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A Microbial Mat of a Large Sulfur Bacterium Preserved in a Miocene Methane-Seep Limestone

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A Miocene methane-seep limestone from the Romagna Apennine (Pietralunga, Italy) was found to contain an extraordinarily well-preserved microbial mat consisting of filamentous fossils. Individual filaments of the lithified Pietralunga mat are 50 to 80μm in diameter and resemble the sulfide-oxidizing bacteriumBeggiatoa. Mats of sulfur bacteria are common around modern methane-seeps, but have not yet been reported from ancient seep limestones. This is thought to be related to the conditions prevailing in metabolically active mats of sulfur bacteria that do not favor carbonate formation. The preservation of the Pietralunga mat was most likely caused by a sudden change from oxidizing to anoxic conditions, leading to the rapid carbonate precipitation induced by anaerobic oxidation of methane. Lipid biomarkers specific for archaea and sulfate-reducing bacteria linked with the anaerobic oxidation of methane co-occur with compounds derived from methanotrophic bacteria and ciliates. These findings confirm a close proximity of oxic and anoxic conditions, as required for the growth of sulfide-oxidizing bacteria in the methane-based ecosystem. The lack of earlier reports on fossilized thiotrophic mats in seep limestones is most likely related to the rarity of environmental changes rapid enough to preserve the filaments rather than to a lower frequency of thiotrophic mats around methane-seeps in the geological past.
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Keywords: Tertiary; bacteria; biomarker; carbonates; chemolithotrophic sulfide oxidation; methane-seeps

Document Type: Research Article

Affiliations: 1: Research Center for Ocean Margins, Bremen University, Bremen, Germany 2: Center for Geosciences, Göttingen University, Göttingen, Germany 3: Marine Geology Division, Institute of Marine Sciences, CNR Bologna, Italy 4: Department of Geological Sciences, University of Alabama, Tuscaloosa, Alabama, USA 5: Institute of Biogeochemistry and Marine Chemistry, Hamburg University, Hamburg, Germany

Publication date: 01 June 2004

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