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Research statement of the Marine Biogeochemistry
and Electrochemistry Group IntroductionThe biogeochemistry of trace elements in the oceans is controlled by the competing processes of particle scavenging and uptake and releases by marine micro-organisms, and is moderated by chemical speciation. Several elements (iron, cobalt, zinc and copper) are essential to microorganisms, and are thought to limit primary production in the oceans. Other elements (molybdenum, nickel, manganese, cadmium, and others) play important biological roles or have been shown to behave as biogenic elements. The chemical speciation regulates the geochemistry of these elements and their availability to micro-organisms. For these reasons, chemical speciation is fundamental to understanding the chemical processes in the oceans. When we started this research suitable methods for the study of speciation still had to be developed. Electroanalytical methods are uniquely suitable to investigate these processes, and this area has become a major research interest of this group to determine trace elements and their chemical speciation in natural waters. Of related interest are the redox chemistry of metals and sulfide, the production of metal specific ligands (siderophores, phytochelatins and metallothioneins) by marine algae and bacteria, and the relation between metal speciation and enzyme activity such as the speciation of zinc and cobalt and the activity of carbonic anhydrase. Electroanalytical developmentsNew methods (cathodic stripping voltammetry (CSV) and chronopotentiometry) have been pioneered by this group to determine trace elements and their chemical speciation in sea water. We discovered recently that the sensitivity (already very high) can be enhanced by catalysis to amplify the reduction current. CSV lends itself to automation using flow analysis techniques, which has led to the development of a metal-autoanalyser which determines metal concentrations directly in sea water pumped on-board a ship, with chronopotentiometric and voltammetric detection. A useful spin-off is that the same methods can be used to determine toxic metals in biological materials such as blood and molluscs. As a result of these developments it is now possible to determine the concentration of some twenty elements (including aluminium, antimony, cadmium, chromium, cobalt, copper, iron, lead, molybdenum, nickel, platinum, selenium, uranium, tin, titanium, vanadium, and zinc, and nitrogen compounds such as ammonia, nitrite and nitrate) in seawater using CSV, and investigate the speciation of many of these. Methods were developed to determine several organic compounds including purines, folic acid, sulfur-containing amino acids and various thiol compounds (cysteine and glutathione) in seawater by CSV. Marine chemical findingsOur measurements are among the first to demonstrate that the biogenic metals (copper, zinc, iron, and cobalt) occur organically complexed in the oceans. Our measurements have shown that organic complexation reactions control the transport of copper, nickel and zinc through estuaries, and that dissolved metal concentrations in estuarine and coastal waters follow dynamic patterns. Unexpectedly anionic elements such as antimony, molybdenum, and uranium were also shown (by CSV) to occur to a significant extent as non-labile species in estuarine waters. A voltammetric study of the geochemistry of platinum in the Indian Ocean conclusively showed that this element has a geochemical behaviour reminiscent to that of manganese in those waters. Our investigations have shown that titanium and aluminium occur as unknown non-labile species, either colloidal or organically complexed, in the oceanic water column. We used our methods to determine the complex stability of sulfide with several metals in seawater, and found that the complex with copper is much more stable than expected. Probably the most important finding of the last decade in oceanography has been that lack of iron limits oceanic productivity (the Iron Hypothesis). Our finding that iron is organically complexed provides the explanation for its apparent poor availability, and moderates the Iron Hypothesis to a combination of lack of iron and unavailability. Photochemical effects and the existence of transient species were investigated in the field, showing that the redox chemistry of chromium appears to be controlled both by photochemical and biological processes in the upper water column, causing the presence of significant amounts of chromium(III) where it is thermodynamically unexpected. We demonstrated that the biologically important folic acid and glutathione occur dissolved in the oceanic water column of the NE Atlantic. Further work has now shown the importance of thiols like glutathione on the chemical speciation of copper suggesting that thiols may well account for at least part of the ligands we see in natural waters. The complexation reactions likely regulate the rate at which these metals are transferred through the membranes of microorganisms. For this reason we looked at interactions with microorganisms and found that marine algae (the important bloom forming coccolithophore Emiliania huxleyi) release iron-binding ligands in response to iron additions. This work stimulated the use of seawater cultures without modifier additions. Future ResearchMetals and their complexation reactions continue to be important. Iron will be the stuff of theses for some time to come. Our recent work extends the iron hypothesis to include cobalt which may limit specific groups of algae. We have received three major contracts from the EU (480 kEuro) to continue this work until 2003, in oceanic and coastal waters, and in remote mountain lakes. These (and other) findings were made possible by advances in (electro-)analytical chemistry, in which we played a leading role. Much can be done to further improve the electroanalytical detection, and a major potential growth area is in-situ detection. Recent developments in new materials and in micro-machining have enabled the development of microelectrodes suitable for such in-situ measurements. Our group is engaged in experiments to develop probes based on arrays of individually addressable microelectrodes. These probes will be used to determine gradients (of sulfide and metals) at the water/sediment interface, by insertion in the porewaters. This work is supported by industry (EcoChemie and Metrohm) with instrumentation. It is likely that similar micro-array-electrodes can be used for multi-DNA-probes with direct electrochemical detection of matching DNA segments, which may have important health implications as well as enable detection of microorganisms in the marine environment. Research funds contributing to investigationsOur research has benefited from financial support by the Natural Environment Research Council, the Royal Society, the European Union, contracts with the Admiralty, British Nuclear Fuels and British Gas, and collaboration with electrochemical companies (Metrohm and EcoChemie).
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