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Autotrophic Carbon Fixation at a Shallow-water Hydrothermal System: Constraining Microbial Activity, Isotopic and Geochemical Regimes

Collaborators:
Dionysis Foustoukos (Co-PI, Geophysical Lab, Carnegie Inst., Washington DC)
Costa Vetriani (Co-PI, Rutgers University)
Solveig Bühring (MARUM Bremen, Germany)
Nadine Le Bris (Banyuls-sur-Mer, France)

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 CO2fixation 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. To fill these gaps, we propose 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. Specifically, we will address the following hypotheses:

  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 fixation

To address these hypotheses we propose 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, we propose to use the shallow-water vents off Milos (Greece) as a natural laboratory to perform these studies. The data generated in this study will allow us to constrain 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, which does not only have implications for assessing the importance of carbon fixation in extant ecosystems, but it will also provide a tool to improve the interpretation of isotopic values in the geological record.

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This project is just starting and is funded by NSF Biological Oceanography. This project is also part of a collaboration with Dr. Solveig Bühring who received an Emmi Noether award by the German Science Foundation to establish a group focusing on the geomicrobiology of shallow-water hydrothermal vents.