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

铁(Fe)是海洋微生物生长的关键营养物质，影响碳循环和气候系统，但铁不容易溶于海水，因此其有效性限制了大部分表层海洋的浮游植物生长。铁进入表层海洋的最重要方式之一是通过风吹来的尘埃沉积；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。尽管已知这一重要性，但粉尘的溶解机制和使铁保持在溶液中的有机分子的身份仍然知之甚少。这项研究将侧重于利用实验室和现场实验，更好地了解海水中存在的天然有机分子如何促进撒哈拉沙漠粉尘中铁的释放，微生物对粉尘产生的有机分子，与粉尘溶解相关的铁的同位素分馏，以及最终配体调节溶解在确定粉尘在海洋铁循环中的作用中的作用。对于外展活动，提倡者将辅导来自非洲/亚洲的难民高中生数学和科学，以帮助他们融入美国学校系统，并通过参加圣彼得堡科学节与公众讨论科学。作为该项目的一部分，三名研究生和本科生实习生将得到支持和培训。来自南佛罗里达大学和俄勒冈州立大学的科学家将在百慕大的都铎山塔收集北大西洋天然粉尘，收集一年的周期，并表征粉尘每周的体积和水溶性铁同位素和元素组成。然后，代表撒哈拉和非撒哈拉(人为影响)季节的适当粉尘亚样将被溶解在海水中，并由具有一系列铁结合稳定常数的配体(天然配体以及确定的海洋铁载体)进行溶解。还将与从墨西哥湾和北大西洋百慕大大西洋时间序列地点收集的自然生物群落一起进行粉尘培养。研究人员将表征每个实验中释放的铁的数量和同位素特征，培养过程中微生物群落对粉尘的反应(DNA、细胞计数和鉴定)，以及用于实验和培养过程中产生的天然有机物中存在的配体。通过对非生物实验和生物实验的比较，可以调查粉尘溶解的生物效应的大小。这些实验将使研究人员能够从机理上了解影响粉尘溶解速度的因素，特别是不同特定有机配体在促进和稳定海洋表层铁释放方面的作用，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条件，具体测试以下假设：由于与有机配体的络合，从尘埃中净释放的铁的同位素含量很高，但这种分馏的大小取决于结合强度和不同配体的存在。这一过程研究的结果将为诸如GEOTRACES的大规模实地研究的模型和解释提供一个框架，并增强研究社区解释气溶胶和海洋铁同位素数据的能力。这一奖项反映了NSF的法定使命，并通过使用基金会的智力价值和更广泛的影响审查标准进行评估，被认为值得支持。

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.