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Microbial Oxidation of Marine Hydrocarbons: Quantifying Rates of Methane, Ethane, Propane, and Butane Consumption

  • Author(s): Mendes, Stephanie Diana
  • Advisor(s): Valentine, David L
  • et al.
Abstract

Natural gases have many environmental consequences if released into the atmosphere due to their high global warming potentials. However, these gases are subject to oxidation by microorganisms, which act as an effective biofilter that limits the atmospheric input. Organisms responsible for this oxidation are present in a wide range of environments and hydrocarbon compositions. To quantify the rate at which a microbial community oxidizes and regulates the flux of hydrocarbons into an environment, a combination of chemical tracer techniques is typically applied. This work applied both stable and radioactive tracers to monitor the oxidation capacity of the microbial community in both water column and marine sediment, with hydrocarbon concentrations generally ranging from low (coastal systems near hydrocarbon reservoirs) to medium (natural seeps) to high (oil spills).

Samples containing low and medium natural gas concentrations were collected from the world's largest natural marine seep field, Coal Oil Point, located offshore of Santa Barbara, California. Natural gas emanating from Coal Oil Point first diffuses into sediment porewater before migrating into the overlying water column to form a dissolved plume. Novel radioactive tracers were developed and applied to assess the timing by which microbes metabolize these gases within both the water column and sediment. A three phase study was employed to track the dissolved hydrocarbon plume within the water column. Phase 1 synthesized tritiated ethane, propane, and butane using Grignard reagents and tritiated water; Phase 2 systematically assessed the experimental conditions, wherein the indigenous microbial community was found to rapidly oxidize ethane, propane, and butane; Phase 3 applied radioactive tracers to track microbial oxidation within a dissolved hydrocarbon plume. Spatial and temporal patterns of ethane, propane, and butane oxidation down current from the hydrocarbon seeps demonstrated that >99% of these gases were metabolized within 1.3 days following initial exposure. Estimates based on the observed metabolic rates and carbon mass balance calculations suggest that ethane-, propane-, and butane-consuming microorganisms may transiently account for a majority of the total microbial community within impacted waters.

Sediment studies focused on the metabolism of ethane oxidation from two individual seeps within the Coal Oil Point field: Campus Point Mounds and Patch Seep. Ethane oxidation was quantified in slurry incubations under anaerobic conditions, which indicated that >97% of the gas was removed after 43 days when incubated near in situ temperature and gas concentrations. This study was the first to quantify anaerobic ethane oxidation within cold seep sediments. Total ethane consumption rates from Campus Point Mounds and Patch Seep were calculated to be 2 µM day-1 and 0.012 µM day-1, respectively, revealing that anaerobic oxidation in sediment is a dynamic process capable of modulating ethane's release into the ocean and atmosphere. Ethane oxidation was also quantified in whole core profiles. These profiles showed elevated ethane oxidation rates to depths of 11 cm beneath the sea floor, but rates below detection in the underlying zone of low sulfate.

Samples containing high natural gas concentrations were collected from the Deepwater Horizon oil spill, both during and following the nearly three-month hydrocarbon release. Microbial oxidation of ethane and propane was tracked using stable isotope (13C) tracers. Within 20 weeks of the onset of hydrocarbon release, the microbial communities had "bloomed" to consume elevated concentrations of ethane and propane, and subsequently "busted" following a prolonged period of starvation. This short time interval for complete natural gas consumption contradicts initial predictions based on previous methane studies, and indicates that oxidation rates can accelerate in environments of high hydrocarbon concentrations.

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