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Appl. Environ. Microbiol. doi:10.1128/AEM.02810-06
Copyright (c) 2007, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Salinity constraints on subsurface Archaeal diversity and methanogenesis in a sedimentary rock rich in organic matter

Department of Microbiology, University of Massachusetts Amherst, Amherst, MA 01003; Department of Geosciences, University of Massachusetts Amherst, Amherst, MA 01003; Department of Geology, Amherst College, Amherst, MA 01002

^* To whom correspondence should be addressed. Email: spetsch{at}geo.umass.edu.

	Abstract

The diversity of microorganisms active within sedimentary rocks provides important controls on the geochemistry of many subsurface environments. In particular, biodegradation of organic matter in sedimentary rocks contributes to the biogeochemical cycling of carbon and other elements, and strongly impacts the recovery and quality of fossil fuel resources. In this study, Archaeal diversity was investigated along a salinity gradient spanning 8-3490 mM Cl^- in a subsurface shale rich in CH₄ derived from biodegradation of sedimentary hydrocarbons. Shale pore waters collected from wells in the main CH₄-producing zone lacked electron acceptors such as O₂, NO₃^-, Fe³⁺, or SO₄^2-. Acetate was detected only in high-salinity waters, suggesting that acetoclastic methanogenesis is inhibited above 1000 mM Cl^-. Most-probable-number (MPN) series revealed differences in methanogen substrate utilization (acetate, trimethylamine or H₂/CO₂) associated with chlorinity. Greatest methane production in enrichment cultures was observed in incubations at or close to the native porewater salinity of each inoculum. RFLP analyses of Archaeal 16S rRNA genes from seven wells indicated links between Archaeal communities and pore water salinity. Archaeal clone libraries constructed from sequences from 16S rRNA genes isolated from two wells reveal phylotypes similar to a halophilic methylotrophic Methanohalophilus species and a hydrogenotrophic Methanoplanus species at high salinity, and a single phylotype closely related to Methanocorpusculum bavaricum at low salinity. These results show that several distinct communities of methanogens persist in this subsurface, CH₄-producing environment, each adapted to particular conditions of salinity and preferential substrate use, and each inducing distinct geochemical signatures in shale formation waters.

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