Gas Hydrates

coldseep_hyd2.jpgLow molecular weight hydrocarbons (methane through pentane), hydrogen sulfide, and carbon dioxide form ice-like clathrates with water called "gas hydrates." Frequently, methane is the dominant gas, so these structures are referred to as "methane hydrates" or "methane clathrates." Submarine methane hydrates comprise a large reservoir of labile organic carbon, representing one of the most important reservoirs of organic carbon on Earth. Methane hydrate forms under conditions of methane saturation, high pressure, and low temperature by the inclusion of gas molecules, primarily methane, but also higher hydrocarbon gases up to pentane, into a lattice of water molecules. The resulting solid is stable at temperatures below about 7 ºC and pressures greater than about 50 bar. The widespread occurrence of gas hydrates became apparent when deeply buried layers, known as bottom simulation reflectors (BSR), were detected by seismic and deep-sea drilling studies. BSR features are widely distributed at the base of continental margins at sub-bottom depths of about 300m. In the Gulf of Mexico, hydrates occur as shallow layers and vein-filling plugs that breach the sediment interface and slowly decompose in contact with seawater. The "extreme" nature of the hydrate niche includes low temperature and high pressure, limited carbon sources (C1-C5), and desiccation (little available free water). Thus microbes living in this environment require special adaptations.

Little is known about the types of microorganisms dwelling in hydrate environments, their phylogenetic diversity, taxonomy, ecology, or ecophysiology. We are testing the hypotheses that hydrate microbes inhabit distinct microbial niches requiring specialized adaptation, that these niches have predictable temporal and spatial characteristics, and that there is dynamic interaction between microbial activity and the geochemistry of the system. We use molecular biological and organic geochemical techniques to assess the microbial community composition and identify microorganisms previously unidentified. Rates of processes are determined using classic biogeochemical methods.

Methane hydrates are found along continental margins around the world and represent a unique extreme environment that could serve as a novel niche for microbial life. The picture on the left shows a hydrate breaching the surface of the sediment, and the picture on the right shows a close up view of the hydrate surface. The hydrate surface is very uneven because hydrates are somewhat unstable and dissolution may cause pitting on the surface. Hydrates are either white or orange in color; the orange coloration results from the incorporation of oil in the ice lattice. These structure II gas hydrates are rich C2-C6 alkanes, hydrogen sulfide, and carbon dioxide.

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