A) Mercury related research

1. Transformation of Hg species in the co-culture of aerobic with anaerobic bacteria

The work includes obtaining data on metabolic reactions between typical anaerobic mercury methylators and aerobic strains of marine bacteria, which are combined in the suspended and aggregated form. The use of microbiological and chemical analytical methods will provide a picture of mechanisms of bio-uptake and transformation of mercury inside such complexes. The student will work interdisciplinary in the nexus between analytical chemistry and microbiology.

Mentor: dr. Aleš Lapanje (ales.lapanje@ijs.si)

2. Isotopic fractionation of mercury species in natural waters: development and validation of analytical methods 

Mercury (Hg) is a toxic metal that occurs naturally in the Earth's crust. In natural waters Hg exists in different forms such as inorganic mercury (Hg(II)), dissolved elemental mercury (DEM), mono methyl mercury (MeHg) and dimethyl mercury (DMeHg). In uncontaminated natural aquatic systems, Hg is present at trace levels ranging from a few pg/L to few ng/L. The chemical form of Hg in aquatic systems is strongly influenced by redox conditions, pH, and the concentrations of inorganic and organic complexing agents. Other factors such as temperature, solar radiation, chemical composition (salinity in particular), the presence of suspended (organic) matter and microorganisms, also play important roles. It is important to quantify how much Hg is retained within an ecosystem that can be further methylated and bioaccumulated. The determination and speciation of mercury is commonly performed using spectrometric techniques. Since the discovery of isotopic fractionation of mercury, mass dependent fractionation (MDF) and mass independent fractionation (MIF) of mercury isotopes are used as a tracer to understand the mercury cycle. MIF is a powerful tool in understanding the Hg transformations and reaction mechanisms. The master student will develop and validate an analytical approach for the pre-concentration and measurements of Hg isotopes in natural waters using separation techniques (ion-exchange chromatography approach). Determination of isotopic fractionation in MeHg, DEM and Hg(II) will provide a useful information about Hg transformations in the atmosphere and aquatic environments.

Mentors: dr. Ermira Begu (ermira.begu@ijs.si), dr. Igor Živković (igor.zivkovic@ijs.si)

3. Development and validation of analytical methods for the estimation of isotopic fractionation of Hg and MeHg in human samples

The main forms of mercury (Hg) exposure in the general population are methylmercury (MeHg) from seafood, inorganic mercury (Hg(II)) from food), and mercury vapor (Hg(0)) from dental amalgam fillings. Mercury is toxic in all forms. However, MeHg is considered as the most toxic form of Hg in the environment due to its ability to enter the food chain where it bioaccumulates and biomagnifies. MeHg is efficiently adsorbed from the gastro-intestinal tract, and it passes the blood-brain and placenta barriers. It primarily affects the central nervous system. The damage caused by MeHg is irreversible, which is not the case for inorganic Hg. Since the early 1960s, growing awareness of environmental mercury pollution (e.g., the Minamata tragedy resulting from MeHg poisoning) has stimulated the development of more accurate, precise and efficient methods of determining mercury and its compounds in a wide variety of human samples matrices. Mercury isotope signature is a useful tool for understanding Hg sources, and processes and transformations in human tissues. The master student will conduct study on real human samples, characterized by relatively low Hg concentrations. The master student will develop and validate a method for mass dependent fractionation (MDF) and mass independent fractionation (MIF) of mercury isotopes in human samples. This work will include Hg separation techniques based on ion-exchange chromatography to obtain valuable insights in biological processes related to Hg isotope fractionation in human body.

Mentors: dr. Ermira Begu (ermira.begu@ijs.si), dr. Igor Živković (igor.zivkovic@ijs.si)

4. Estimation of isotopic fractionation of Hg and MeHg in fish: development and validation of analytical methods 

Mercury (Hg) is considered a global pollutant due to its ability to undergo long-term transport in the atmosphere, its transformations between different species, and its ability to enter and accumulate in the food chain. The biogeochemical transformation, toxicity, bioavailability, mobility, and fate of Hg relies on its total concentration but also on its chemical forms. The consumption of fish is the primary source of methyl mercury (MeHg) exposure to humans. Therefore, an accurate determination of Hg and MeHg in fish is of great importance. However, even more important is the isotopic composition of the element in fish. Recent analytical advancements, especially in multicollector inductively coupled plasma mass spectrometry (MC-ICPMS), have allowed for stable isotopic studies of many elements, including Hg. By knowing the isotopic signature of the element, an interpretation of possible sources and transformations of Hg is possible. The main goal of the master thesis will be to develop and validate a method for isotopic fractionation in fish samples using the state-of-the-art method (MC-ICPMS). The goal is to determine isotopic fractionation in both MeHg and inorganic Hg to provide an explanation for the origin of inorganic Hg(II) and organic MeHg in fish. This study will contribute to expand our current knowledge about Hg pathways at regional and global levels.

Mentors: dr. Ermira Begu (ermira.begu@ijs.si), dr. Igor Živković (igor.zivkovic@ijs.si)

5. Influence of nutrient concentrations and biologically derived methylating agents on mercury methylation in the marine environment 

Mercury (Hg) is a global contaminant that can travel across the globe through the atmosphere, where it is readily oxidized to inorganic mercury (Hg(II)). Hg(II) that is deposited to the surface ocean and land can be reduced back to elemental mercury and re-mitted to the atmosphere. The other option is mercury methylation to the most toxic Hg species – methylmercury (MeHg). Even though there is evidence for mercury methylation in surface waters, exact mechanisms are still not well understood. Mercury in surface water can be methylated through the activity of heterotrophic bacteria during organic matter remineralization. This has been observed by the correlation between the concentrations of MeHg and nutrients in seawater. In addition, there are indications that low nitrate and phosphate concentrations can influence microbial metabolisms and possibly Hg methylation. Marine plankton is a known producer of various compounds that behave like methylating agents. The exact extent of these compounds to mercury methylation during remineralization of marine plankton in the marine environment is not known. The goal of this master thesis will be to develop and validate spectrophotometric methods for the determination of nutrient concentrations in seawater (silicate, ammonium, nitrate, nitrite, and phosphate). This will allow accurate determination of nutrients in seawater and estimation of relations of these nutrients with MeHg in seawater. Small scale mesocosm experiment will be created to test whether the amendment of nutrients, methylating agents, and specific inhibitors of nutrient cycles to seawater from the Gulf of Trieste can indeed promote/inhibit Hg methylation.

Mentor: dr. Igor Živković (igor.zivkovic@ijs.si)

6. Determination of deposition rate constants of different mercury fractions in precipitation

Mercury (Hg) is a global contaminant that is distributed across the globe through atmospheric transfer. Mercury in the atmosphere can exist in the form of gaseous oxidized mercury (GOM), gaseous elemental mercury (GEM), methylmercury (MeHg), and particulate bound mercury (PBM). Interconversion between GOM and GEM is influenced by the presence of UV radiation and strong oxidants, such as halogen and hydroxyl radicals. The exact origin of methylmercury in wet deposition is still unknown. The removal of mercury from the atmosphere can be via wet or dry deposition. Even though the determination of mercury in wet deposition is common, it is still not well characterized in means of elimination rates from the atmosphere itself. The concentration of mercury species in precipitations can vary significantly between different regions and within time, reflecting the local changes in environmental atmospheric mercury concentrations. The main goal of this master thesis would be to calculate the deposition rate constants for different mercury fractions from the atmosphere during several rain events. The objective is to see whether different rain events have the same deposition rate constants, which would indicate similar sources of mercury and/or processes in the atmosphere. Different rain events will be compared to estimate the influence of wind and droplet size on elimination efficiency and rate constants.

Mentor: dr. Igor Živković (igor.zivkovic@ijs.si)