Research Projects

Mercury in the Hackensack

Since the fall of 2017, when Rob and Zosia received funding from the Hudson River Foundation for a project “Evaluation of the impacts of human and climate disturbance on mercury dynamics and bioaccumulation in benthic and pelagic organisms in the Hackensack sub-estuarine system” we have focused on the Hackensack sub-estuary of the Hudson River system. The research has involved fieldwork to date with two sampling trips completed. Samples are still being analyzed and data compiled but the results show interesting trends, and will be presented at the International Conference on Mercury as a Global Pollutant (ICMGP) in Kraow, Poland in 2019. Upcoming laboratory mesocosm experiments are being planned to complete the project, and we have obtained an annular flume that will be used to generate sediment resuspension for the studies examining the impacts of sediment transport on mercury (Hg) and methylmercury (MeHg) in different marsh environments. The primary focus is examining the role of sediment resuspension and transport in the fate and mobility of total Hg and MeHg within the system and the potential for re-contamination of areas, that have been remediated, from external local sources, and the role of vegetation in mediating net Hg methylation. The main objectives of the project are: 1) to determine Hg and MeHg contamination levels at three sites within the Hackensack River, including one at the mouth of Berry’s Creek, to ascertain whether Berry’s Creek could serve as a source of the observed recontamination up and downstream from its convergence with the Hackensack River; 2) investigate the patterns of MeHg concentrations in the Hackensack River ecosystem, by assessing total and MeHg in various benthic and pelagic organisms found at sampling sites, along with ancillary variables that will help identify sources; 3) examine how the water flow conditions, i.e. regular tidal current vs. storm-induced river flow, can influence the mobility of inorganic Hg and MeHg from sediment into the water column; 4) assess the effectiveness of Spartina vs. Phragmites marshlands in trapping contaminated particulate transported from other reaches by the river, and in the production of MeHg from inorganic Hg inputs; and 5) employ phytoplankton uptake experiments to determine key factors driving Hg and MeHg bioavailability to phytoplankton, the entry point to aquatic food webs in the system. The research was recently highlighted y the university’s Office of Research.

US GEOTRACES Pacific Meridional Transect: Determination of the Air-Sea Exchange of Inorganic and Methylated Mercury in the Anthropogenically-Impacted and Remote Pacific Ocean

The research, funded by the NSF Chemical Oceanography Program, is aimed at addressing key parameters outlined in GEOTRACES documents, and, in particular, is: 1) helping to assess the anthropogenic impact of Hg from the atmosphere to the Pacific Ocean, while informing more generally about the sources of Hg and methylmercury (MeHg) in the sampled air, precipitation and aerosols (natural vs anthropogenic); 2) quantifying the evasion of Hg species (elemental (Hg(0)) and dimethylmercury (DMeHg)) and the distribution of Hg and MeHg in the atmosphere and in wet deposition, which will help assess the variation in deposition during the cruise (GP15 in figure), and constrain the estimation of the net input of Hg to the surface ocean; and 3) quantifying the air-sea exchange of methylated Hg (both MeHg and DMeHg) which will lead to a better understanding of the importance of deposition as a source of mixed layer MeHg. The research is designed to test the following hypotheses concerning the air-sea exchange of Hg, and the sources and sinks for ocean Hg: 1) atmospheric Hg deposition is the primary factor controlling the evasional flux of Hg(0) to the atmosphere in the North and tropical Pacific Ocean; 2) evasion of DMeHg is a small component of the gas exchange flux but its degradation in the atmospheric boundary layer is an important source of atmospheric MeHg; 3) scavenging of gas and aerosols is the main source for the MeHg found in wet deposition; and 4) halogen chemistry plays a substantial role in Hg cycling within the marine atmospheric boundary layer and elevated ionic Hg concentrations will be found in regions with low ozone. Samples were collected during the 2018 GEOTRACES cruise in the atmosphere continuously using a Tekran speciation unit as well as with ion exchange membranes at lower resolution for reactive gaseous Hg (RGHg) and particulate Hg, and aerosols were also collected using hi-volume samples, along with wet deposition. Aerosol and deposition samples have been analyzed for total Hg and MeHg. Continuous dissolved gaseous Hg was sampled underway, and also surface samples collected to determine methylated Hg. Water column profile samples of total MeHg were also collected. The measurements made relied on well-developed, routinely used techniques but also built on new approaches to examine important questions concerning the cycling of methylated Hg across the ocean-air interface. Method development prior to the cruise ensured that the collections at sea, both underway measurements and samples collected for later analysis at the University of Connecticut, will yield significant and detailed results. Sample analysis is currently underway and the continuous measurements are still being processed, although most atmospheric samples have been analyzed. Preliminary results for dissolved gaseous Hg are shown in the figure at left. More detailed analysis of these data are underway. Presentations of the results will be made at the ICMGP in Krakow, Poland in Sept 2019, and at the Chemical Oceanography Gordon Research Conference in July 2019. The results from the research will provide data required by the GEOTRACES program and enhance our understanding of the cycling of Hg and methylated Hg species at the sea-air interface. This information is vital to understanding the long-term consequences of anthropogenic Hg releases on MeHg levels in seafood, and associated impacts on human and wildlife health, and also inform the Minamata Convention. A PhD student, and undergraduate students, are being educated and gaining research experience during the project, which will be a focus of the graduate student’s thesis.

Methylmercury Production and Fate in Response to Multiple Environmental Factors

This project, a sub-project of a multi-investigator project with Dartmouth College “Sources and protracted effects of early life exposure to arsenic and mercury”, funded through the NIH Superfund Program (4/2014-2019, with more funding pending) is using experimental approaches, field studies, and modeling to investigate the combined and interactive effects of environmental changes in temperature, salinity, and organic carbon concentration (OC) on the fate of MeHg in marine ecosystems. The collaborative nature of the project is that most of the geochemical analyses and related studies are completed at UConn while the focus of the biological analysis and bioaccumulation studies are done at Dartmouth, within Celia Chen’s research group. Plankton bioaccumulation studies are being completed in Nick Fisher’s lab at Stony Brook as another aspect. For more details on the overall project click here.

More details and preliminary results are detailed below. The research, which builds on previous work completed under the Superfund program, is focused on examining the factors controlling MeHg production, in both the water column and sediments, and the net input from sediments to the water column, and how these influence bioaccumulation in coastal food webs, and ultimately impact human exposure to MeHg. Interactions between environmental factors are being evaluated by adapting various approaches to distinguish additive from non-additive effects. Experimentally parameterized, field validated Hg methylation and demethylation assays, bioaccumulation and exposure models are being developed to predict the impact of complex environmental alterations on MeHg fate in marine ecosystems. The major hypotheses of the project are: 1) MeHg production, and flux from sediments to the water column, will decrease due to the combined and interactive effects of increasing carbon loading to sediments, temperature, and salinity on MeHg production and mobility; 2) In estuarine field sites spanning a range of comparable OC, salinities, and Hg inputs, Hg methylation rates and %MeHg in sediments from southern latitudes will be higher than those from more northern latitudes; 3) Bioaccumulation of MeHg in phytoplankton will decrease due to combined and interactive effects of increasing dissolved OC, temperature and salinity on MeHg bioavailability, phytoplankton growth and metabolism; 4) The trophic transfer of MeHg from primary producers through primary to secondary consumers will increase due to combined and interactive effects of the major variables – the cumulative impact of the changes expected due to increasing OC, temperature, and salinity will decrease MeHg production but increase bioaccumulation in the food web; and 5) The cumulative impact of the changes expected due to increasing OC, temperature, and salinity will result in a decrease in human exposure to MeHg through coastal seafood consumption.

Studies to date include a detailed mesocosm study, recently published (Buckman et al., 2019; see publications) which examined the interactions between sediment organic content and temperature on the bioaccumulation of MeHg into amphipods, oysters and plankton, and examination of the differences in the extent of MeHg production and flux from the sediment to the water column. Field studies within estuaries along the east coast of the US (Delaware to Maine) in 2015 and 2016 have been aimed at better understanding the links between sediment and water column characteristics and sediment MeHg production, and flux (both particulate and dissolved), and how the complexation of Hg within the dissolved and particulate phase impacts its methylation. Results from this work is published (Taylor et al., 2019; Seelen et al., 2018). These studies build on our previous work within the region (Balcom et al., 2015; Gosnell et al., 2016; Chen et al., 2014). Studies are also focused on better understanding the bioavailability and MeHg content of different water column particulate fractions as our previous work has shown that forage fish MeHg burdens correlate better with water column concentrations than with those of bulk surface sediment (Chen et al., 2014). We are investigating this link in greater detail in recent studies. Additionally, in 2016, studies focused on examining Superfund (contaminated sites) to examine how Hg and MeHg cycling may differ in these ecosystems compared to less impacted locations. A number of papers have been recently published, and presentations related to this work have been made at national and international meetings, with further papers at ICMGP in Krakow in Sept 2019..

One study (Seelen et al., 2018), looked at examining the composition of the resuspended material compared to that of the bulk surface seResuspension2diment (0-4 cm). Sediments collected from the Delaware River from high organic (HOC) and low organic matter (LOC) locations from two sites (SB and WB) were used for sediment resuspension experiments using Gust erosion devices. The collected resuspended sediment was analyzed for Hg, MeHg and ancillary information including C and N stable isotopes. Overall, the resuspended sediment did not closely match the bulk sediment (see figure. bulk meHg indicated by dotted lines) and better reflected the composition of the resuspended particulate. This study further suggests that bulk sediment characteristics are not reflective of what is entering the base of the food chain in estuaries. Another study, completed by Sofi Jonsson, a research scientist from Sweden, and Nash Mazrui, a PhD student, in 2015 examined how complexation of Hg to different dissolved ligands and particulate fractions impacted the rate of Hg methylation. Results suggest that Hg bound to DOC is more bioavailable that Hg bound to other ligands, and that Hg bound to iron sulfide is the least bioavailable. A paper related to this work has been submitted for publication. Results from one experiment are shown in the figure. The methylation rate was higher in the LOC sediments compared to the HOC sediments in all cases. Furthermore, the Hg pre-equilibrated to DOC (HgII-DOM in the figure) was higher than that of an inorganic Hg solution spike (HgII(aq)) and also when the aqueous spike and DOC were added simultaneously but not pre-equilibrated (HgII(aq)+DOM in the figure).

Examining the Role of Nanoparticles in the Formation and Degradation of Methylated Mercury in the Ocean

This project is a collaboration with Jing Zhao in the Chemistry Department began in summer 2016 and is funded through the Environmental Chemistry program at NSF. It is currently on a no cost extension. Ongoing research is examining the role of cadmium selenide (CdSe) and other nanoparticles (NPs) in the transformations of methylmercury (MeHg). Published results (Wang et al., 2019) examined the interactions of inorganic Hg and MeHg with various types of NPs, using various techniques. See here. The overall project is focused on understanding how environmental surfaces, and in particular, nanoparticles influence the formation and degradation of methylated Hg species (MeHg and dimethylmercury (DMeHg)). The idea is built on our initial studies with iron sulfide minerals (FeS) and thiols which have shown that these can mediate the conversion of MeHg into DMeHg through a methyl transfer reaction (Jonsson et al., 2016). Formation and/or precipitation of HgS is the other product of this reaction.


The overall motivation for these studies is that while DMeHg is found throughout the ocean water column there is little understanding of its formation. Additionally, while MeHg formation appears to be microbially mediated, it is not known whether DMeHg is formed by biotic or abiotic processes. We hypothesized that nanoparticles (NPs) would mediate reactions in the marine environment that are important pathways for the abiotic formation of DMeHg and the decomposition of MeHg. We have examined the interactions of inorganic Hg and MeHg with cadmium sulfide (CdS) and selenide (CdSe) and found an interaction which is manifest in a decrease in the fluorescence of the NP solutions – see figure. Further study of the interactions using XPS and ICP-MS has shown that the interaction is not between the added Hg and the NP core, and there is no replacement of Cd with Hg, but that the interaction is only at the NP surface. These findings have important implications for understanding the role of natural and manufactured NPs in the environment on the bioavailability and transformation of MeHg. Given the interactions found, it is less likely that formation of DMeHg will occur in the presence of NPs, but this is being further studied. Current results suggest that at lower pH, the surface of the NPs becomes more active, perhaps due to the dissociation of the capping agent and we have evidence that a reaction occurs, leading to a decrease in the MeHg concentration. We have not yet ascertianed the exact nature of the product, which is volatile, as we do not obtain a mass balance in solution after acidifying. We propose to further examine this as a potential abiotic formation of DMeHg. Further studies will focus on using natural and manufactured nanoparticles (FeS, CdS, CdSe and HgS), coated with different organic compounds, and with DOC, and how the form and surface characteristics may impact the rate of reaction, and what type of thiols may be important in these reactions under environmentally-relevant conditions. We also examine other potential abiotic degradation pathways for MeHg that do not lead to DMeHg formation.


Collaborative Research: Transformations and Mercury Isotopic Fractionation of Methylmercury by Marine Phytoplankton

This project, which is funded by NSF Chemical Oceanography is a collaboration with Nick Fisher at Stony Brook more and John Reinfelder at Rutgers more began in fall 2016. The major goals of the collaborative research project are focused on addressing the following hypotheses: 1) Reduction of ionic Hg (HgII) within algal cells and algal formation of dimethylmercury (Me2Hg) and/or elemental Hg (Hg0) from methylmercury (MeHg) are important pathways for the formation of volatile Hg species in the surface ocean, and coccolithophores and possibly other haptophytes account for production of most of the volatile Hg; 2) Production of Me2Hg occurs through reactions of MeHg with thiols and Se-proteins within cells, and through its abiotic interaction with particulate or nanoparticulate sulfides within the water column; and 3) Transformations of Hg species within phytoplankton impart distinct Hg isotopic signatures on residual Hg in phytoplankton that are unique for HgII and MeHg and different from those of abiotic photochemical reactions. To test these hypotheses, our groups are examining the transformations and isotopic fractionation of Hg by marine phytoplankton in laboratory and field experiments. The studies at UConn are focused mostly on abiotic transformations, although field studies examining methylation in coastal waters are also being completed.

Additionally, studies are focused on understanding more clearly the factors influencing the uptake of MeHg into phytoplankton. Preliminary results are shown below for a study that looked at the uptake of MeHg in the presence of two marine DOC samples extracted from coastal waters (Orrington and Sawyer Island), and compared the rates to published data in the literature. As shown in the figure on the left, the recent study results are consistent with Luengen et al. (2012) that the uptake rate decreases with DOC concentration, but with substantial scatter in the data. Also, uptake in the presence of cysteine was much lower than with DOC in the previous study. When the recent data and the cysteine data are plotted against the total reduced S content of the DOC (right figure), the relationships is more consistent, reinforcing the expectation that it is not the total DOC content but rather the reduced S content that determines the extent of binding of MeHg to the DOC, and therefore the impact on bioaccumulation. More studies are underway to examine how the character of the DOC influences uptake and transformations of MeHg.

The overall studies will examine further the factors controlling uptake and the the extent of biologically mediated and abiotic production of volatile Hg species, and the transformation of MeHg into other Hg forms. While prior studies of the Mason group have focused on the photochemical formation of Hg0 and the degradation of MeHg, the current studies are focused on field measurements of transformations in the presence of microbes.