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

地球的能量流动--从动物到植物，再到微生物--是由电子转移决定的，也就是将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。光合作用可能是最常见的这种反应，二氧化碳和水中的电子被用来制造氧气和糖。然而，在产生氧气的新陈代谢出现之前，早期生命的新陈代谢受到不同的电子转移反应的支配--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。作为这一早期进化的结果，金属作为酶的基本元素，仍然在微生物生命中发挥着核心作用。其中一种重要的金属是锰，它特别擅长捐赠和接受电子，因为它在海洋中可以有三种不同的形式。锰的一种形式是固体锰氧化物，它非常活泼，几乎和氧本身一样是一种氧化剂。这种形式的锰完全是由细菌产生的，但我们仍然对细菌如何做到这一点，或者为什么会这样做知之甚少！理解这些锰反应的一个好地方是在氧气不存在的地区，因为在那里，锰反应可能占主导地位。由于人为来源的营养物质过度浓缩，墨西哥湾的氧气浓度一直在稳步下降。该系统是研究低氧条件下锰反应的理想系统，因此在这个项目中，将确定锰在不同氧气水平下的反应。来自罗德岛大学和德克萨斯农工大学的科学家还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，以更好地了解锰可能如何经历一个完整的反应循环。了解金属在海洋中的循环方式是理解地球上生命的核心，因为金属的库存在地球历史上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录像师将被纳入邮轮活动，将科学与公共宣传联系起来。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任职人数不足的少数群体，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已经开发出在海洋环境中解开锰氧化还原循环的化学技术，但尚未在不同的环境系统中彻底应用这些新方法，或将锰的形态形成和循环与其他元素的循环相结合。在这里，科学家们建议对墨西哥湾的锰进行全形态分析，评估锰(III)-L化合物的形成、分布和生物可利用性，以及微生物在促进锰(III)-L和氧化锰形成中的作用。特别是，科学家们试图研究锰(III)-L络合物在季节、盐度和氧气梯度上的稳定性，并表征与陆地和生物锰(III)结合的配体。我们假设，锰的循环，特别是在季节性缺氧区和亚缺氧区，与其他氧化还原敏感物种的循环错综复杂但神秘地联系在一起，包括氮和有机碳化合物。这项研究将研究墨西哥湾的这些动态，它有(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)动态和季节性氧气制度的梯度。所有这些梯度都深刻地影响着锰和其他元素的氧化还原化学，因此必须考虑到这些梯度，以创建一个准确的框架来理解耦合的氧化还原循环。在进行这项研究时，我们不仅将广泛阐明海洋锰循环，还将突出以前神秘的化学动力学，这些动力学驱动着脆弱的沿海生态系统中氧气最小区域的形成和消散。这一奖项反映了NSF的法定使命，并通过使用基金会的智力优势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.