Biology Theses and Dissertations
Permanent URI for this collectionhttp://hdl.handle.net/1903/2749
Browse
4 results
Search Results
Item INTERACTIONS BETWEEN NATURAL ORGANIC MATTER COMPOSITION AND MERCURY TRANSPORT IN A BOREAL WATERSHED(2010) Ghorpade, Sarah; Heyes, Andrew; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Natural organic matter (NOM) composition affecting mercury (Hg) transport along a watershed transect were examined in the Lake 658 system at the Experimental Lakes Area, Canada. This watershed was dosed with an enriched stable isotope of Hg, allowing for distinction between recently deposited and historic Hg. Newly deposited Hg was not detected in significant quantities in upland flow or the lake, and occurred only in upper soil horizons, indicating that Hg has not reached steady state 8 years following deposition. Characterization of dissolved phase NOM was conducted by molecular weight fractionation, and analysis of absorbance properties and lignin phenols. Low molecular weight compounds were more mobile in the upland, while high molecular weight fractions contained more Hg. Spectral properties were not consistent predictors of Hg, but supported findings on molecular weight distribution. Source material composition, as indicated by lignin phenols, did not vary widely and was not correlated with Hg.Item Mercury methylation in dissimilatory iron reducing bacteria(2007-08-13) Kerin, Elizabeth; Suzuki, Marcelino; Gilmour, Cynthia; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Microbial mercury methylation is an integral factor controlling methylmercury concentrations within aquatic ecosystems. This thesis explores the phylogenetic distribution and biochemistry of methylation among the dissimilatory iron-reducing bacteria (DIRB). When distribution of methylation capacity among DIRB was examined, strains of Geobacter spp., which are closely related to mercury-methylating, sulfate-reducing Deltaproteobacteria, methylated mercury while reducing iron or other substrates. In contrast, no tested strains of the Gammaproteobacteria genus Shewanella produced methylmercury above abiotic controls. Mercury methylation by the cobalamin-dependent methionine synthase (MetH) pathway was examined. Heterologous expression of G. sulfurreducens metH in E. coli was used to evaluate involvement of MetH in methylation. Methylation by a clone expressing MetH and a non-expressing control clone was tested in vivo and in vitro. Methylation by the expressing clone was not significantly higher than either the control or abiotic assays in either experiment, suggesting that MetH is not involved in methylation in G. sulfurreducens.Item THE ROLE OF ORGANIC MATTER IN THE DISSOLVED PHASE SPECIATION AND SOLID PHASE PARTITIONING OF MERCURY(2006-01-24) Miller, Carrie Lynn; Mason, Robert; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The interaction of mercury (Hg) and methylmercury (MeHg) with organic matter is extremely important in the dissolved phase speciation and solid phase partitioning of Hg and MeHg in aquatic systems. This study shows, that under oxic conditions Hg and MeHg will likely associated with Fe oxides through an indirect association with organic matter, while under sulfidic conditions, solid phase Fe sulfide will dominate the complexation of Hg to the solid phase. As a result of the association of Hg with Fe solids, which undergo dynamic changes at redox interfaces in aquatic systems, the distribution of Hg on particles is likely changing at redox boundries, areas that have been shown as active zones of methylation. Redox zones are also going to be important in controlling the mobility of MeHg from the site of production to areas in aquatic systems in which uptake by biota occurs. Although the dissolved phase speciation of Hg has been shown as an important factor in Hg methylation, as a result of the diffusive uptake of neutral Hg-sulfide into bacterial cells, this speciation had previously not been measured. Hg forms stronger bonds with reduced sulfide relative to dissolved organic matter (DOM), therefore, it was not previously thought that DOM was important in the speciation of Hg under sulfidic conditions. Using modified octanol-water partitioning extractions and centrifugal ultrafiltration, the speciation of Hg in sulfidic natural samples and laboratory solutions was examined. It was shown that the concentration of neutral Hg-sulfide complexes are lower than predicted by thermodynamic models, as a result of an interaction of these species with DOM. It is proposed that the interaction of Hg with DOM is not a complexation, but rather, a partitioning of neutral Hg-sulfide complexes into hydrophobic portion of the DOM. Thermodynamic constants were calculated for this interaction and applied to model the speciation of Hg in natural samples. The concentration of neutral Hg-sulfide is lower than models previously predicted, as a result of the DOM interaction. Since the concentration of neutral Hg-sulfide affects methylation, DOM could impact the rate of Hg methylation in aquatic systems.Item THE INVESTIGATION OF MERCURY REDOX CHEMISTRY IN NATURAL WATERS AND THE DEVELOPMENT OF A NEW METHOD FOR INCUBATION EXPERIMENTS(2005-04-26) Whalin, Lindsay; Mason, Robert P; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The redox processes that control Hg speciation in natural waters are poorly understood and study results often disagree, primarily a consequence of varied and often flawed methodologies. An incubation method was developed utilizing PFA Teflon® bag reaction vessels to reduce sources of error, and additions of isotopically labeled Hg to quantify rate constants. With low measures of error and duplicate bag reproducibility, this method was applied via incubations of natural waters in ambient sunlight to test three theories; 1) Hg oxidation and reduction are photochemically mediated, 2) Hg reduction is correlated to [DOC], and 3) Hg oxidation is enhanced by halides. The former was proven through the detection of redox chemistry during daylight and its absence in the dark. Results indicate the importance of both redox processes in natural waters, but failed to prove the latter two theories, potentially due to greater than expected [DOC] in one experiment.