Transformations and Bioavailability of Mercury in Aquatic Ecosystems

<p>NON-TECHNICAL SUMMARY: <br/>Mercury isa potentially serious public health concern due to its accumulation in aquatic and terrestrial food chains. The consumption of marine and freshwater fish containing elevated concentrations of mercury by women of child-bearing age has been linked to adverse health outcomes for their children (Oken et al., 2005; Grandjean and Perez, 2008). The goal of this project is to examine the biological and abiotic mechanisms that lead to the mobilization, transformation, and bioaccumulation of mercury in subsurface, estuarine, and marine environments. Understanding the fate of mercury in some of the most densely populated states in the U.S. will link process studies focused on biological cycling, speciation, and bioaccumulation to environmental management of the nation's aquatic natural resources. This project addresses the USDA National
Institutes for Food and Agriculture national research priority area of Food Safety in that it will improve the knowledge base needed to "reduce the incidence of food-borne illness and provide a safer food supply." It also supports the mission of the NJAES in that it will contribute to the development of effective management strategies related to mercury in New Jersey's coastal environment thereby protecting natural resources, fisheries, and public health.
<p>APPROACH:<br/> Bioavailability of MeHg at the base of marine food webs The bioavailability of MeHg at the base of aquatic food webs will be examined in bacteria and phytoplankton in laboratory experiments. Laboratory studies will use monospecific bacteria and marine phytoplankton cultures and the relative transport kinetics of MeHg bound to inorganic and organic ligands, especially thiols. Bacteria accumulation will be examined using strains transformed with plasmids containing genes for oranomercurial lyase, which converts MeHg to Hg(II), and mercuric reductase, which reduces Hg(II) to Hg(0). The removal of purgeable Hg(0) during uptake experiments will be used to quantify MeHg transport. For phytoplankton bioavailability studies, cells will be subjected to a thiol wash procedure to remove cell-surface bound MeHg allowing the quantification of intracellular MeHg. Since
MeHg accumulation in freshwater phytoplankton has been shown to vary with nutrient limitation, we will examine the effects of nitrogen limitation on MeHg uptake kinetics and steady-state accumulation in cultured species. Hg isotopic fractionation during the photochemical reduction of Hg(II) and MeHg in freshwater and marine aquatic systems The isotopic fractionation of Hg stable isotopes will be examined in photochemical incubations of Hg(II) and MeHg in freshwater and seawater under a variety of environmental conditions. Freshwater incubations will be conducted at acidic and neutral pH, under oxic and anoxic conditions. For both freshwater and seawater incubations, various organic ligands will be included to assess the role of Hg speciation on photochemical reduction and isotopic fractionation and the wavelengths and intensities of UV and visible light will be varied. Since
preliminary results show large MIF associated with photochemical reduction, we will explore Hg isotopic fractionation during photochemical Hg(II) reduction and MeHg degradation in much greater detail. We will also examine the photochemical reduction and associated isotopic fractionation of Hg and MeHg bound to intracellular components of living phytoplankton cells and their exudates. Hg isotopic fractionation associated with the microbial methylation of inorganic Hg We have developed a system to collect sufficient quantities of MeHg from environmental and laboratory samples for Hg isotopic analysis. An aqueous phase ethylation procedure for MeHg analysis was adapted to separately trap MeHg and Hg(II) (Fig. 1). The apparatus was scaled up to accommodate higher masses of MeHg with a high capacity GC column. An offline trapping set-up is employed using gold traps and a thermally
controlled desorption into permanganate. The system is in the final phase of optimization and currently separates and traps MeHg with recoveries of 80% to 97%. Hg isotopic fractionation during Hg(II) methylation will be determined using pure cultures of sulfate and iron-reducing bacteria and methanogenic archaea will be determined. Pure cultures including D. desulfuricans ND132, an incomplete oxidizer (Jay et al., 2002), Desulfococcus multivorans, a complete oxidizer (Ekstrom et al., 2003), and G. sulfurreducens PCA (Kerin et al., 2006), with documented Hg methylation activities will be employed. The methanogen, Methanospirillum hungatei, which was recently shown to methylate Hg (Yu et al. in revision), will also be examined. This choice of strains will cover the known metabolic diversity among bacteria that methylate Hg. We propose to focus on strain PCA as conditions that facilitate
high yields of MeHg production in resting cell assays have been established (Schaefer and Morel, 2009). Resting cell methylation assays may avoid complications from the effect of increasing cellular biomass on Hg(II) bioavailability and isotopic fractionation as we have seen in our reduction experiments (Kritee et al., 2008). High methylation yields, whereby 10% to 80% of the added 5 nM Hg(II) is methylated in four hours are critical to obtaining enough MeHg for precise Hg isotope ratio determination by MC-ICP-MS. In addition, a crude cell extract methylation assay is available (Schaefer and Morel, 2009) for strain PCA and could be instrumental in experiments that will determine how transport through the cell envelope impacts fractionation. Strain PCA reduces Hg(II) to Hg(0) in a process that depends on the availability of both electron donor and acceptor (Wiatrowski et al., 2006).
Because of these requirements it is unlikely that the production of Hg(0) will compete with methylation in the employed resting cell assays. Efforts will be made to translate the results of this project for use by other scientists and environmental or public health professionals through the publication of papers and the presentation of results at meetings and workshops. In addition, results will be communicated to students in the form of coursework modules. The success of these efforts will be assessed through the evaluation of the impact of published work (citations) and presentations (formal and informal feedback) and through student evalutions of course effectiveness.