The project, a collaboration between investigators at the University of Delaware and Oregon Health and Science University, will develop a sensitive method for measuring the concentration of total Mn and Mn(III) at sub-nanomolar levels in ocean waters, both in oxic environments and in reduced zones. The proposed method is a modification of a published method, which has nM sensitivity. The improvement in the detection limit of this method and the demonstration of its ability to measure Mn(III) at environmental levels (<0.5 nM)is the major goal of this one year project. Two different approaches will be investigated, and preliminary evidence suggests that both approaches are viable alternatives. Use of a long path optical cell will increase sensitivity of the current detection system, and preconcentration is another approach that can be used. An alternative is the use of fluorescence detection, and this will also be investigated. The various methods will be tested using referrence materials, including those collected during recent oceanographic intercalibration exercises. Additionally, the methods will be tested during short cruises and through laboratory culture experiments.
Broader Impacts: Manganese redox chemistry impacts a number of global biogeochmeical cycles, and therefore understanding these interactions has broad implications. The project will train graduate students, and the results will be desseminated through a number of educational and public outreach activities. This research will continue international collaborations developed during previously funded work.
Manganese has been thought to exist in two primary forms in marine waters and sediments. The soluble and reduced form is manganese(II), with two electrons removed from the metal. The insoluble and oxidized form is manganese dioxide with four electrons removed from the metal so is a manganese(IV) compound. Manganese dioxide is comparable to rust when iron oxidizes. Previous analytical chemistry methods to measure the forms of manganese have been based on filtration, but these methods did not discriminate other soluble forms. The element manganese is important in a variety of geochemical and biological processes vital to the health of the earth and is one of the most abundant actively cycled transition metals in the Earth’s crust. Manganese is an important nutrient for all plants as it is an essential metal in the photosynthetic apparatus that produces the oxygen that surface organisms need to breathe as well as providing energy to make organic matter or plant material from atmospheric carbon dioxide. In surface sediments, solid manganese oxides can be used to decompose natural organic matter when oxygen is no longer present. Thus manganese is an important element in the cycling of carbon and other elements. In this collaborative study, researchers from the University of Delaware and the Oregon Health and Science University showed that another soluble form of manganese exists in the marine environment, especially in waters from the surface layers (first few centimeters) of marine sediments (the image shows a sediment box core from the Saint Lawrence estuary on the left). This new form is manganese(III), which is an intermediate between manganese(II) and manganese dioxide. The research team developed a new analytical chemistry method to determine soluble manganese(III) and manganese(II), which combined make up the total soluble manganese. The team made measurements in sediments from salt marshes and the Saint Lawrence estuary with Canadian colleagues. The graph in the middle of the image shows that the manganese(III) is found in the upper 10 centimeters of the core between the surface manganese dioxide layer and the deeper manganese(II) layer. Manganese(III) is also observed at high concentrations (70 micromolar or up to 4 parts per million). The results further indicate that soluble manganese(III) can be as much as 90% of the total soluble manganese in these systems. Below 10 centimeters, manganese(II) is the only soluble manganese form. The production or formation of manganese(III) can occur by several pathways including the oxidation of soluble manganese(II) in the presence of oxygen by microbes and the reduction of manganese dioxide by organic matter, hydrogen sulfide and other reduced chemical compounds, often coupled with microbial metabolic processes. Soluble manganese(III) is important because it can oxidize other reduced chemicals by accepting an electron becoming managense(II), or reduce other oxidized chemicals by donating an electron and becoming manganese dioxide. This chemical behavior indicates that it can be a potent catalyst in the environment. Better knowledge of the manganese cycle, especially of manganese(III) is important to our understanding of the formation of the ocean’s low oxygen (suboxic) zones including the equatorial Pacific Ocean, the Arabian Sea and the Black sea as well as ocean sediments. Many other important oceanographic processes are coupled to manganese chemistry including the oxidation of hydrogen sulfide, organic carbon and ammonia, and the formation of extensive manganese oxide mineral deposits including nodules and crusts; this research grant has provided improved quantitative understanding of manganese’s role in the marine environment.
