Collaborative Research: Autotrophic carbon fixation at a shallow-water hydrothermal system: Constraining microbial activity, isotopic and geochemical regimes

目前，只有有限的信息的身份和活动的微生物进行CO2固定在原地，尽管事实上，这些生物体形成的基础上，其各自的生态系统。已知能够自养生长的代表存在于几乎所有主要的原核生物群中，这些生物通过为异养生物提供持续的有机碳供应而在生态系统中发挥重要作用。 存在于极端环境中的微生物利用除Calvin-Benson-Bassham（CBB）循环之外的CO2固定途径。目前，已知五种可供选择的自养CO2固定途径。 不同的碳固定途径导致所产生的生物质的不同同位素特征，这是由于羧化酶在轻碳（12 C）和重碳（13 C）之间的同位素区分。因此，关于微生物群落主要利用的碳固定途径的推断也可以基于现存系统以及地质记录中有机物的稳定碳同位素组成来进行。 然而，目前很少有人知道的系统学和分馏的程度，在碳固定的原核生物，据我们所知，没有研究存在，系统地研究了不同的碳固定途径的操作之间的关系，这是如何反映在稳定的碳同位素组成的自然系统。 这是一个为期2年的跨学科，国际研究计划，采用了强大的尖端研究工具的组合，旨在提高我们对自养碳固定及其化学和同位素特征的理解，沿着自然热液系统的环境梯度。本文提出了以下假设：1.微生物的多样性呈现沿着热和氧化还原梯度，以及CO2固定率，将反映对原位温度和地球化学条件的适应2。利用CBB循环的自养CO2固定的微生物将代表在较高温度下的化能自养群落的较小百分比，其中利用替代CO2固定途径的微生物占主导地位3。 生物质和特定生物标志物分子的同位素值将沿着热梯度和氧化还原梯度从特征在于较高热液流体通量和因此较高温度的区域到周围较冷的区域变化，PI将使用多学科方法来描述不同碳固定途径的相对贡献，沿着通过将宏基因组分析与以下相结合来评估环境梯度：1）评估参与碳固定的特定关键基因的频率和表达，以及2）测量碳固定率。 这些数据将与生物质、DIC和特定烃/脂质的稳定C同位素组成的测定相结合。 米洛斯（希腊）附近的浅水喷口由于其容易进入、环境梯度完善和背景资料广泛，将被用作进行这些研究的天然实验室。 智力优势。在这项研究中产生的数据将允许在自然系统中的自养碳固定和生物质和特定生物标志物分子（例如CH 4，C2+烷烃，脂质）的同位素特征之间的关系的约束。这对评估碳固定在现存生态系统中的重要性具有影响，它还将提供一种工具，以改进对地质记录中同位素值的解释。这是美国和外国机构之间的跨学科和合作努力，为建立网络和促进国际合作创造了独特的机会。这也将有利于参与的学生（1研究生，几个本科生）和博士后。多年来，PI参与了几项教育和公共宣传活动，这些活动已经接触到数百万人。最后，该项目符合一些多学科和国际倡议的重点，参与者是这些倡议的积极成员（例如，SCOR热液能源和海洋碳循环工作组; CIW的深层碳观测站）。

Currently, there is only limited information on the identity and activity of the microorganisms carrying out CO2-fixation in situ, despite the fact that these organisms form the basis of their respective ecosystems. Representatives that are able to grow autotrophically are known to exist in almost all major groups of prokaryotes, and these organisms play essential roles in ecosystems by providing a continuous supply of organic carbon for heterotrophs. Microorganisms present in extreme environments utilize CO2- fixation pathways other than the Calvin-Benson-Bassham (CBB) cycle. At present, five alternative autotrophic CO2 fixation pathways are known. Different carbon fixation pathways result in distinct isotopic signatures of the produced biomass due to the isotopic discrimination between light (12C) and heavy (13C) carbon by the carboxylating enzymes. Thus, inferences about the carbon fixation pathway predominantly utilized by the microbial community can also be made based on the stable carbon isotopic composition of the organic matter, in extant systems as well as in the geological record. However, at present little is known about the systematics and extents of fractionation during carbon fixation by prokaryotic organisms, and to our knowledge no studies exist that have systematically studied the relationship between the operation of different carbon fixation pathways and how this is reflected in the stable carbon isotopic composition in a natural system. This is a 2-year interdisciplinary, international research program that employs a powerful combination of cutting-edge research tools aiming to improve our understanding of autotrophic carbon fixation and its chemical and isotopic signature along environmental gradients in a natural hydrothermal system. The following hypotheses are addressed: 1. The diversity of microorganisms present along a thermal and redox gradient, and rates of CO2 fixation, will reflect adaptation to in situ temperatures and geochemical conditions 2. Microorganisms utilizing the CBB cycle for autotrophic CO2-fixation will represent a smaller percentage of the chemolithoautotrophic community at higher temperatures, where microorganisms utilizing alternative CO2-fixation pathways dominate 3. Isotopic values of biomass and specific biomarker molecules will vary along a thermal and redox gradient from zones characterized by a higher hydrothermal fluid flux and thus higher temperatures to the surrounding, cooler areas, corresponding to the physiology of the microorganisms utilizing different pathways for carbon fixationThe PIs will use a multidisciplinary approach to delineate the relative contribution of the different carbon fixation pathways along an environmental gradient by combining metagenomic analyses coupled with: 1) an assessment of the frequency and the expression of specific key genes involved in carbon fixation, and 2) with the measurement of carbon fixation rates. These data will be integrated with the determination of stable C isotopic composition of biomass, DIC, and specific hydrocarbons/lipids. Due to its easy accessibility, well-established environmental gradients, and extensive background information, the shallow-water vents off Milos (Greece) will be used as a natural laboratory to perform these studies. Intellectual Merit. The data generated in this study will allow constraints on the relationship between autotrophic carbon fixation and the resulting isotopic signatures of biomass and specific biomarker molecules (e.g. CH4, C2+ alkanes, lipids) in a natural system.. This has implications for assessing the importance of carbon fixation in extant ecosystems, and it will also provide a tool to improve the interpretation of isotopic values in the geological record.Broader Impacts. This is an interdisciplinary and collaborative effort between US and foreign institutions, creating unique opportunities for networking and to foster international collaborations. This will also benefit the involved students (1 graduate, several undergraduates) and a postdoc. The PIs have been involved in several educational and public outreach activities over the years that have reached literally millions of individuals. Finally, the project fits with the focus of a number of multi-disciplinary and international initiatives, in which PIs are active members (e.g. SCOR working group on Hydrothermal energy and the ocean carbon cycle;and Deep Carbon Observatory at CIW).