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

目前，关于在现场进行二氧化碳固定的微生物的身份和活动的信息有限，尽管这些微生物构成了各自生态系统的基础。已知能够自养生长的代表生物存在于几乎所有主要的原核生物群体中，这些生物通过为异养生物提供持续的有机碳供应，在生态系统中发挥着至关重要的作用。极端环境中的微生物利用卡尔文-本森-巴沙姆(CBB)循环以外的二氧化碳固定途径。目前已知五种可供选择的自养CO2固定途径。由于羧化酶对轻碳(12C)和重碳(13C)的同位素区分，不同的碳固定途径导致产生的生物质具有不同的同位素特征。因此，在现存系统和地质记录中，也可以根据有机质的稳定碳同位素组成来推断微生物群落主要利用的固碳途径。然而，目前对原核生物固碳过程中分馏的系统学和程度知之甚少，据我们所知，还没有研究系统地研究不同固碳途径之间的操作之间的关系，以及这种关系如何反映在自然系统中稳定的碳同位素组成中。这是一个为期两年的跨学科的国际研究计划，它采用了尖端研究工具的强大组合，旨在提高我们对自然热液系统中自养碳固定及其沿环境梯度的化学和同位素特征的理解。提出了以下假设：1.沿着热和氧化还原梯度存在的微生物的多样性，以及固定二氧化碳的速率，将反映对就地温度和地球化学条件的适应2.利用CBB循环进行自养二氧化碳固定的微生物将在较高温度下的化学自养群落中所占比例较小，其中利用替代固定二氧化碳途径的微生物占主导地位3.生物量和特定生物标志物分子的同位素值将沿着热和氧化还原梯度变化，这些区域的特点是热液流量较高，因此温度较高，到周围较冷的地区，与利用不同固碳途径的微生物的生理学相对应的PI将使用多学科方法来描绘不同固碳途径在环境梯度上的相对贡献，方法是结合元基因组学分析：1)评估参与固碳的特定关键基因的频率和表达；2)与固碳率的测量相结合。这些数据将与生物量、DIC和特定碳氢化合物/脂类的稳定碳同位素组成的测定相结合。由于米洛斯(希腊)附近的浅水喷口很容易进入，建立了良好的环境梯度，并提供了广泛的背景信息，因此将被用作进行这些研究的天然实验室。智力上的功绩。这项研究中产生的数据将限制自然系统中自养碳固定与生物量和特定生物标志物分子(例如CH4、C2+烷烃、脂类)所产生的同位素特征之间的关系。这对评估碳固定在现有生态系统中的重要性具有重要意义，也将提供一种工具来改进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。这也将使相关学生(1名毕业生，数名本科生)和一名博士后受益。多年来，私人投资机构参与了几项教育和公共宣传活动，这些活动确实惠及数百万人。最后，该项目符合一些多学科和国际倡议的重点，在这些倡议中，私营部门是积极的成员(例如，SCOR关于热液能源和海洋碳循环的工作组；以及国际地球物理研究所的深层碳观测站)。

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).