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

更好地了解大气中的二氧化碳（CO2）浓度及其与地球的关系？美国的温度对气候研究至关重要。来自冰芯的记录表明，二氧化碳分压（pCO2）与全球温度相关——高的pCO2意味着温暖的地球。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）。对1ma以前的二氧化碳分压的所有估计都依赖于其他追踪二氧化碳的间接测量。这些间接度量称为代理。该项目将测试一种新的二氧化碳分压替代物，称为古生菌（EpsilonAr）替代物，它基于从生活在海洋系统中的微生物中回收的脂类（一类生化物质）。EpsilonAr代理的基础来自一些有限的实地观测和一个理论模型。当前研究的主要目标是通过控制实验室实验和研究记录了最近冰期-间冰期旋回的海洋沉积物，来检验EpsilonAr代用物背后的理论。该项目跨越学科界限，需要地球生物学方法以及同位素分析和沉积记录的应用。该项目将培养一名博士后研究员和两名暑期本科生和/或高中实习生。将采用实验室实验和自然样品相结合的方法来验证海洋Thaumarchaeota脂质中记录的碳同位素分馏反映生长过程中细胞外CO2浓度的假设。Thaumarchaeota使用3-羟基丙酸/4-羟基丁酸（3HP/4HB）碳固定途径，其中主要酶乙酰/丙酰辅酶a羧化酶对底物HCO3-具有特异性。目前的假设是，Thaumarchaeota依赖于CO2扩散，然后是细胞内对HCO3的催化。该项目的初始重点是对模式分类单元亚硝基海洋亚硝酸盐（Nitrosopumilis maritimus）进行控制实验，以测试碳同位素效应及其对二氧化碳依赖（通过改变pH值、溶解无机碳（DIC）和生长速率）的定量模型。结果将与预测对二氧化碳不敏感的3HP/4HB分类群Metallosphaera sedula的对照实验进行比较。该项目的第二阶段将检查预测的EpsilonAr的同位素敏感性是否在亚热带大西洋的冰期-间冰期沉积记录中观察到。将通过比较观测到的EpsilonAr对CO2的响应和增长率与现有理论框架来评估结果，并根据需要更新和完善理论以反映实验和观测结果。该奖项反映了美国国家科学基金会的法定使命，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.