Methanogenesis in cold marine sediments is a globally important process leading to methane hydrate deposits, cold seeps, physical instability of sediment, and atmospheric methane emissions. We employed a multidisciplinary approach that combined culture-dependent and -independent analyses with geochemical measurements in the sediments of Skan Bay, Alaska (53°N, 167°W), to investigate methanogenesis there. Cultivation-independent analyses of the archaeal community revealed that uncultivated microbes of the kingdoms Euryarchaeota and Crenarchaeota are present at Skan Bay and that methanogens constituted a small proportion of the archaeal community. Methanogens were cultivated from depths of 0 to 60 cm in the sediments, and several strains related to the orders Methanomicrobiales and Methanosarcinales were isolated. Isolates were psychrotolerant marine-adapted strains and included an aceticlastic methanogen, strain AK-6, as well as three strains of CO 2 -reducing methanogens: AK-3, AK7, and AK-8. The phylogenetic positions and physiological characteristics of these strains are described. We propose a new species, Methanogenium boonei, with strain AK-7 as the type strain.More than 85% of the ocean's organic carbon is deposited in shallow, anoxic marine sediments (8). Methanogens play an important role in the degradation of this organic matter by converting low-molecular-weight compounds into methane. In marine sediments, methane is present in dissolved form in the pore water, as free gas, or trapped within methane hydrates. Molecular and isotopic data suggest that most of the methane produced in marine sediments is of biogenic origin (31); however, methanogens that inhabit permanently cold marine sediments have been poorly characterized.Several studies have investigated the microbial diversity in cold marine sediments (12,43,46); however, the role that these microbes play in mediating geochemical processes is still not completely understood. In addition to methanogens, uncultivated lineages of other Archaea have been identified in marine sediments (22,29,32,37,41,67). These include phylotypes belonging to the marine benthic groups B (MBG-B), C, and D and the anaerobic methane-oxidizing (ANME) archaeal groups ANME-1 and ANME-2.Skan Bay has previously served as a model system for studying low-temperature microbial processes in marine sediments (2,3,47,51,66). The sediments contain clear environmental gradients (66), and the complete redox sequence from oxygen to methane occurs within 0.5 m of the surface, allowing for good depth resolution of microbial activities (20). Previous analyses of Skan Bay sediments have shown that microbes mediate important geochemical processes, including sulfate reduction, methane production, and the anaerobic oxidation of methane (AOM) (66). The specific aim of this study was to identify and cultivate Archaea involved in methane cycling at Skan Bay. Cultivation-independent and -dependent techniques were employed to characterize the archaeal populations at various depths and therefore along...
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