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

地球上的能量流——从动物到植物，再到微生物——是由电子转移决定的，即电子从一个分子或元素转移到另一个分子或元素以获得能量。光合作用可能是这类反应中最常见的，来自二氧化碳和水的电子被用来制造氧气和糖。然而，在产生氧气的代谢出现之前，早期生命的代谢是由不同的电子转移反应控制的——其中大多数涉及金属，因为它们能够很容易地提供和接受电子。由于这种早期进化的结果，金属作为酶的基本元素，在微生物生命中仍然发挥着核心作用。其中一种重要的金属是锰（Mn），由于锰在海洋中可以以三种不同的形式存在，它特别擅长提供和接受电子。锰的一种形式是固体锰氧化物，它的反应性很强，几乎和氧本身一样强。这种形式的锰完全是由细菌产生的，但我们对细菌是如何产生锰的，以及为什么产生锰的知之甚少！理解这些Mn反应的一个好地方是在没有氧气存在的地方，因为在那里Mn反应可能占主导地位。由于人为来源的营养物质过度富集，墨西哥湾地区的氧浓度一直在稳步下降。该系统非常适合在低氧条件下检测Mn反应，因此在本项目中，将确定Mn在不同氧水平下的反应方式。来自罗德岛大学和德克萨斯农工大学的科学家们也将专门针对制造这些固体锰氧化物的细菌，试图了解其形成机制。最后，科学家们将尝试测量锰的来源和去向，以更好地了解锰是如何经历一个完整的反应周期的。了解金属在海洋中的循环方式对了解地球上的生命至关重要，因为在地球的历史上，金属的库存一直受到化学变化的影响，比如氧气条件的变化。一名科学传播学学生、一名海上艺术家和一名摄像师将被纳入邮轮活动，将科学与公众外展联系起来。本项目将资助3名研究生、2名本科生和2名早期职业研究人员。该项目将侧重于招募未被充分代表的少数民族，以介绍STEM研究和实地工作。目前，来自罗德岛大学和德克萨斯农工大学的科学家们已经开发出化学技术来解决海洋环境中锰氧化还原循环的问题，但尚未将这些新方法彻底应用于不同的环境系统中，或者将锰的形态和循环与其他元素的形态和循环结合起来。在这里，科学家们建议对墨西哥湾的锰进行充分的物种鉴定，评估Mn(III)-L化合物的形成、流行和生物利用度，以及微生物在促进Mn(III)-L和Mn氧化物形成中的作用。特别是，科学家们试图检查Mn(III)-L配合物在季节，盐度和氧气梯度中的稳定性，并表征陆地和生物Mn（III）结合配体。我们假设Mn的循环，特别是在季节性缺氧和缺氧区，与其他氧化还原敏感物种（包括氮和有机碳化合物）的循环有着复杂而神秘的联系。本研究将研究墨西哥湾的这些动态，它具有以下方面的梯度：(1)季节性循环的生产力；(2)盐度和通过其支流的陆地输入；(3)动态和季节性氧气状态。所有这些梯度都深刻地影响着Mn和其他元素的氧化还原化学，因此必须考虑到这一点，以创建一个准确的框架来理解耦合氧化还原循环。在进行这项研究时，我们不仅将广泛阐明海洋锰循环，我们还将强调以前神秘的化学动力学，这些化学动力学驱动脆弱的沿海生态系统中氧气最小带的形成和消散。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.