Lipid D/H ratios as a proxy for microbial metabolism

氢的两种稳定同位素——质子(H)和氘(D)的相对丰度变化，是地球科学家可用的有用的天然示踪剂之一。例如，极地冰芯D/H比值的变化可以说是过去冰期/间冰期旋回的最佳记录。但目前对它们在生物圈中的行为的了解相当有限。我们知道，相对于D，植物优先吸收H，因此相对于环境水，有机物质在D中被强烈消耗。然而，尚不清楚这些同位素在消费者（动物或微生物）体内的行为，以及同位素信号如何在生态系统中传播。反过来，如何解释对地球科学界至关重要的古代有机质D/H记录也不为人所知。古代有机记录只记录水文循环（即气候），还是也记录生态系统的变化？这一建议从微生物消费者的角度解决了这个基本问题：细菌、古细菌和以食用有机物质为生的微观真核生物。拟议的研究试图回答这些问题：它们如何改变有机物中的D/H比率，以及这种变化是否与它们的代谢类型相关。这个问题将通过在明确的条件下使用培养物来解决，然后直接测量代表沉积有机记录的脂质分子的D/H比。一些支持的生化测量将被用来帮助我们理解为什么这些变化存在的机制基础。更广泛的影响包括开发一个可访问网络的数据库，用于存档和搜索生物D/H数据，涉及研究生和本科生，并将研究成果纳入国际地理课程以及标准期刊出版物和会议报告。测量单个脂质D/H比的方法已经存在了十多年，但几乎所有这些努力都只针对植物和浮游植物，目的是重建古气候。然而，最近的数据表明，在我们周围的世界里，同位素分馏的复杂性要高得多，这对其他类型的研究也很有用。异养微生物脂质äD值的变化高达500？这些信号似乎是由宿主代谢控制的，而不是生长底物的D/H比。如果是这样的话，自然形成的脂质中的氢同位素就有可能用于追踪现代和古代环境中的微生物代谢，这是一个重要的科学目标。本提案的目的是开始了解除植物以外的生物中这种变异的范围和机制基础，并最终将该方法推向成为地球生物学的有用工具。为此，PI将开展三个相关的研究方向：i) PI将开展广泛的、基于培养的微生物D/H分异调查，包括细菌、古生菌和真核生物，使用一系列有氧和无氧代谢。预计研究约10-15种不同的生物，每种生物在一系列条件下和不同的基质上生长。ii)为了更深入地了解这种分离的机理，PI将采用更有针对性的生化研究，使用NAD(P)H和转氢酶活性的丰度测量来了解影响细胞还原等价物同位素组成的关键生化过程。iii) PI将开发氨基酸的化合物特异性测量方法，这将使我们能够检查一类更能代表散装生物质的化合物，以及更能诊断生物来源的特定蛋白质。

Variations in the relative abundance of the two stable isotopes of hydrogen, protium (H) and deuterium (D), are one of the useful natural tracers available to Earth Scientists. For example, changes in the D/H ratio of polar ice cores are arguably the best record of past glacial/interglacial cycles. But current understanding of their behavior in the biosphere is rather limited. It is known that plants preferentially take up H relative D, and so organic matter is strongly depleted in D relative to environmental water. However, it is not known how these isotopes behave in consumers, either animals or microbes, and how isotopic signals propagate through ecosystems. In turn, it is not well known how to interpret ancient records of organic matter D/H, of primary importance to the Earth Science community. Do ancient organic records only record the hydrologic cycle (i.e. climate), or do they also record changes in ecosystem? This proposal tackles that fundamental problem from the standpoint of microbial consumers: bacteria, archaea, and microscopic eukaryotes that live by eating organic materials. The proposed research seeks to answer the questions of how they alter D/H ratios in organic matter, and whether such changes are correlated with their type of metabolism. The problem will be addressed through the use of cultures under well-defined conditions and then directly measuring the D/H ratios of lipid molecules that would be representative of the sedimentary organic record. A number of supporting biochemical measurements will be used to help us understand the mechanistic basis for why such variations exist. The broader impacts consist of developing a web-accessible database for archiving and searching biologic D/H data, involving graduate and undergraduate students, and incorporating research results into an International Geobiology Course as well as standard journal publications and conference presentations. Methods for measuring the D/H ratios of individual lipids have existed for more than a decade, yet almost all such efforts have been directed exclusively towards plants and phytoplankton, with the goal of paleoclimate reconstruction. Recent data suggest, however, that there is a much richer complexity of isotopic fractionations in the world around us, and that it could also be useful for other types of investigations. Variations in the äD values of heterotrophic microbial lipids of up to 500? have been observed, and these signals appear to be controlled by host metabolism, rather than the D/H ratio of growth substrates. If so, H isotopes in naturally occurring lipids could potentially be used to trace microbial metabolism, both in modern and ancient environments, an important scientific goal. The objective of this proposal is to begin to understand the scope and mechanistic basis of such variations in organisms other than plants, and to ultimately move the methodology toward being a useful tool for geobiology. To do this, the PI will undertake three related lines of research: i) The PI will undertake a broad, culture-based survey of microbial D/H fractionations, including bacteria, archaea, and eukaryotes using a range of aerobic and anaerobic metabolisms. The anticipates studying ~10-15 different organisms, each grown under a range of conditions and on different substrates. ii) To obtain a deeper mechanistic understanding of such fractionations, the PI will employ more targeted biochemical studies using abundance measurements of NAD(P)H and transhydrogenase enzyme activity to understand key biochemical processes affecting the isotopic composition of cellular reducing equivalents. iii) The PI will develop compound-specific measurements for amino acids, which will allow us to examine a class of compounds more representative of bulk biomass, as well as specific proteins that are more diagnostic for biotic sources.