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

锰（Mn）是海洋生物生长的重要微量营养物质。过去，所有溶解在海水中的锰被认为只以一种化学形式存在：锰（II）。最近在水和沉积物中很少或没有氧气的研究表明，Mn（III）可以是溶解Mn的主要形式，如果附着在称为配体的特定有机分子上，甚至可以存在于含氧水中。本研究将进一步研究这些发现，旨在量化不同氧浓度下Mn（III）循环的化学和微生物过程。这项研究将比较布罗德基尔河湿地、切萨皮克湾和下圣劳伦斯河口的实地地点；测量已知具有强氧梯度和不同有机碳源的水柱和沉积物，这些有机碳源可以改变稳定溶解Mn（III）的配体的可用性和结合强度。在某些化学形式中，Mn倾向于表现得像铁，所以这项研究可能有更广泛的意义，可以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）的关注日益增加，特别是在沿海地区，了解Mn（III）与有机配体在氧梯度上的反应对于了解生物体的Mn可利用性非常重要。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。各种各样的公共宣传活动计划与K-12教师将被选为研究游船的参与者。Mn（III）直到最近才被认为是海水中Mn的重要氧化还原态。以前，人们普遍认为所有通过0.2或0.4µm过滤器的Mn都是溶解的Mn(II)，而保留的部分是固体Mn（III， IV）氧化物。在黑海、波罗的海和切萨皮克湾的研究表明，可溶性锰（III）可以达到水柱中溶解锰的100%。Mn（III）可以与有机配体配合物存在于含氧海水中。该项目将量化和约束围绕弱和强Mn（III）配体在垂直和水平氧梯度上转化的机制。要研究的野外地点包括具有各种有机碳源和氧动力学的系统：下圣劳伦斯河口、切萨皮克湾和布罗德基尔河湿地河口。这项研究将继续为我们目前对沿海系统中锰生物地球化学的理解所发生的根本转变提供信息。研究结果也适用于涉及Fe（III）配体转化的氧化还原过程，因为Mn和Fe在相似的环境条件下倾向于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.