation rates across large gradients in oxygen utilization in the oligotrophic central Pacific Ocean. Overall, methylation rates over 24h incubation periods were comparable to those previously published from Arctic and Mediterranean waters despite differences in productivity between these marine environments. In contrast to previous studies that have attributed Hg methylation to heterotrophic bacteria, we measured higher methylation rates in filtered water compared to unfiltered water. Furthermore, we observed enhanced demethylation of newly produced methylated mercury in incubations of unfiltered water relative to filtered water. The addition of station-specific bulk filtered particulate matter, a source of inorganic mercury substrate and other possibly influential compounds, did not stimulate sustained methylation, although transient enhancement of methylation occurred within 8h of addition. The addition of dissolved inorganic cobalt also produced dramatic, if transient, increases in mercury methylation. Our results suggest important roles for noncellular or extracellular methylation mechanisms and demethylation in determining methylated mercury concentrations in marine oligotrophic waters. Methylation and demethylation occur dynamically in the open-ocean water column, even in regions with low accumulation of methylated mercury.

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Article

Determination of monomethylmercury from seawater with ascorbic acid-assisted direct ethylation

2014-01 , Munson, Kathleen M. , Babi, Diana , Lamborg, Carl H.

We developed a technique to measure monomethylmercury (MMHg) concentrations from small volumes (180 mL) of seawater at low femtomolar concentrations using direct ethylation derivitization, decreasing the required volume by 90% from current methods while maintaining a 5 fM detection limit. In this method, addition of ascorbic acid before derivitization of MMHg allows for full recovery of MMHg from the seawater matrix without the need for sample distillation or extraction. The small sample size and relative ease of detection are ideal both for shipboard as well as shore-based measurements of preserved MMHg samples. Combined with shipboard determination of dimethylmercury (DMHg) and elemental mercury (Hg(0)), this method can be used to determine full marine mercury speciation.

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Article

Mercury species concentrations and fluxes in the Central Tropical Pacific Ocean

2015-03-25 , Munson, Kathleen M. , Lamborg, Carl H. , Swarr, Gretchen J. , Saito, Mak A.

The formation of the toxic and bioaccumulating monomethylmercury (MMHg) in marine systems is poorly understood, due in part to sparse data from many ocean regions. We present dissolved mercury (Hg) speciation data from 10 stations in the North and South Equatorial Pacific spanning large water mass differences and gradients in oxygen utilization. We also compare the mercury content in suspended particles from six stations and sinking particles from three stations to constrain local Hg sources and sinks. Concentrations of total Hg (THg) and methylated Hg in the surface and intermediate waters of the Equatorial and South Pacific suggest Hg cycling distinct from that of the North Pacific gyre. Maximum concentrations of 180 fM for both MMHg and dimethylmercury (DMHg) are observed in the Equatorial Pacific. South of the equator, concentrations of MMHg and DMHg are less than 100 fM. Sinking fluxes of particulate THg can reasonably explain the shape of dissolved THg profiles, but those of MMHg are too low to account for dissolved MMHg profiles. However, methylated Hg species are lower than predicted from remineralization rates based on North Pacific data, consistent with limitation of methylation in Equatorial and South Pacific waters. Full water column depth profiles were also measured for the first time in these regions. Concentrations of THg are elevated in deep waters of the North Pacific, compared to those in the intermediate and surface waters, and taper off in the South Pacific. Comparisons with previous measurements from nearby regions suggest little enrichment of THg or MMHg over the past 20 years.

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Article

Mercury in the anthropocene ocean

2014-03 , Lamborg, Carl H. , Bowman, Katlin , Hammerschmidt, Chad R. , Gilmour, Cindy , Munson, Kathleen M. , Selin, Noelle , Tseng, Chun-Mao

The toxic metal mercury is present only at trace levels in the ocean, but it accumulates in fish at concentrations high enough to pose a threat to human and environmental health. Human activity has dramatically altered the global mercury cycle, resulting in loadings to the ocean that have increased by at least a factor of three from pre-anthropogenic levels. Loadings are likely to continue to increase as a result of higher atmospheric emissions and other factors related to global environmental change. The impact that these loadings will have on the production of methylated mercury (the form that accumulates in fish) is unclear. In this article, we summarize the biogeochemistry of mercury in the ocean and use this information to examine past impacts that human activity has had on the cycling of this toxic metal. We also highlight ways in which the mercury cycle may continue to be affected and its potential impact on mercury in fish.

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