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Abundance, diversity, and activity of anaerobic propane- and butane-oxidizing microorganisms at a hydrothermal hydrocarbon seep

Jeff Seewald (Co-PI, MC&G, WHOI)
Friedrich Widdel (Max-Planck-Institute for Marine Microbiology, Bremen, Germany)
Florin Musat (Max-Planck-Institute for Marine Microbiology, Bremen, Germany)
Ulrike Jaekel (Max-Planck-Institute for Marine Microbiology, Bremen, Germany)

In many sedimentary environments, hydrocarbons can reach high concentrations and are potentially important substrates for microbial communities. One of the most spectacular places where large quantities of hydrocarbons are naturally introduced into the biosphere is the deep-sea hydrothermal vent site in Guaymas Basin, Gulf of California. Because anaerobic biodegradation of hydrocarbons has profound implications for our understanding of factors that regulate life and carbon cycling on earth because it greatly expands the range of geologic environments that may support life, it has been the focus of numerous investigations in recent years. At present, little is known about an aerobic and even less about an anaerobic degradation of these hydrocarbons, although there is geochemical evidence for the microbial utilization of propane, n-butane, and n-pentane under anoxic conditions at gas seeps. Recently, cultures have been obtained with the involvement of the PI from Guaymas Basin that are able to anaerobically degrade propane and butane as sole growth substrates under sulfate-reducing conditions. Currently, however, we know little regarding their ecological importance, i.e., their abundance, distribution, and activity in situ. Furthermore, in view of the limitations of cultivation techniques, an even greater diversity is expected to be responsible for oxidation of gaseous hydrocarbons in situ. Our proposed studies are designed to test the following general hypotheses:

1. Organisms capable of anaerobic degradation of propane and butane are an important  part of the microbial communities at Guaymas Basin and contribute to carbon cycling in these sediments.

2. The distribution of these organisms in the sediment is not uniform, with highest numbers in zones of active fluid flow and supply of low molecular weight hydrocarbons.

3. The natural diversity of organisms responsible for anaerobic oxidation of gaseous hydrocarbons in situ is not adequately represented among the currently isolated and enriched cultures.

To address these hypotheses we are proposing to collect sediment cores and vent fluids for microbiological and geochemical studies from the sediment-covered hydrothermal system at Guaymas Basin. Fluorescence in situ hybridization (FISH) in combination with geochemical analyses will be performed to assess the abundance and distribution of the previously enriched and isolated organisms. We will also utilize stable isotope probing (SIP) to identify microorganisms carrying out anaerobic propane and butane oxidation in situ, and enrich and isolate novel organisms utilizing these hydrocarbons under a wider range of conditions. The principal intellectual merit of this project is two-fold. First, this project will for the first time combine culture-independent and -dependent methods with geochemical analyses to comprehensively assess the ecological role and importance of microorganisms capable of anaerobic degradation of gaseous hydrocarbons other than methane. Second, based on previous work in Guaymas Basin as well as other hydrocarbon seeps and hydrothermal environments, we will likely discover new major lineages of microorganisms involved in this process. The broader impacts of this project include the web-based dissemination of data, facilitating the exchange of ideas, and promoting educational and outreach activities. We will train undergraduate and graduate students in modern methods of microbial ecology, geobiology, and geochemistry.


This project is funded by the NSF Microbial Observatory and Microbial Interactions and Processes program.

At locations where hydrothermal fluids seep out of the sediment in Guaymas Basin, Gulf of California, extensive mats composed of filamentous sulfur-oxidizing bacteria of the genus Beggiatoa and other microbes develop on top of the sediment. These organisms utilize reduced sulfur-compounds contained in the hydrothermal fluids and oxygen and/or nitrate contained in the entrained seawater. Below these mats, anaerobic microbes utilize organic matter either produced by these microbes or through thermal alteration of the organic rich sediments. In other locations, hydrocarbons, including ethane, propane, and butane, formed deep-down in the sediments under high pressure and temperatures are seeping out of the sediment, supporting a diverse, yet poorly characterized suit of microbes specialized in degrading these compounds anaerobically