Development and Intercomparison of Methodologies to Measure Dissolved Ferrous Iron in Seawater

微量金属铁是整个海洋中的一种关键微量营养素，在调节初级生产中起着重要作用。铁（化学符号Fe）在环境中可以以不同的电离状态存在，主要以还原形式Fe（II）和氧化形式Fe（III）存在，这影响了其在水柱中的化学行为，并最终影响了研究人员准确测量其丰度的能力。化学海洋学界迫切需要精确测量水柱中的铁（II），因为目前使用的方法可能会高估从某些深度收集的水样中的浓度。在本研究中，南加州大学的研究人员将重新开发一种未充分利用的方法来测量Fe(II)，并对两种方法进行评估和相互比较。这项工作将使铁（II）浓度的估计得以改进，为未来的使用提供一个可靠的方法，并阐明当前方法可以有效使用的条件。该项目将涉及大量的本科生研究机会，研究将与来自印度的国际研究人员合作进行。在有氧或硝酸盐存在的情况下，Fe（II）在海水中热力学上是不稳定的，但在地表水和氧最低区都有报道。对铁（II）测量的严格评估很重要，因为它的化学和生物利用度与铁（III）非常不同。铁（II）形成弱配合物，弱水解，易溶于海水。铁（III）形成强配合物，难溶，强水解。在铁限制条件下，即使少量的铁作为铁（II）存在，也可以提高生物利用度的数量级。在纳摩尔到亚纳摩尔水平上测定铁（II）的最广泛使用的方法涉及铁（II）催化鲁米诺氧化，一种化学发光（CL）反应。这种方法有潜在的人工产物，特别是在光区，其他减少物种可能会干扰。研究人员将开发一种新的、改良版的已发表的CL替代方法，涉及用合成螯合剂络合铁（II）的预富集。虽然这种替代方法容易产生人工制品和污染，限制了其在开放海洋中的价值，但新的方法创造了一种独立于鲁米诺方法的准确和敏感的协议的可行途径。随着新方法的发展，该项目将与CL方法进行相互比较，CL方法将在实验室、缺氧中生态系统、加利福尼亚海岸外的沿海时间序列站和热带北太平洋东部进行。

The trace metal iron is a key micronutrient throughout the oceans that plays an important role in regulating primary production. Iron (chemical symbol Fe) can exist in different ionization states in the environment, primarily as the reduced form Fe(II) and the oxidized form Fe(III), which impacts its chemical behavior within the water column, and ultimately the ability of researchers to accurately measure its abundance. There is an urgent need in the chemical oceanography community for accurate measurements of Fe(II) in the water column, as currently used methods may overestimate concentrations in water samples collected from certain depths. In this study, researchers from the University of Southern California will significantly redevelop an underutilized methodology to measure Fe(II), and carry out an evaluation and inter-comparison between the two methodologies. This work will enable the refinement of estimates of Fe(II) concentration, present a robust methodology for future use, and clarify conditions where the current methodology can be used effectively. This project will involve strong undergraduate research opportunities, and the research will be conducted in collaboration with an international researcher from India.Fe(II) is thermodynamically unstable in seawater in the presence of oxygen or nitrate, yet its presence has been reported both in surface waters and in oxygen minimum zones. A rigorous assessment of Fe(II) measurements is important because its chemistry and bioavailability are so different than Fe(III). Fe(II) forms weak complexes, is weakly hydrolyzed and highly soluble in seawater. Fe(III) forms strong complexes, is sparingly soluble and strongly hydrolyzed. The presence of even a modest fraction of Fe as Fe(II) under Fe-limited conditions could increase bioavailability by orders of magnitude. The most widely used methodology to determine Fe(II) at nanomolar to sub-nanomolar levels involves the Fe(II) catalyzed oxidation of luminol, a chemiluminescent (CL) reaction. There are potential artifacts with this methodology, especially in the euphotic zone, where other reduced species might interfere. Researchers will develop a new, modified version of a published alternative method to CL involving the preconcentration of Fe(II) complexed by a synthetic chelator. While this alternative method is prone to artifacts and contamination that has limited its value in the open ocean, new approaches create a plausible path to an accurate and sensitive protocol that is independent of the luminol method. Following the new method development, this project will implement inter-comparisons with the CL approach, which will be conducted in the laboratory, in an anoxic mesocosm system, at a coastal time-series station off the California coast, and in the Eastern Tropical North Pacific.