Project: Collaborative Research: Transformations of soluble Mn(III) along horizontal and vertical oxygen gradients

NSF Award Abstract:
Manganese (Mn) is an important trace nutrient for biological growth in marine organisms. In the past, all Mn dissolved in seawater was thought to exist in only one chemical form: Mn(II). Recent work in waters and sediments with little or no oxygen has shown that Mn(III) can be the dominant form of dissolved Mn and can even be present in oxygenated water if attached to specific organic molecules called ligands. This research will further investigate these discoveries, aiming to quantify the chemical and microbiological processes responsible for Mn(III) cycling under varying oxygen concentrations. The research will compare field sites in the Broadkill River wetland, the Chesapeake Bay, and the Lower St. Lawrence Estuary; measuring the water column and sediments known to have strong oxygen gradients and different organic carbon sources that could change the availability and binding strength of ligands that would stabilize dissolved Mn(III). In some chemical forms, Mn tends to act like iron, so this research may have broader implications by helping marine chemists to understand more about iron cycling in similar oxygen gradients. With growing concerns over diminished oxygen concentrations (hypoxia) in the ocean, and particularly in coastal regions, understanding the reactions of Mn(III) with organic ligands across oxygen gradients could become important for understanding Mn availability to organisms. The project includes support for the participation and mentoring of one graduate student and two postdoctoral researchers, and there will be a U.S.-Canada collaboration. A variety of public outreach activities are planned with a K-12 teacher to be selected as a participant on a research cruise.

Mn(III) has only recently been recognized as an important redox state for Mn in seawater. Previously, it was widely accepted that all Mn that passes through a 0.2 or 0.4 µm filter is dissolved Mn(II) while the retained portion is solid Mn(III, IV) oxide. Research in the Black Sea, the Baltic Sea, and the Chesapeake Bay has shown that soluble Mn(III) can be up to 100% of the dissolved Mn in the water column. Also, Mn(III) can exist as complexes with organic ligands in oxygenated seawater. This project will quantify and constrain the mechanisms surrounding weak and strong Mn(III) ligand transformations across vertical and horizontal oxygen gradients. Field sites to be studied include systems with a variety of organic carbon sources and oxygen dynamics: the Lower St. Lawrence Estuary, Chesapeake Bay, and Broadkill River wetland estuary. This research will continue to inform the fundamental shift that is taking place in our current understanding of Mn biogeochemistry in coastal systems. The results should also be applicable to redox processes involving Fe(III) ligand transformations, since Mn and Fe tend to exhibit similar redox chemistry under similar environmental conditions.