Continental margins are localized between the limit of the continental shelf and the abyssal plain.
Surrounding nearly all continents is a shallow extension known as the continental shelf.
Active margins are found on the leading edge of a continent where subduction occurs. And a passive margin is the transition between oceanic and continental crust which is not an active plate margin.
The continental shelf regions also contain a high amount of marine sediment accumulation.
The rising of mud, fluid and gas (mainly reduced compounds CH4 and H2S) at seep ecosystems in continental margins, allowed the development of highly specialized chemotrophic communities which are good indicators of active gas seeps. Large amount of methane are stored in sediments of continental margins as dissolved in pore water, free gas, and as crystalline gas hydrate. Origin of methane at cold seep is determined by carbon isotopic fractionation. Thus origin is biogenic, thermogenic or mix. Only 3% of the methane from subsurface reservoir reach the atmosphere. Molecular and isotopic data suggest that most of the methane produces in marine sediment is of biogenic origin. And biogenic methane is a product of organic matter degradation by methanogens. However, the studies of methanogens that inhabit permanently cold marine sediment are limited.
In anoxic sediments controlled by diffusion fluxes, all the methane produced by methanogenesis is oxidized in the methane-sulphate transition zone. Most of the methane which rises from subsurface reservoirs is consumed by anaerobic microorganisms inhabiting sulphate-penetrated sediment layer before it reaches the seafloor. This microbially mediated Anaerobic Oxidation of Methane (AOM) controls the emission of the greenhouse gas methane from the ocean to the atmosphere. The metabolic process of AOM is proposed to be a reversed methanogenesis coupled to the reduction of sulphate involving methanotrophic archaea (ANME archaea) and sulphate-reducing bacteria (SRB). ANME archaea and SRB are assumed to interact syntrophically and form microbial consortia. Recent study revealed that AOM process dominates in marine sediments and consumes 60 to 100% of the methane flux. This metabolic pathway produces also large amount of hydrogen sulphur (H2S) which mediate thiotrophic communities like symbiotic animal and filamentous bacteria.
In addition, in the oxic zone of the sediment (first millimetres), methane could be oxidized by aerobic methanotrophs (Mox, Methane aerobic oxidation). Methanotrophic bacteria consumes around 80% of the methane produced in subsurface sediment.
In the frame of the European program HERMES, our objectives within the UMR6197 laboratory are the investigation of diversity and biogeography of methanogens at seep ecosystems in continental margins.
Our main objectives are :
- the identification of key microorganisms and their biogeochemical pathways at cold seeps and anoxic ecosystems
- and the characterization ofdominant microbial populations mediating methanogenesis at seep in connection to deep biosphere.
More information on microbiology in extreme environments in Mediterranean Sea is available at the MEDECO cruise website and more specifically file n°4.