Collaborative Research: Manganese as a key reactant in the expanding low oxygen zones of the Gulf of Mexico, USA

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The Earth’s flow of energy – from animals, to plants, to microbes – is dictated by electron transfers, namely, moving electrons from one molecule or element to another to gain energy. Photosynthesis is perhaps the most familiar such reaction, where the electrons from carbon dioxide and water are used to make oxygen and sugar. However, before the appearance of oxygen-generating metabolisms, early life metabolism was governed by different electron transfer reactions – most of these involving metals because of their ability to readily donate and accept electrons. As a result of this early evolution, metals still play a central role in microbial life, as essential elements in enzymes. One such important metal is manganese (Mn), which is particularly good at donating and accepting electrons given that it can be in three different forms in the ocean. One form of Mn is a solid Mn-oxide, which is very reactive and almost as strong an oxidant as oxygen itself. This form of Mn is completely generated by bacteria, but we still know little about how bacteria do this, or why! One good place to understand these Mn reactions is in areas where oxygen is not present, because there, Mn reactions may dominate. The Gulf of Mexico is a region where oxygen concentrations have been decreasing steadily due to over-enrichment of nutrients from anthropogenic sources. This system is ideal for examining Mn reactions under low oxygen conditions, so in this project, how Mn reacts under different levels of oxygen will be determined. Scientists from the University of Rhode Island and Texas A&M University will also specifically target the bacteria that make these solid Mn oxides, to try and understand the mechanism of formation. Finally, the scientists will try to measure where the Mn is coming from and where it is going, to get a better idea of how Mn may undergo a complete reaction cycle. Understanding how metals cycle in the ocean is central to understanding life on Earth, as the inventory of metals has been subject to shifts in chemistry, like changing oxygen conditions, over the Earth’s history. A science communications student, an artist at sea, and a videographer will be incorporated into cruise activities to link the science with public outreach. This project will support three graduate students, undergraduate students, and two early career researchers. This project will focus on recruitment of underrepresented minorities to provide an introduction to STEM research and field work. At present, scientists from the University of Rhode Island and Texas A&M University have developed the chemical techniques to deconvolute manganese redox cycling in marine environments but have yet to thoroughly apply these new methods in diverse environmental systems or to couple manganese speciation and cycling with that of other elements. Here, the scientists propose to fully speciate manganese in the Gulf of Mexico, evaluating the formation, prevalence, and bioavailability of Mn(III)-L compounds and the role of microbes in facilitating Mn(III)-L and Mn oxide formation. In particular, the scientists seek to examine the stability of Mn(III)-L complexes across seasonal, salinity and oxygen gradients and to characterize both terrestrial and biotic Mn(III)-binding ligands. We hypothesize that Mn cycling, particularly in seasonally anoxic and suboxic zones, is intricately but enigmatically linked to the cycling of other redox sensitive species, including nitrogen and organic carbon compounds. This study will study these dynamics in the Gulf of Mexico, which has gradients in (1) productivity on a seasonal cycle, (2) salinity and terrestrial input via its tributaries and (3) a dynamic and seasonal oxygen regime. All of these gradients profoundly impact the redox chemistry of Mn and other elements and thus must be taken into account to create an accurate framework for understanding coupled redox cycling. In conducting this research, not only will we broadly elucidate the marine manganese cycling, we will also highlight previously cryptic chemical dynamics that drive the formation and dissipation of oxygen minimum zones in fragile coastal ecosystems.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.