Gas hydrates and hydrocarbon seeps are a significant global environmental force, which could potentially disrupt the flow of carbon or nutrient-delivery processes and lead to fundamental changes in the diversity and function of the openocean ecosystem. Microorganisms play a critical role in the formation and degradation of gas hydrates. The Gulf of Mexico is one of few places on Earth, where the dynamics of hydrocarbon flux and hydrate formation or dissolution have been studied extensively. A sea-floor observatory is being established in the Mississippi Canyon region (site MC 118) in the Gulf of Mexico to monitor in space and time the dynamics and environmental impact of gas hydrates. Little research has been done on microbiology or biogeochemistry at this site. One of our research objectives is to trace the carbon flow mediated by microorganisms in the hydrate environment using an integrated approach of lipid biomarkers and carbon-isotope signatures. The goal here is to summarize microbiological and geochemical research that has been conducted in the Grand Canyon region of another gas hydrate location in the Gulf of Mexico and provide an overview on research to be performed at MC 118.
Marine gas hydrates are formed at high pressure and low temperature on continental slopes/rises and are often associated with hydrocarbon vents, seeps, or mud volcanoes. Modern gas hydrates and seeps have been found globally atdeep-sea spreading centers, convergent plate boundaries, and passive continental margins (26, 32, 33). Relic gas hydrateand seep-locations have been found in geologic formations as old as the Devonian period (43). It is estimated that approximately 1019 g of methane carbon currently resides in gas hydrates-roughly 3000 times the modern atmospheric methane concentration (25-27). In addition, mud volcanos andseeps may generate 10.2-12.6 Tg (1012 g) yr−1 of CH4 from onshore and shallow offshore oceans (10, 31).
These estimates emphasize the potential importance of methane gas hydrates, mud volcanoes, and cold hydrocarbon seeps in global carbon cycle that needs to be re-evaluated based on this large dynamic hydrocarbon capacitor (8). Forexample, hydrocarbon seeps and vents emit large volumes of 13C-depleted gases, which affect the carbon pool and isotopic signatures of the oceanic system (8, 17, 33). Prominent negative 13C excursions are observed to co-occur with past intervals (Eocene to Precambrian) of abrupt environmental changes, which are attributed to methane oxidation from marine gas hydrates (3, 9, 13, 15, 19, 20).
Microorganisms play an essential role in mitigating methane release from marine systems into the atmosphere (29, 44, 51). For example, Reeburgh (44) estimates that more than 80% of methane produced each year in the anoxic marine sediments is consumed through the zone of sulfate reduction. Significant advances have been made in recent years in understanding the microbial communities intercepting and consuming methane from anoxic environments, particularly in methane seeps and associated gas hydrates (4, 6, 12, 14, 22- 24, 36, 39-40, 54-56).
