![\"Seelen](\"https:\/\/today.uconn.edu\/wp-content\/uploads\/2023\/11\/IMG_6040-300x225.jpg\")
UConn Department of Marine Sciences<\/a> researcher and Associate Research Professor Zofia Baumann<\/a> explains that the project was started by Emily Seelen Ph.D. \u201918, now a post-doctoral researcher at the University of Southern California, when she was a graduate researcher in Professor Robert Mason\u2019s<\/a> lab. Seelen worked on one of Mason\u2019s funded projects and also received an NSF graduate fellowship that also enabled a study abroad collaboration with co-authoring chemists Erik Bj\u00f6rn<\/a>, Ulf Skyllberg<\/a>, and Van Liem-Nguyen from Ume\u00e5 University in Sweden, well-known for their research on mercury, who had the facilities and expertise needed for the detailed mercury and sulfur chemical analysis of samples collected in the coastal zone of the northeastern United States. Urban W\u00fcnsch from the Technical University of Denmark provided expertise in organic matter characterization.<\/p>\n The researchers analyzed the chemical characteristics and sulfur forms in NOM samples and performed experiments to examine the factors influencing the degree of association of methylmercury with the NOM, and the chemical factors that control the extent of the association. They also probed the mechanisms controlling the bioavailability of methylmercury in tiny organisms at the base of the food web, called phytoplankton. The experiments analyzing extracted dissolved organic matter and examining the methylmercury interactions were done in Bj\u00f6rn\u2019s lab, and experiments with phytoplankton uptake were performed at UConn, Baumann explains. The samples for this study were collected in conjunction with other funded research through the NIH Superfund Program which partnered Mason\u2019s research group with Research Professor Celia Chen and other investigators at Dartmouth College, and the analysis of samples from this study provided further support for this work.<\/p>\n \u201cThe uptake of methylmercury from seawater into phytoplankton is the largest concentration step, because the phytoplankton act like sponges; they are so efficient,\u201d says Baumann. \u201cWe are seeing that phytoplankton can concentrate methylmercury up to a million times, possibly more under certain conditions, compared to seawater, and every other transition up the food chain, say from phytoplankton to zooplankton, it’s never this large, more like a factor of ten.\u201d<\/p>\n Baumann explains that her role was to instruct Seelen on how to perform the phytoplankton uptake experiments, drawing from her previous work tracing how contaminants, like heavy metals, move through the environment.<\/p>\n Water samples were collected from 20 different sites along the Eastern Coast of the United States, including estuary and open ocean sample locations. Besides measuring dissolved and particulate mercury and methylmercury in the water, characteristics such as salinity, dissolved organic carbon (DOC), and DOM were measured. The researchers suspected that methylmercury has a high affinity for sulfur, so DOM was chemically characterized in further detail to understand sulfur compounds called thiols. The chemists then performed thermodynamic and kinetic reaction experiments to understand how strongly the thiols impacted mercury\u2019s bioavailability to phytoplankton.<\/p>\n Seelen explains, \u201cA major finding was that the binding affinity of different NOM was similar across sites, and the ability of the NOM to bind methylmercury was more determined by the number of thiol groups per gram of NOM, which varied between river water and the coastal waters, where it was lowest. It was highest in salt marsh environments.\u201d<\/p>\n Also, in terms of the uptake into phytoplankton, the kinetic interactions between methylmercury and NOM were rapid enough that this did not impact the methylmercury assimilation, and thus the thiol content was the major factor driving its impact on bioaccumulation.<\/p>\n DOM is abundant in the ocean and binds mercury very effectively, acting like a shield that prevents uptake into phytoplankton. The study showed that the offshore waters had much lower thiol content in NOM, so relatively more methylmercury is taken up by the plankton.<\/p>\n \u201cWe found a strong relationship between location and thiol content and were able to demonstrate that it\u2019s very likely that the thiol contents in NOM are the key parameter controlling the availability of methylmercury to phytoplankton, rather than the NOM or DOC concentration,\u201d says Mason.<\/p>\n The researchers focused on estuaries and the river-estuary-coastal ocean continuum because these are areas rich in nutrients and biomass and therefore support an abundance of biological productivity, and as a result are regions where there is abundant seafood and potential for human exposure.<\/p>\n \u201cThis is why it\u2019s essential to understand how mercury is cycling across these different environments,\u201d says Bj\u00f6rn. \u201cWe concluded that terrestrial organic matter derived from plant matter in marshes is tightly controlling methylmercury bioavailability, versus in the coastal ocean, where the NOM is derived from phytoplankton and local sources, resulting in the methylmercury being more bioavailable.\u201d<\/p>\n New England is home to many estuaries historically impacted by mercury pollution<\/a> and all have elevated concentrations of mercury, that are largely inorganic and not bioavailable. The same is true of Sweden and many locations around the world. Even though a small fraction of the inorganic mercury becomes bioavailable, and converted to methylmercury, the concentration is still much higher than in the open ocean. Therefore, it’s consequential to understand what’s going on in these estuaries because there is a large pool of mercury that can eventually enter the food web, and many humans rely on coastal seafood as a source of protein, and therefore are potentially exposed to elevated levels of methylmercury.<\/p>\n Not much can be done about the legacy mercury pollution, but future emissions can be decreased, and this is a focus of an international treaty, the Minamata Convention on Mercury<\/a>. The convention came into force in 2017 after being ratified by 50 countries and there are now 147 parties to the convention, who have committed to reduce mercury use and emissions, and protect human health.<\/p>\n![\"Seelen](\"https:\/\/today.uconn.edu\/wp-content\/uploads\/2023\/11\/Seelen_picture1-1-300x225.jpg\")