Collaborative Research: In Situ Oxidation Rates of Methane Injected from Seafloor Gas Seeps

点击翻译按钮获取中文摘要

Thousands of natural gas seeps have been discovered as streams of bubbles rising up from the seafloor just offshore from coastlines around the world. Extensive fields of seeps, largely releasing the same methane gas that we use to heat our homes, have recently been found about a hundred miles east of North Carolina’s Cape Hatteras at water depths as shallow as 300 feet. It is estimated that at least tens of thousands of these hydrocarbon-rich seeps occur on continental margins around the world. The seeps, usually seen as large rising plumes by sonar systems on research ships, inject huge, but poorly quantified, amounts of methane into overlying waters as they rise through the water column. A primary question is how much of the methane, a potent greenhouse gas, actually reaches the atmosphere. This question is the subject of much current research funded by several Federal research agencies including the National Science Foundation and the Department of Energy. Oceanographers believe that the methane (and other gases included in the bubble streams such as ethane and propane) are either transported away by ocean currents or consumed by microorganisms specially adapted to live with hydrocarbons as their main carbon source. This proposal seeks to determine the importance of microbial consumption in controlling methane distributions in the deep ocean and how the consumption rates depend on concentrations of methane, oxygen and other chemicals, as well as in situ pressures and temperatures. Previous studies in laboratories aboard research ship using samples returned to the surface suggest that microbial consumption of methane from the seeps may lag for a week after its injection into the water column and thus that physical dispersion by currents may dominate deepwater methane dynamics through dilution. However, other measurements suggest that there is no lag before aggressive microbial oxidation of injected methane begins. We have proposed to conduct in situ measurements of microbial methane consumption and related microbial community structure in bottom waters at several coastal and continental margin sites off the North Carolina coast and the northern Gulf of Mexico, where numerous natural seeps have also been observed. We will use new technologies developed after the release of massive quantities of methane during the Deepwater Horizon disaster in the Gulf. The sites offer varying concentrations of methane and other chemical and physical conditions such as oxygen concentrations and temperature that will allow us to test specific hypotheses about the role of microbial processes. Through performing the experiments with instruments right on the seafloor next to the seeps we can remove much of the uncertainty surrounding previous shipboard measurements. We will use newly developed seafloor landers equipped with advanced laser methane sensors that are capable of multi-week measurements while assaying dissolved oxygen, dissolved inorganic nitrogen and the microbial community for the presence of methane-consuming methanotrophs and their activity using advanced genomics techniques that can reveal the nature of methanotrophic responses to ambient methane concentrations. Successful collection of in situ methane consumption rate data and associated microbial community changes should prove important for modeling ocean methane dynamics over a range of oceanographic conditions including seep-enriched bottom waters and bottom waters impacted by accidental hydrocarbon releases.Graduate and undergraduate students supported by the project will gain critical skills in laboratory and field settings and will also benefit from frequent interactions with established researchers from diverse fields. Team members will participate in hands-on undergraduate education and training through developing individualized research projects leading to honors theses, presentations at national meetings and excellent graduate school placements. Graduate and undergraduates will also participate in K-12 science outreach efforts that help to attract and inform the next generation of oceanographers. The team will work with the University of North Carolina Morehead Planetarium and Science Center and participate directly in the North Carolina Science Festival. Through media contacts made from TED talks, exciting results will be broadly disseminated to the public. The project will have immediate relevance for understanding microbially mediated responses to hydrocarbon inputs from accidental releases along the Southeast Atlantic margin where oil and gas exploration are a constant topic of state and national policy discussions. Hundreds of recently discovered gas seeps along the continental margin offshore of Cape Hatteras, North Carolina plus thousands in the northern Gulf of Mexico, inject huge amounts of dissolved methane into overlying shelf and slope waters through dissolution of rising bubble plumes. The fate of the methane is largely controlled by a balance between microbial oxidation and advective transport away from seep sources. The efficacy of microbial oxidation likely depends on concentrations of methane, oxygen and ambient dissolved inorganic nitrogen (DIN), as well as in situ pressures and temperatures. Recent shipboard aerobic methane oxidation rate (AMOR) measurements suggest that microbial consumption of seep methane may lag for a week and thus physical dispersion could dominate deepwater methane dynamics through dilution. However, methane stable carbon isotopic measurements in bottom waters suggest that there is no lag before aggressive oxidation of injected methane begins. We propose to conduct in situ measurements of AMOR while simultaneously investigating its microbial drivers at representative North Carolina and Gulf of Mexico continental margin sites featuring numerous active bubble seeps. These sites offer varying concentrations of methane and DIN; performing the experiments in situ will remove much of the uncertainty of shipboard rates. We will conduct the measurements utilizing newly developed benthic lander systems equipped with advanced laser methane sensors that are capable of multi-week AMOR measurements while assaying dissolved oxygen, DIN and the microbial community for the presence of methanotrophs (through metagenomes) and their activity (through metatranscriptomes) that can reveal the nature of methanotrophic responses to ambient methane concentrations.The project will test key hypotheses about deep-sea methane dynamics including determining if there are significant lags in microbial responses after exposure to elevated methane concentrations, determining the relationships of AMOR to methane and DIN concentrations and investigating the response times and magnitude of methanotrophs to spatial and temporal variability in methane concentration. Successful collection of in situ AMOR and associated microbial community data will prove important for modeling ocean methane dynamics over a range of oceanographic conditions including seep-enriched bottom waters and accidental hydrocarbon releases.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.