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Applied and Environmental Microbiology, April 2007, p. 2271-2283, Vol. 73, No. 7
0099-2240/07/$08.00+0     doi:10.1128/AEM.02685-06
Copyright © 2007, American Society for Microbiology. 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,¹ California Institute of Technology, Division of Geological and Planetary Sciences, 1200 East California Boulevard, Pasadena, California 91125-7800,² Penn State Astrobiology Research Center and Department of Geosciences, Pennsylvania State University, 239 Deike Building, University Park, Pennsylvania 16802,³ International University of Bremen, Research II, Campusring 1, D-28759 Bremen, Germany⁴

Received 16 November 2006/ Accepted 20 January 2007

The deep anoxic shelf of the northwestern Black Sea has 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 the 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 incubation, beta microimaging, secondary ion mass spectrometry, and catalyzed reporter deposition fluorescence in situ hybridization, was applied to sections of mat obtained from the large reef structure to locate hot spots of methanotrophy and to identify the responsible microbial consortia. In addition, 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 was the main carbon source. Regions dominated by ANME-1 had isotope signatures that were significantly heavier (–66.4 ± 3.9 [mean ± standard deviation; n = 7]) than those of the more central regions dominated by ANME-2 (–72.9 ± 2.2 ; n = 7). Incorporation of ¹⁴C from radiolabeled CH₄ or CO₂ revealed one hot spot for methanotrophy and CO₂ fixation close to the surface of the mat and a low assimilation efficiency (1 to 2% of methane oxidized). Replicate incubations of the mat with ¹⁴CH₄ or ¹⁴CO₂ revealed that there was interconversion of CH₄ and CO_2. The level of CO₂ reduction was about 10% of the level of anaerobic oxidation of methane. However, since considerable methane formation was observed only in the presence of methane and sulfate, the process appeared to be a rereaction of anaerobic oxidation of methane rather than net methanogenesis.

Published ahead of print on 2 February 2007.

Publication no. GEOTECH-258 of the R&D program GEOTECHNOLOGIEN.

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