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

锰是海洋生物生长的一种重要的微量营养物质。过去，所有溶解在海水中的锰都被认为只以一种化学形式存在：锰(II)。最近在含氧量很少或没有氧气的水域和沉积物中的研究表明，锰(III)可以是溶解的锰的主要形式，如果与称为配体的特定有机分子结合，甚至可以存在于含氧水中。这项研究将进一步调查这些发现，旨在量化在不同氧气浓度下导致Mn(III)循环的化学和微生物过程。这项研究将对布罗德基尔河湿地、切萨皮克湾和圣劳伦斯下河口的野外场地进行比较；测量已知具有强烈氧气梯度和不同有机碳源的水柱和沉积物，这些碳源可能会改变配体的有效性和结合强度，从而稳定溶解的锰(III)。在某些化学形式中，锰的行为倾向于铁，因此这项研究可能会有更广泛的影响，帮助海洋化学家更多地了解类似氧梯度下的铁循环。随着人们对海洋中氧浓度降低(缺氧)的日益关注，特别是在沿海地区，了解跨氧梯度的有机配体与锰(III)的反应对于了解生物对锰的有效性可能变得重要。该项目包括支持一名研究生和两名博士后研究人员的参与和指导，并将有美国和加拿大的合作。计划了各种公共推广活动，一名K-12教师将被选为研究巡航的参与者。直到最近，锰(III)才被认为是海水中锰的重要氧化还原状态。以前，人们普遍认为，通过0.2或0.4微米滤光片的所有锰都是溶解的锰(II)，而保留部分是固体锰(III，IV)氧化物。在黑海、波罗的海和切萨皮克湾的研究表明，可溶的锰(III)可以达到水柱中溶解的锰的100%。此外，在含氧海水中，Mn(III)可以与有机配体形成络合物存在。这个项目将量化和限制围绕着跨垂直和水平氧梯度的弱和强Mn(III)配体转换的机制。要研究的现场包括具有各种有机碳源和氧气动力学的系统：下圣劳伦斯河口、切萨皮克湾和布罗德基尔河湿地河口。这项研究将继续使我们目前对沿海系统中的锰生物地球化学的理解发生根本性的转变。这些结果也应该适用于涉及Fe(III)配体转换的氧化还原过程，因为在相似的环境条件下，Mn和Fe往往表现出相似的氧化还原化学。

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.