CAREER: The Biogeochemical Controls on Hydrogen-Isotope (D/H) Fractionations in Lipids

职业：脂质中氢同位素 (D/H) 分馏的生物地球化学控制

• 批准号: 0645502
• 负责人: Alex Sessions
• 金额: $ 59.86万
• 依托单位: California Institute of Technology
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2007
• 资助国家: 美国
• 起止时间: 2007-03-01 至 2012-02-29
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0645502&HistoricalAwards=false
• 关键词: CAREER; Biogeochemical; Controls; Hydrogen; Isotope

Intellectual Merit: Compound-specific analyses of the hydrogen isotope ratio (D/H) of organic compounds are enjoying a rapid boom in popularity. These analyses are being applied to widespread topics of biogeochemical interest, including generation of paleoclimate records, identifying sedimentary organic matter sources, studying bioremediation of pollutants, identifying ancient metabolic pathways, understanding petroleum sources, and many others. One result of this rapid expansion is that the generation of organic D/H data has greatly surpassed our fundamental understanding of how to interpret such data. For example, paleoclimate records are based on the assumption that the large D/H fractionation between environmental water and plant-wax lipids is constant, yet we do not know either the biochemical basis for that fractionation or the extent to which it may vary. This proposal encompasses a number of related studies with the single goal of understanding basic biogeochemical controls on organic D/H ratios.Four areas of particular importance will be pursued. First, we will seek to understand the biochemical basis for photosynthetic D/H fractionations. The main question is whether the splitting of H2O by photosynthesis, or the reduction of carbon skeletons during biosynthesis, is the primary determinant of lipid D/H ratios. Variability in the net fractionation, and the potential for H exchange during biosynthesis, will also be investigated. We will use experimental studies of cyanobacteria and photosynthetic purple sulfur bacteria in culture to identify key biochemical steps and associated fractionations. The results will have profound importance for the interpretation of D/H climate proxies. Second, we will attempt to elucidate sources of lipid H in heterotrophic microbes. In particular, we will investigate whether the D/H ratio of lipids primarily reflects that of their food (organic substrates) or water. It is experimentally difficult to distinguish between the unknown proportion of the two sources and the two unknown fractionations associated with use of each source. Our novel approach will be to study organisms that utilize organic substrates with no hydrogen, such as oxalate. This will allow us to measure the net heterotrophic fractionation with respect to water, and then to accurately assess the uptake of organic H by a variety of other heterotrophs, and for a variety of substrates.Third, we will develop a comprehensive dataset of temperature-dependent equilibrium D/H fractionations between organic H and water. This data will immediately be useful to a wide range of studies, for example in ascertaining whether or not particular samples have been affected by H exchange and thus could serve as a proxy for paleoclimate. Our approach will combine experimental equilibration with ab initio molecular modeling to produce accurate estimates for a wide range of organic moieties. Experiments will follow a new approach we have recently tested, in which exchange of (-carbonyl positions is measured indirectly. These experiments are amenable to ketones and carboxylic acids, and will serve as key calibration points for subsequent computational estimates of hydrocarbons.Finally, we will seek to develop a robust empirical relationship between leaf-wax D/H and environmental aridity. In doing so, we will examine a variety of factors that potentially influence the net lipid/water fractionation, including water-use efficiency and water transport, variability in biochemical fractionations, seasonal changes in metabolism, and others.Broader Impacts: These studies will also provide a framework for an outreach program at local high schools and colleges. The goal is to engage young students with meaningful scientific research on a problem that matters to them: climate change. We will set up independent research projects for young students at their home schools, supervised by their own teachers. They will be provided with introductory lectures, develop hypotheses and research plans, prepare lipid extracts, vacuum extract leaf waters, submit samples for isotopic analysis in my lab, and then analyze their own data. This approach has numerous benefits, including i) the data they produce will be scientifically useful and pertinent to an important topic; ii) it uses one of the simplest possible isotopic systems (D/H evaporation) to teach concepts of paleoclimate science; iii) the scientific questions are suitable for division into small units approachable by individual students; iv) methods are safe and accessible to untrained students, allowing them to participate in nearly the complete research process; v) it provides an efficient way to export the technical expertise of Caltech to local schools, leveraging the time of students and teachers to simultaneously draw students into science while also producing useful research.

智力优势：有机化合物氢同位素比（D/H）的化合物特异性分析正在迅速普及。这些分析正被应用于地球化学感兴趣的广泛主题，包括古气候记录的生成，确定沉积有机质来源，研究污染物的生物修复，确定古代代谢途径，了解石油来源等。这种快速扩展的一个结果是，有机D/H数据的生成大大超过了我们对如何解释这些数据的基本理解。例如，古气候记录是基于这样的假设，即环境水和植物蜡脂之间的大D/H分馏是恒定的，但我们不知道这种分馏的生化基础或它可能变化的程度。该建议包括一些相关的研究，其唯一目标是了解对有机D/H比的基本生物地球化学控制。首先，我们将试图了解光合作用D/H分馏的生化基础。主要的问题是，是否通过光合作用分解H2O，或在生物合成过程中碳骨架的减少，是脂质D/H比的主要决定因素。净分馏的变化，以及在生物合成过程中H交换的潜力，也将进行研究。我们将使用蓝细菌和光合紫硫细菌在培养的实验研究，以确定关键的生化步骤和相关的分馏。研究结果对D/H气候代用指标的解释具有重要意义。其次，我们将试图阐明在异养微生物中的脂质H的来源。特别是，我们将研究脂质的D/H比是否主要反映其食物（有机底物）或水的D/H比。实验上很难区分两个源的未知比例和与每个源的使用相关的两个未知分数。我们的新方法将是研究利用不含氢的有机底物（如草酸盐）的生物。这将使我们能够测量相对于水的净异养分馏，然后准确地评估各种其他异养生物的有机H的吸收，并为各种substrates.Third，我们将开发一个全面的数据集的温度依赖性平衡D/H分馏有机H和水之间。这些数据将立即用于广泛的研究，例如确定特定样品是否受到H交换的影响，从而可以作为古气候的代表。我们的方法将结合联合收割机实验平衡与从头计算分子建模，以产生广泛的有机部分的准确估计。实验将遵循我们最近测试的一种新方法，其中（-羰基位置的交换是间接测量的。这些实验是服从酮和羧酸，并将作为随后的计算估计的碳氢化合物的关键校准点。最后，我们将寻求开发一个强大的经验叶蜡D/H和环境干旱之间的关系。在此过程中，我们将研究各种可能影响净脂质/水分馏的因素，包括水的利用效率和水的运输，生化分馏的变化，代谢的季节性变化等。更广泛的影响：这些研究还将为当地高中和大学的推广计划提供一个框架。其目标是让年轻学生参与对他们来说很重要的问题的有意义的科学研究：气候变化。我们将为年轻学生在他们的家乡学校设立独立的研究项目，由他们自己的老师监督。他们将被提供介绍性讲座，发展假设和研究计划，准备脂质提取物，真空提取叶沃茨，提交样品在我的实验室进行同位素分析，然后分析他们自己的数据。这种方法有许多好处，包括i）它们产生的数据将在科学上有用，并与重要主题相关; ii）它使用最简单的同位素系统之一（D/H蒸发）教授古气候科学的概念; iii）科学问题适合于划分为学生个人易于理解的小单元;方法是安全的，未经培训的学生可以使用，使他们能够参与几乎完整的研究过程; v）它提供了一种有效的方式来输出加州理工学院的技术专长，以当地学校，利用学生和教师的时间，同时吸引学生进入科学，同时也产生有用的研究。