Award: OCE-1155385

Scientists have discovered that a particular form of soluble manganese (Mn) found in marine sediments is significantly more abundant than previously thought. This research, carried out by researchers at the University of Delaware, the Oregon Health and Science University and McGill University in Canada, transforms our understanding of the role of Mn in ocean biogeochemistry. An essential element for life, Mn plays a critical role in photosynthesis and the biogeochemical cycles of nutrients, carbon, nitrogen and iron. Improved understanding of soluble Mn could shed light on the complex connections between biology, geology and chemistry in ocean environments. It may also advance our understanding of deep-sea Mn nodule formation and organic matter bacterial decomposition in the ocean's low oxygen (O2) environments. Like other trace metals, Mn exists in multiple oxidation states: Mn(II), Mn(III) and Mn(IV). However, manganese was thought to exist primarily in two forms in marine waters and sediments; soluble Mn(II) and solid Mn(IV). Previous analytical methods did not discriminate other soluble forms, especially Mn(III). With updated techniques to improve the detection limit for dissolved or soluble Mn(III), we have shown that soluble Mn(III) accounts for up to 100 % of the total dissolved Mn found in marine sediments and waters. Soluble Mn(III) is important because it can oxidize other reduced chemicals by accepting an electron becoming soluble Mn(II), or reduce other oxidized chemicals by donating an electron and becoming solid manganese dioxide [Mn(IV)]. This chemical behavior indicates that it can be a reactive intermediate or catalyst in the Mn cycle which is coupled to other elemental cycles in the environment (see Figure 1 and text below). In accomplishing our objectives, we were able to measure soluble Mn(III) in all of the diverse systems studied, which included the sedimentary porewaters of the Lower Saint Lawrence Estuary (LSLE) and the water column of the Chesapeake Bay that have low or nondetectable oxygen concentrations. Data from both field and laboratory incubation studies demonstrated that soluble Mn(III) could form from the oxidation of soluble Mn(II) by bacteria in the presence of downward diffusing oxygen and the reduction of solid Mn(IV) oxides. In the Chesapeake Bay, soluble Mn(III) reacted with hydrogen sulfide (H2S) to form harmless elemental sulfur and Mn(II), which was re-oxidized by bacteria to form both soluble Mn(III) and solid Mn(IV) oxides. The coupling of all these reactions is a catalytic cycle between manganese, sulfur and oxygen that prevents toxic H2S from reaching surface waters. Preliminary data in fully oxygenated waters of the Lower Saint Lawrence Estuary and the Broadkill River Estuary of southern Delaware indicate that soluble Mn(III) can account for up to 66% and 100% of the total dissolved Mn(III), respectively. As part of this work, we were able to provide information on the nature of the organic ligands binding to soluble Mn(III), and its effect on the reactivity and stability of soluble Mn(III) in the environment. We found that both weak and/or strong Mn(III)L complexes exist in a variety of environments ranging from in the presence of low concentrations of hydrogen sulfide to nondetectable concentrations of hydrogen sulfide, and from nondetectable oxygen to fully oxygenated waters. Also, we were able to develop a method to measure the strength of Mn(III) binding which allows us to quantify weak versus strong organic ligands binding Mn(III). The porewaters of the Lower Saint Lawrence Estuary provide predominantly weaker ligands derived from organic matter decomposition whereas the oxic water column of the LSLE and the surface oxygenated waters of the Broadkill River estuary provide stronger ligands, which are of humic origin. Our research indicates that there is a competition between Mn and Fe for ligands to keep them soluble. This could affect the uptake of Fe by phyt...