Research Projects

The Effect of Terrestrial Organic Matter on Coastal Mercury Cycling (NSF Chemical Oceanography)

The project is collaborative research with colleagues at Dartmouth College (Vivien Taylor and Celia Chen). The study was designed to examine the effects of climate-driven increases in terrestrial organic matter (OM) concentrations in the Gulf of Maine (GoM) waters on the complex and often competing processes influencing: 1) mercury (Hg) cycling and distribution; 2) the formation of methylated Hg (both monomethyl (MeHg) and dimethylmercury (DMeHg)); and 3) MeHg uptake into microplankton. Our central hypothesis is that higher terrestrial OM inputs, and likely hightened co-transport of Hg and MeHg, and concomitant shifts in coastal OM amount and quality will cause increases in Hg inputs, MeHg production and influence its bioaccumulation. Terrestrial OM is thought to be a conduit for transfer of Hg from watersheds to coastal and offshore waters and controls the formation of aqueous MeHg by providing a microenvironment that supports microbial Hg methylation. Increasing dissolved OM concentrations have been associated with decreased bioaccumulation of MeHg into phytoplankton, the primary bioaccumulation step for uptake of MeHg into seafood. However, our preliminary results show this association is mediated by OM source more than concentration, as indicated by the sulfur content of the OM. Furthermore, mixotrophic microplankton could bioaccumulate MeHg complexed with OM while feeding heterotrophically. Mixotrophs can seasonally dominate microplankton assemblages in coastal waters, and the impact of heterotrophic feeding modes on MeHg uptake at the base of the food web has not been adequately explored. The proposed research aimed at studying the effects of OM dynamics on Hg and MeHg cycling and bioaccumulation through 1) field surveys and shipboard experiments in the GoM, where significant increases in terrestrially-sourced OM have caused a “yellowing” effect, and 2) through laboratory microcosm experiments using autotrophic and mixotrophic microplankton taxa under contrasting carbon acquisition modes and OM characteristics and concentrations. The field program is built around 5 proposed cruises in the GoM and the laboratory studies will include both uniculture uptake experiments and mesocosm studies.

The first cruise was completed in April 2023 – after being delayed from Nov 2022 – and Rob, Hannah, Sophia and Melissa were on board. We collected water samples throughout the water column at 15 stations in the GoM and in the Penobscot River, which is historically contaminated with Hg from industrial sources. Dissolved and particulate samples were collected for total Hg and MeHg, and on-board analysis was completed for dissolved gaseous Hg species, both Hg0 and DMeHg. Additionally, large particulate samples were collected using McLane pumps and plankton nets for the analysis of Hg natural isotope abundance. an underway Hg0 sampler also collected samples continuously during the cruise. Size fractionated seston samples were also collected. Colleagues from Bigelow Laboratory were also measuring OM characteristics and other ancillary parameters on the cruise.

Prior to the cruise, phytoplankton uptake experiments with a mixotrophic dinoflagellate were done examining the pathways of MeHg accumulation. Further experiments are planned.

US GEOTRACES GP-17- OCE and -ANT Sections: External Sources, Cycling and Processes Affecting Mercury Speciation in the South Pacific and Southern Oceans (NSF Chemical Oceanography)

The major goals of the project were to participate in two oceanographic cruises that form part of the GEOTRACES GP17 activities (GP17-OCE in 2022/23 and GP17-ANT in 2023/24), and do ancillary sampling and analysis at the institutions covered by this collaborative award to ensure the success of the project. The overall goals of the collaborative study were based on the priorities of the GEOTRACES program and associated activities, and the gaps in knowledge concerning mercury (Hg) ocean biogeochemistry. To achieve the aims of the research, measurements are being made of all the Hg species: total Hg (HgT), elemental Hg (Hg0), methylmercury (MMHg) and dimethylmercury (DMHg), in the dissolved and particulate phases, as appropriate, and in the atmosphere. The UConn and USGS studies focused on collecting samples for examining the air-sea exchange of Hg and methylated Hg, and the isotopic composition of these fractions. The other collaborators are UC-Santa Cruz and Wright State University. The collaborative group proposed a program with the major objectives being focused by the following hypotheses: 1) Atmospheric deposition is the primary external Hg source and the major factor controlling the evasion rate of Hg0 to the atmosphere in the South Pacific and Southern Oceans; 2) While continental, deeper ocean margins and hydrothermal inputs are not important Hg sources, these inputs dominate external fluxes of MMHg; 3) MMHg formed in ice and at the sediment-water interface is an important source to the Southern Ocean; 4) The fraction of total Hg as methylated Hg is related to primary production and associated deeper water remineralization such that the South Pacific gyre will have some of the lowest relative concentrations in the global ocean; 5) Localized external inputs of Hg, such as glacial melt, are not exceptionally high (unlike in Greenland), but the flux associated with increased freshwater inputs is observable. Likewise, fluxes of Hg into the ocean from sediment porewaters will be low, but locally important on the Antarctic shelf; and 6) Dissolved total Hg isotopic composition in surface water will elucidate the relative contributions of dry and wet deposition, as well as glacial ice melt, across a gradient of climatic conditions from the Southern Pacific to the Southern Ocean. Hypotheses 1-4 are relevant to both cruises while hypotheses 5 & 6 relarted primarily to GP17-ANT.

The first cruise (GP17-OCE) has been completed at the end of the current funded year and preliminary results and data suggest that the cruise was a success in terms of achieving the major objectives. Preliminary underway data is shown above. preparations for the second cruise are already beginning.

Methylated Mercury Sources and Cycling in the High Latitude North Atlantic (NSF Chemical Oceanography)

This project was designed to addresses key questions regarding the benthic mercury (Hg) contribution in the ocean and its potential impact on our understanding of the biogeochemical cycling of Hg species, both inorganic and organic Hg, their inputs to the deep ocean, and the sources and cycling of methylated Hg (TMeHg) in the subpolar North Atlantic. The research was focused by a GEOTRACES Process Cruise around Iceland on which we participated in summer 2021. Our research was formulated around the hypotheses that: 1) Benthic sediments are a net source of TMeHg to the deep water in this region and will dominate the TMeHg supply into the North Atlantic; 2) While sediment may release inorganic Hg due to various biochemical and physical processes, sediments will be a net sink for THg; and 3) Overall, the seas around Iceland are a net sink for total Hg but a source for TMeHg to the deep North Atlantic Ocean, with the sources being primarily from shelf- and sediment-derived TMeHg inputs. The hypotheses wee trsted by making radioisotopic and Hg measurements on the cruise and analysis of this data in conjunction with the known hydrography and water flow patterns in this region of the ocean.

The GEOTRACES expedition (64PE474, July-August 2021) allowed for various natural radiotracer approaches to be used to quantify the various exchange fluxes, and while the post-doc on the cruise was involved in these on-board activities, she was also be able to collect samples of dissolved and particulate THg and TMeHg, and also process samples for sediment and porewater Hg species. The radiotracer methods used allowed for better estimation of benthic Hg fluxes than has been done previously. Analysis of data to date show that the benthic fluxes of dissolved MeHg and total Hg can be quantified with the 224Ra/228Th disequilibrium approach. The rate of particulate Hg redeposition was also traced with the excess 234Th of the top sediment layer. Results indicate that the sediment on the sides of Greenland-Iceland Rise was a main source of Hg species to this region. The strong underwater current of the Denmark Strait likely carries these Hg inputs southwards into the North Atlantic. For the sampling stations on the top of the rise, excess 224Ra and excess 234Th relative to their parents were observed in the sediment column, suggesting the redeposition of the resuspended particles. The concentrations of MeHg and total Hg in the bottom water at stations at the top if the rise were lower than those at the rise side stations, implying that the redeposited particles scavenged considerable Hg from water column. Sediments on the rise is likely a sink for Hg transported from the Arctic but a source for MeHg. The initial results will be presented at the international mercury meeting in July 2022.

Final analysis of samples is ongoing, as is data synthesis, quality assurance and data analysis.

Constraining the Role of Chemical Transformations in the Cycling of Mercury at the Arctic Ocean Air-Sea Interface (NSF Polar Programs)

The project is focused on examining the air-sea exchange of mercury (Hg) forms, both inorganic (ionic (HgII) and elemental (Hg0) and organic forms (monomethyl (MeHg) and dimethyl Hg (DMeHg)), in the Arctic Ocean and the role of sea ice in mediating the gas exchange of Hg0 and methylated Hg, and how ice cover influences the extent of the chemical reactions in surface waters and Hg gas exchange. Additionally, given the region of the study, the interaction between the sediment and water column, and the distribution of Hg species throughgout the water column were also examined. The study has provided information on how climate change and other factors are affecting the levels of MeHg in the water, and thus the marine food chain, and how this could impact human exposure now and in the future. To enhance understanding, radon (Rn), an inert gas, was also be measured by our collaborators to allow for the separation of the role of gas exchange versus the role of chemical reactions for the Hg forms. Other radioisotopes were also measured to enhance understanding of other aspects of the Hg cycle. The hypotheses driving this research were primarily examined during a cruise in the Arctic Ocean using field measurements, which occurred in May/June 2021, and through on-going laboratory experiments during the cruise, and experiments at the University of Connecticut (UConn). The hypotheses are: 1) Gas evasion of elemental Hg0 is not the primary sink for water column Hg in the Arctic Ocean; 2) The relatively low net production of Hg0 in the Arctic reflects lower UV light levels and ice cover, low productivity and low bioavailable HgII for reduction; and 3) Gas evasion of DMeHg is not the primary sink for methylated Hg and in situ decomposition is the major loss mechanism in Arctic seawater.

To examine these hypotheses in detail, the following objectives have been completed: 1) During a late spring cruise in the Arctic Ocean, along a transect from open water, through the marginal ice zone and into completely covered ice regions, high resolution measurements of dissolved Hg0 and atmospheric Hg speciation (Hg0, reactive gaseous Hg (RGHg) and aerosol Hg (PHg)), as well as the concentrations of Hg species in the water column and sediments, and in sea ice were made. Determination of surface water DMeHg also occurred; 2) Samples were collected to assess the Rn deficit with respect to its parent, radium, in surface waters for the estimation of gas exchange rates and how this deficit changes with ice cover; 3) Underway ancillary data on water temperature, salinity, fluorescence, air temperature, UV, and PAR, wind speed and direction, were collected as needed for flux estimation and data interpretation, as well as ancillary samples for the measurement of Hg speciation (Hg0 , MeHg, DMeHg) and related parameters in surface waters, surface snow and sea ice; 4) Atmospheric precipitation and aerosol samples were collected during the cruise; 5) On-board incubation experiments, and further pre- and post-cruise studies, have been completed to examine the photochemical transformations, and the microbially-mediated (dark) redox processes influencing inorganic Hg and methylated Hg speciation, and the controlling factors; and 6) this information is being used to produce a revised budget for inorganic Hg and methylated Hg cycling within the mixed layer of the Arctic Ocean using our data and results from the literature.

This project is currently on a no-cost extension and will end at the end of 2023. The following publications are related to this project.

He, Yipeng and Shi, Xiangming and Huffman, Wesley W. and Lamborg, Carl H. and Mason, Robert P.. (2022). Description of a Dimethylmercury Automatic Analyzer for the High-Resolution Measurement of Dissolved Gaseous Mercury Species in Surface Ocean Waters.  Environmental Science & Technology. 56  (18) 13076 to 13084. 

Dastoor, Ashu and Angot, Hélène and Bieser, Johannes and Christensen, Jesper H. and Douglas, Thomas A. and Heimbürger-Boavida, Lars-Eric and Jiskra, Martin and Mason, Robert P. and McLagan, David S. and Obrist, Daniel and Outridge, Peter M. and Petrova, Mariia V. and Ryjkov, Andrei and St. Pierre, Kyra A. and Schartup, Amina T. and Soerensen, Anne L. and Toyota, Kenjiro and Travnikov, Oleg and Wilson, Simon J. and Zdanowicz, Christian. (2022). Arctic mercury cycling.  Nature Reviews Earth & Environment. 3  (4) 270 to 286. 

Marissa C. Despins, Robert P. Mason, Ana M. Aguilar-Islas, Chad R. Hammerschmidt, Silvia E. Newell. Linked mercury methylation and nitrification across oxic sub-polar regions. Submitted to Frontiers in Environmental Chemistry, in revision .

Nine presentations at meetings have been made at national and international conferences on the mercury studies. The project supported the thesis work of Yipeng He and Marissa Despins and will form part of the thesis research of Hannah Inman. The direct collaborators on the project were Dave Kadko (deceased), Mark Stephens (FIU) and Doug Hammond (USC). Other researchers on, or receiving data from the cruise, included Penny Vlahos and students (UConn), and Laurie Juranek (Oregon Univ.) and Bob Pickart (WHOI)

Role of Nanoparticles in Mercury Cycling (NSF Chemistry)

This project was a collaboration with Jing Zhao in the Chemistry Department began in summer 2016 and was funded through the Environmental Chemistry program at NSF. Research examined the role of cadmium selenide (CdSe) and other nanoparticles (NPs) in the transformations of methylmercury (MeHg). Published results  are Shi et al. (2021) which examined the transformations of inorganic and MeHg in the presence of cysteine-capped CdSe NPs, and built on from Wang et al., 2019, who 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.