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

地球的能量流-从动物到植物，再到微生物-是由电子转移决定的，即将电子从一个分子或元素移动到另一个分子或元素以获得能量。 光合作用可能是最常见的反应，二氧化碳和水的电子被用来制造氧气和糖。然而，在产生氧的代谢出现之前，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，因为它们能够容易地捐赠和接受电子。由于这种早期的进化，金属仍然在微生物生命中发挥着核心作用，作为酶的基本元素。其中一种重要的金属是锰（Mn），它特别擅长提供和接受电子，因为它在海洋中可以以三种不同的形式存在。Mn的一种形式是固体Mn氧化物，其是非常活泼的并且几乎与氧本身一样强的氧化剂。这种形式的锰完全由细菌产生，但我们仍然对细菌如何做到这一点知之甚少，或者为什么！理解这些Mn反应的一个好地方是在不存在氧的区域，因为在那里，Mn反应可能占主导地位。墨西哥湾是一个氧气浓度一直在稳步下降的地区，由于过度富集的营养物质从人为来源。该系统非常适合在低氧条件下检查Mn反应，因此在本项目中，将确定Mn在不同氧气水平下的反应。 来自罗得岛大学和得克萨斯州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，以更好地了解锰如何经历一个完整的反应循环。 了解海洋中的金属如何循环对于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，例如氧气条件的变化。 一名科学传播学生、一名海上艺术家和一名摄像师将被纳入游轮活动，将科学与公众宣传联系起来。该项目将支持三名研究生，本科生和两名早期职业研究人员。 该项目将侧重于招聘代表性不足的少数民族，以介绍STEM研究和实地工作。目前，来自罗得岛大学和德克萨斯A M大学的科学家已经开发出了在海洋环境中解卷积锰氧化还原循环的化学技术，但尚未将这些新方法彻底应用于不同的环境系统或将锰的形态和循环与其他元素的形态和循环耦合。在这里，科学家们建议在墨西哥湾充分物种化锰，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和氧化锰形成中的作用。特别是，科学家们试图研究Mn（III）-L复合物在季节，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。我们推测，锰循环，特别是在季节性缺氧和亚氧区，是错综复杂的，但神秘地链接到其他氧化还原敏感物种，包括氮和有机碳化合物的循环。本研究将研究墨西哥湾的这些动态，墨西哥湾在以下方面具有梯度：（1）季节性周期的生产力，（2）通过其支流的盐度和陆地输入，以及（3）动态和季节性氧气状况。所有这些梯度都深刻地影响了Mn和其他元素的氧化还原化学，因此必须考虑到创建用于理解耦合氧化还原循环的准确框架。在进行这项研究时，我们不仅将广泛阐明海洋锰循环，我们还将突出以前神秘的化学动力学，推动脆弱的沿海生态系统中氧气最小区的形成和消散。该奖项反映了NSF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.