Award: OCE-1558692

Manganese (Mn) is an essential element for life, and its cycling between different soluble and solid forms has significant impact on the cycling of many other elements including carbon, nitrogen, oxygen, iron and sulfur. Mn plays a critical role in photosynthesis as it is required for the oxygen-evolving enzyme system in plants. Mn is also an important mineral resource, being found in deep-sea manganese nodules and crusts. This collaborative project, involving researchers primarily at Oregon Health & Science University and the University of Delaware, and in part at 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 mainly on the conversion between soluble Mn (primarily in the +II oxidation state) and solid Mn (or Mn oxides, primarily in the +IV oxidation state) and has not considered the intermediate +III oxidation state. Recently, we have learned that Mn(III) can be complexed by certain chemicals, compounds that keep Mn(III) in a soluble form (called a Mn(III) complex). Soluble Mn(III) is important because it 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). It is through these processes that Mn cycling can significantly influence the chemistry of the surrounding environment. Mn(III) complexes can accumulate and be abundant in Mn-rich waters; they have been largely overlooked as a form of Mn in aquatic systems. Our research provides new insights into the distribution and behavior of Mn(III) in aquatic marine systems. We developed techniques to measure Mn(III) complexes and Mn oxides at sub-nanomolar levels. We have shown that Mn(III) is ubiquitous, sometimes the most abundant soluble form of Mn. It occurs as both strong and weak complexes which are more or less stable, respectively. Both classes of Mn(III) complexes can coexist in the same waters. Soluble Mn(III) can be particularly abundant in waters containing humic substances, which can complex Mn(III). In low-oxygen and anoxic systems, Mn(III) complexes can be relatively stable. At the oxic-anoxic interface, however, Mn cycling between Mn(II), Mn(III), and Mn(IV) is very dynamic. Here, Mn(III) complexes can form during the oxidative and reductive pathways alike, 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 column of the NW Atlantic Ocean during cruises conducted in two different years. The profiles of dissolved Mn(II), dissolved Mn(III) and particulate Mn oxides represent a unique dataset, as they are the first that depict the distribution of Mn(III) in the water column in the Atlantic Ocean. Mn(III) concentrations are highest just below the euphotic zone near the ocean surface, in the oxygen minimum zone at middepth, and increasing toward the bottom. Mn(III) complexes contributed 10-20% of the generally uniform total dissolved Mn concentration in the deep ocean. Both soluble and particulate oxidized manganese are significant components of the deep-water manganese pool and likely play a prominent role in oceanic redox chemistry and organic carbon remineralization. In addition to field research, laboratory studies focused on the role of bacteria to mediate reactions involving Mn(III). We showed that Pseudomonas putida strain GB-1 can oxidize both weak and strong Mn(III) complexes and form Mn(IV) oxide. This bacterium has 3 different enzymes capable of oxidizing Mn(II) to Mn(IV), but only two of them, both copper-containing enzymes, are capable of oxidizing complexed Mn(III) as well. We also isolated Mn(II)-oxidizing bacteria from the St. Lawrence River estuary. During Mn(II) oxidation by these microorganisms, Mn(III) complexes formed as an intermediate when weak or strong complexing agents were present. Manganese is key in photosynthesis, and the bioavailability of manganese directly influences phytoplankton growth, which in turn can affect higher levels of the food chain. Our research provided a broader understanding of the distribution and behavior of different forms of Mn, especially Mn(III) and the pathways involved in its formation and removal, in several different aquatic systems. Our research provides new insights into the Mn cycle, which is anticipated to become more important with climate change and the concomitant decrease in the oxygen content and pH of the oceans. Overall, the project provided training for two postdocs/senior research associates, a M.S. student, an undergraduate student and a summer undergraduate 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 technical skills. Similarly, the M.S. and undergraduate students gained experience in planning experiments, in making presentations, and--most importantly--in conducting independent research. Last Modified: 06/01/2020 Submitted by: Bradley M Tebo