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The distributions of dissolved trace metals
Many dissolved trace metals such as iron, nickel, cobalt and manganese are essential nutrients for phytoplankton and bacteria. Some metals may also be toxic, depending on the particular chemical species of that metal. This is the case for copper. We are interested in exploring the distributions of dissolved trace metals in seawater, in environments ranging from the open ocean (where these metals are particularly scarce) to nearshore or coastal environments (where many metals are present in relatively high concentrations). Trace metals often play different roles depending on the environment where they are sampled. Iron, for example can limit phytoplankton growth in many regions of the ocean such as the Southern Ocean near Antarctica, or other upwelling regions. However, copper can often be toxic to phytoplankton growth in many harbors and marinas due to its high concentrations from anti-fouling paint on boat hulls. In addition to measurements of total dissolved trace metals, we also measure the organic speciation of these metals, in order to determine their chemical speciation. The different organic species present in seawater can affect how the metal cycles. Some organic forms of copper for example are not toxic, yet the uncomplexed version is detrimental to phytoplankton growth. The organic forms of cobalt in seawater may also be important nutrients such as vitamin B12. Thus, determining the speciation of dissolved trace metals is essential for understanding their role in the marine environment. Techniques we use for these projects are voltammetry (cathodic stripping voltammetry), and inductively coupled plasma mass spectrometry (ICP-MS).
The cycling of siderophores
Siderophores are organic compounds that are produced by microorganisms that have a very strong affinity for iron, or can bind iron very strongly. In the terrestrial environment they are produced by bacteria as a strategy for sequestering iron from their surroundings and taking it up into the cell. We know much less about these compounds, and which organisms can produce them, in seawater. In the past decade, due to the development of highly sensitive mass spectrometry techniques, we have been able to identify siderophores in seawater. With their identification, we can now begin to determine which organisms are producing them and why, and how they effect iron cycling. We are interested in both measuring the distributions of these compounds in seawater, as well experimentally determining which siderophores are produced and consumed by different organisms. Techniques we use in this work are liquid chromatography coupled to inductively coupled plasma mass spectrometry as well as electrospray ionization mass spectrometry.
The uptake and cycling of iron
Iron is an essential nutrient that can limit phytoplankton growth is many regions of the ocean. Iron will often increase primary productivity in these regions, thus effecting the "biological pump" or the drawdown of carbon dioxide from the atmosphere. There are many different chemical forms of iron in seawater, ranging from strong iron-siderophore complexes to weaker iron-humic compounds. These compounds all have a range of bioavailabilites, and this is important to constrain in order to understand the effects of iron inputs on the biological pump. To investigate iron uptake in phytoplankton and bacteria, we use radioactive and stable iron isotope uptake techniques, coupled to either scintillation counting or mass spectrometry techniques.
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