Award: OCE-1558738

Manganese (Mn) is an essential element for life and its cycling between different soluble and solid forms has significant impact on the cycling of other elements including carbon, nitrogen, oxygen, iron and sulfur. Mn plays a critical role in photosynthesis as it is required in plants, phytoplankton and algae for the enzyme system that produces oxygen. Mn is also an important mineral resource, being found in deep-sea manganese nodules and crusts. This research (a collaborative project between researchers at Oregon Health & Science University and the University of Delaware, and in part with colleagues from McGill University in Canada) provides new insights into the biogeochemistry of Mn in the oceans. Previous research into the geochemical cycling of manganese in aquatic systems focused predominately on the cycling between soluble Mn (primarily as Mn in the +II oxidation state), and solid Mn (Mn oxides primarily in the +IV oxidation states) and not on the intermediate +III oxidation state. Recently, we determined that Mn(III) can be complexed by natural organic substances, which keep Mn(III) in a soluble form (termed a Mn(III) complex). Soluble Mn(III) is important because it can be transported by tides and ocean currents and can serve as both an oxidant, whereby it accepts an electron from another substance becoming soluble Mn(II), or a reductant, whereby it reduces other chemicals and becomes a solid [Mn(IV)]. Through these processes, Mn cycling can significantly influence the chemistry of the surrounding environment. Mn(III) complexes can accumulate and be abundant in natural waters; these complexes have been largely overlooked as a form of Mn in aquatic systems. Our research provides new insights into the distribution and behavior of soluble Mn(III) in estuarine and marine systems. We developed techniques to measure Mn(III) complexes and solid Mn oxides at sub-nanomolar levels. We showed that Mn(III) is ubiquitous, and sometimes the most abundant soluble form of Mn. It is found to occur as both weak and strong complexes, which are less or more stable, respectively. Both classes of Mn(III) complexes can be found in the same waters. Soluble Mn(III) can be particularly abundant in waters containing humic materials, which complex the Mn(III). In low oxygen and anoxic systems Mn(III) complexes can be relatively stable, but at the interface between anoxic and oxic conditions, the Mn cycle between Mn(II), Mn(III), and Mn(IV) is very dynamic and Mn(III) complexes can form both during oxidative and reductive pathways with the latter being the more important one. These Mn(III) complexes can then serve as electron donors (reductants) or electron acceptors (oxidants). We measured the different forms of Mn in the water columns of the Chesapeake Bay, Saint Lawrence Estuary (SLE), Broadkill River Estuary DE (BRE), Atlantic and Pacific Oceans. In all systems, Mn(III) complexes make up substantial amounts of the total dissolved Mn. For example, in the oxygenated waters of the SLE and BRE, Mn(III) complexes are as high as 86% and 100% of the total dissolved Mn, respectively, and are a source of Mn(III) to the ocean. Natural humic compounds can bind up to 100% of the Mn(III) complexes in the SLE and the BRE. For oceanic samples, the Mn(III) complexes can be as high as 64% of the total dissolved Mn. Thus, they are a significant component of the deep-water manganese pool indicating that they likely play a prominent role in oceanic redox chemistry and organic carbon remineralization. Even in the Chesapeake Bay under mild reducing conditions, Mn(III) complexes can be as high as 55%. The Mn(III) complexes are not colloidal, as they pass through both 200 and 20 nanometer filters in all systems tested. Our data have changed the paradigm that dissolved Mn in natural waters primarily exists in the +II oxidation state. Manganese is an essential component in photosynthesis, and the bioavailability of manganese directly influences phytoplankton growth, which in turn can affect the food availability to many higher order organisms. Our research provided a broader understanding of the distribution and behavior of different forms of Mn, especially soluble Mn(III) and the pathways involved in its formation and removal, in several aquatic systems. Our research provides new insights into the Mn cycle, which will become more important with changing climate and the concomitant decrease in the oxygen content and pH of the oceans. Overall, the project provided training for one postdoctoral student, a Ph.D. student, an undergraduate student and a summer Research Experiences for Undergraduates (REU) student. The senior personnel gained valuable experience in research, presentation and administrative skills, preparation of manuscripts and mentorship in addition to new scientific skills. Similarly, the Ph.D. and undergraduate students gained experience in planning experiments, in making presentations, and most importantly in conducting independent research including while on research cruises. Data from cruises have been deposited to the public BCO-DMO databases according to NSF policy. Last Modified: 06/02/2021 Submitted by: George W Luther