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Appl. Environ. Microbiol. doi:10.1128/AEM.02685-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Consumption of methane and CO₂ by methanotrophic microbial mats from gas seeps of the anoxic Black Sea

Max Planck Institute for Marine Microbiology, Department of Biogeochemistry, Celsiusstrasse 1, D-28359 Bremen, Germany; California Institute of Technology, Division of Geological and Planetary Sciences, 1200 East California Boulevard, Pasadena, CA 91125-7800, USA; Penn State Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, 239 Deike Building, University Park, PA 16802, USA; International University Bremen, Research II, Campusring 1, D-28759 Bremen, Germany

^* To whom correspondence should be addressed. Email: treude{at}usc.edu.

	Abstract

The deep anoxic shelf of the northwestern Black Sea hosts numerous gas seeps, which are populated by methanotrophic microbial mats in and above the seafloor. Above the seafloor, the mats can form tall reef-like structures composed of porous carbonate and microbial biomass. Here, we investigated spatial patterns of CH₄ and CO₂ assimilation in relation to the distribution of ANME groups and their associated bacteria in mat samples obtained from the surface of a large reef structure. A combination of different methods including radiotracer incubations, beta-microimaging, secondary ion mass spectrometry, and catalyzed reporter deposition fluorescence in situ hybridization was applied to sections of mat obtained from a large reef structure to locate hot spots of methanotrophy and identify the responsible microbial consortia. Furthermore, CO₂ reduction to methane was investigated in the presence or absence of methane, sulfate and hydrogen. The mat had an average ¹³C carbon isotopic signature of -67.1 indicating that methane is the main carbon source. Regions dominated by ANME-1 showed significantly heavier isotope signatures (-66.4, ± 3.9 s.d., n=7) than the more central regions dominated by ANME-2 (-72.9, ± 2.2 s.d., n=7). Incorporation of ¹⁴C from radiolabeled CH₄ or CO₂ indicated one hot spot for methanotrophy and CO₂ fixation close to the surface of the mat, and a low assimilation efficiency of 1-2% of methane oxidized. Replicate incubations of mat with ¹⁴CH₄ or ¹⁴CO₂ revealed an interconversion of CH₄ and CO_2. CO₂ reduction was about 10% of the anaerobic oxidation of methane. However, since considerable methane formation was observed only in the presence of methane and sulfate the process appeared to be rather a re-reaction of AOM than net methanogenesis.

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