"Carbon isotope fractionation in Archaea using the 3HP/4HB pathway: Prospects for paleo-geochemistry and paleo-barometry"

更好地了解大气二氧化碳（CO2）浓度及其与地球的关系？温度对气候研究至关重要。 冰芯记录显示，二氧化碳分压（pCO 2）与全球气温相关-pCO 2高意味着地球温暖。 然而，冰芯记录仅限于地球上最古老的冰的年龄，只有100万年（Ma）。 所有对超过1 Ma的pCO 2的估计都依赖于跟踪CO2的其他间接测量。这些间接测量称为代理。 该项目将测试一种新的pCO 2代用指标，称为pCO 2代用指标（EpsilonAr），该代用指标基于从海洋系统中的微生物中回收的脂质（一类生化物质）。EpsilonAr代理的基础来自于一些有限的实地观测和理论模型。目前研究的主要目标是测试EpsilonAr代用品背后的理论，既使用受控的实验室实验，也通过研究记录最近冰川-间冰期循环的海洋沉积物。该项目跨越学科界限，需要地质生物学方法沿着同位素分析和对沉积记录的应用。 该项目将培养一名博士后研究员和两名暑期本科和/或高中实习生。实验室实验和天然样品的组合将被用来测试海洋Thaumarchaeota脂质中记录的碳同位素分馏反映生长过程中胞外CO2浓度的假设。Thaumarchaeota使用3-羟基丙酸/4-羟基丁酸（3 HP/4 HB）途径进行碳固定，其中主要酶乙酰/丙酰辅酶A羧化酶对底物HCO 3-具有特异性。目前的假设是Thaumarchaeota依赖于CO2扩散，然后通过细胞内催化HCO 3。 该项目的最初重点是对模式分类Nitrosopumilis maritimus进行控制实验，以测试碳同位素效应及其对CO2依赖性的定量模型（通过改变pH值，溶解无机碳（DIC）和生长速率）。 结果将与嗜热菌Metallosphaera sedula的对照实验进行比较，Metallosphaera sedula是一种3 HP/4 HB分类群，预计对CO2不敏感。 该项目的第二阶段将是检查是否在亚热带大西洋的冰川-间冰期沉积记录中观察到预测的EpsilonAr同位素敏感性。 结果将通过比较观测到的EpsilonAr对CO2的响应和增长率与现有的理论框架进行评估，根据需要更新和完善理论以反映实验和观测结果。该奖项反映了NSF的法定使命，并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，被认为值得支持。

A better understanding of atmospheric carbon dioxide (CO2) concentration and its relationship to Earth?s temperature is essential to climate research. Records from ice cores show that the partial pressure of carbon dioxide (pCO2) correlates with global temperatures -- high pCO2 implies a warm Earth. However, ice core records are limited to the age of Earth's oldest ice, which is only ~1 million years (Ma). All estimates of pCO2 from older than 1 Ma rely on other indirect measurements that track CO2. These indirect measurements are called proxies. This project will test a new pCO2 proxy called the Archaea (EpsilonAr) proxy, which is based on lipids (a class of biochemicals) recovered from microbes living in marine systems. The basis for the EpsilonAr proxy originated from some limited field observations and from a theoretical model. The primary objective of the current research is to test the theory behind the EpsilonAr proxy, both using controlled laboratory experiments and by studying marine sediments that record a recent glacial-interglacial cycle. The project crosses disciplinary lines, requiring geobiological methods along with isotopic analyses and applications to the sedimentary record. The project will train a post-doctoral investigator and two summer undergraduate and/or high school interns.A combination of laboratory experiments and natural samples will be used to test the hypothesis that the carbon isotope fractionation recorded in lipids of marine Thaumarchaeota reflects the extracellular CO2 concentrations during growth. Thaumarchaeota use the 3-hydroxypropionate/4-hydroxybutyrate (3HP/4HB) pathway of carbon fixation in which the primary enzyme, acetyl/propionyl-CoA carboxylase, is specific for the substrate HCO3-. The current hypothesis is that Thaumarchaeota are dependent on CO2 diffusion, followed by intracellular catalysis to HCO3 . The initial focus of the project will be to conduct controlled experiments on the model taxon Nitrosopumilis maritimus to test a quantitative model of the carbon isotope effect and its dependence on CO2 (by varying pH, dissolved inorganic carbon (DIC), and growth rate). The results will be compared to control experiments with the thermophile Metallosphaera sedula, a 3HP/4HB taxon predicted to be CO2-insensitive. The second stage of the project will be to examine whether the predicted isotopic sensitivity of EpsilonAr is observed in a glacial-interglacial sedimentary record from the subtropical Atlantic Ocean. Outcomes will be evaluated by comparing the observed EpsilonAr response to CO2 and growth rate with the existing theoretical framework, updating and refining the theory as needed to reflect the experiments and observations.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.