Collaborative Research: Determining the isotopic signature of iron released via ligand-mediated dissolution of atmospheric dust in the surface ocean

铁（Fe）是海洋中微生物生长的重要营养素，影响碳循环和气候系统，但铁不容易溶解在海水中，因此其可用性限制了大部分海洋表面浮游植物的生长。铁到达海洋表面的最重要方式之一是通过风吹尘埃的沉积;然而，为了使这种铁可用于生物生长，它必须溶解并保持在与有机分子结合的溶液中。尽管这种已知的重要性，粉尘溶解的机制和有机分子的身份保持铁在溶液中仍然知之甚少。本研究将侧重于使用实验室和现场实验，以更好地了解如何在海水中存在的天然有机分子，提高撒哈拉沙漠灰尘，响应灰尘，铁的同位素分馏与灰尘溶解的微生物产生的有机分子释放铁，并最终确定在海洋铁循环灰尘的作用配体介导溶解的作用。在推广活动方面，提议者将指导来自非洲/亚洲的难民高中生学习数学和科学，帮助他们融入美国的学校系统，并通过参加彼得堡科学节与公众讨论科学。 作为该项目的一部分，将支助和培训三名研究生和本科生实习生。来自南佛罗里达大学和俄勒冈州州立大学的科学家们将在百慕大的都铎山塔每年收集一次北大西洋的天然尘埃，并描述尘埃的每周体积和水溶性铁同位素和元素组成。然后将代表撒哈拉和非撒哈拉（受气候影响）季节的适当子样本溶解在海水中，溶解由具有一系列Fe结合稳定常数的配体介导（天然配体以及定义的海洋铁载体）。还将对从墨西哥湾和北大西洋百慕大大西洋时间序列站收集的自然生物群落进行尘埃培养。研究人员将表征每个实验中释放的Fe的量和同位素特征，微生物群落对孵育物中灰尘的反应（DNA，细胞计数和鉴定），以及用于实验和孵育物中产生的天然有机物中存在的配体。非生物和生物实验的比较将允许调查的生物效应的大小对粉尘溶解。这些实验将使研究人员能够从机制上了解影响尘埃溶解速率的因素，特别是不同特定有机配体在增强和稳定海洋表面铁释放方面的作用，以及释放的铁的净特征。由此，研究人员将为海洋中的海洋铁同位素循环提供新的限制，特别是测试以下假设：由于与有机配体的络合，灰尘中Fe的净释放是同位素重的，但这种分馏的大小取决于不同配体的结合强度和存在。该过程研究的结果将为GEOTRACES等大规模实地研究的模型和解释提供一个框架，并提高研究界解释气溶胶和海洋铁同位素数据的能力。该奖项反映了NSF的法定使命，并被认为值得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来支持。

Iron (Fe) is a crucial nutrient for microbial growth in the oceans, impacting the carbon cycle and the climate system, but Fe does not dissolve readily in seawater and so its availability limits phytoplankton growth over much of the surface oceans. One of the most significant ways by which Fe reaches the surface oceans is through deposition of wind-blown dust; however, for this Fe to be available for biological growth, it must dissolve and be kept in solution bound to organic molecules. Despite this known importance, the mechanisms of dust dissolution and the identity of the organic molecules keeping Fe in solution remain poorly understood. This study will focus on using laboratory and field experiments to better understand how natural organic molecules present in seawater enhance the release of Fe from Saharan desert dust, the organic molecules produced by microbes in response to dust, the isotopic fractionation of Fe associated with dust dissolution, and ultimately the role of ligand-mediation dissolution in determining the role of dust in the marine Fe cycle. For outreach activities, the proposers would tutor refugee high school students from Africa/Asia in math and science to help them get integrated into the U.S. school system and discuss science with the public by participation in the St. Petersburg Science Festival. Three graduate students and undergraduate interns would be supported and trained as part of this project. Scientists from the University of South Florida and Oregon State University will collect natural North Atlantic dust at the Tudor Hill Tower on Bermuda over a yearly cycle and characterize the weekly bulk and water-soluble Fe isotopic and elemental composition of the dust. Appropriate subsamples of this dust, representing both the Saharan and non-Saharan (anthropogenically-influenced) seasons will then be dissolved in seawater, with dissolution mediated by ligands with a range of Fe binding stability constants (natural ligands as well as defined marine siderophores). Incubations of dust will also be carried out with natural biological communities collected from the Gulf of Mexico and at the Bermuda Atlantic Time Series site in the North Atlantic. The investigators will characterize the amount and isotopic signature of the Fe released in each experiment, the microbial community response to dust in the incubations (DNA, cell counting and identification), and the ligands present in the natural organic matter used for experiments and produced in the incubations. Comparison of the abiotic and biological experiments will allow investigation of the magnitude of the biological effects on dust dissolution. The experiments will allow the investigators to provide a mechanistic understanding of factors that affect dust dissolution rates, especially the role of different specific organic ligands in enhancing and stabilizing the release of iron in the surface ocean, and the net signature of iron released. From this, the investigators will provide new constraints for the marine iron isotope cycle in the ocean, specifically testing the hypothesis that the net release of Fe from dust is isotopically heavy due to complexation with organic ligands, but that the size of this fractionation depends on the binding strength and presence of different ligands. The outcomes of this process study will provide a framework for models and interpretation of large scale field studies such as GEOTRACES, as well as enhancing the research community's ability to interpret aerosol and oceanic iron isotope data.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.