An autotrophic, hydrogenotrophic methanogen, designated strain 17A1 T , was isolated from the profundal sediment of the meromictic Lake Pavin, France. The cells of the novel strain, which were non-motile, Gram-staining-negative rods that measured 2-15 mm in length and 0.2-0.4 mm in width, grew as filaments. Strain 17A1 T grew in a mineral medium and its growth was stimulated by the addition of yeast extract, vitamins, acetate or rumen fluid. Penicillin, vancomycin and kanamycin reduced growth but did not completely inhibit it. Growth occurred at 14-41 6C (optimum 30 6C), at pH 5.0-8.5 (optimum pH 6.5) and with 0-0.4 M NaCl (optimum 0.1 M). The novel strain utilized H 2 /CO 2 and methanol/H 2 as substrates but not formate, acetate, methylamine/H 2 , isobutanol or 2-propanol. Its genomic DNA G+C content was 37.0 mol%. In phylogenetic analyses based on 16S rRNA gene sequences, strain 17A1 T appeared to be a member of the genus Methanobacterium, with Methanobacterium beijingense 8-2 T (96.3 % sequence similarity) identified as the most closely related established species. Based on phenotypic and phylogenetic data, strain 17A1 T represents a novel species of methanogen within the genus Methanobacterium, for which the name Methanobacterium lacus sp. nov. is proposed. The type strain is 17A1 T (5DSM 24406 T 5 JCM 17760 T ).Hydrogenotrophic methanogens appear to be responsible for a large proportion of the methane emissions from freshwater lakes (Conrad, 1999) -emissions that represent a significant proportion (6-16 %) of the global natural emissions of methane (Bastviken et al., 2004). To date, however, only a few methanogens have been isolated from freshwater lake sediments and characterized. These include Methanomethylovorans hollandica, which degrades dimethyl sulphide and methanethiol (Lomans et al., 1999), and Methanosarcina lacustris, which can produce methane from methanol, methylamine or H 2 /CO 2 (Simankova et al., 2001). Lake Pavin in France (GPS coordinates 45u 29.749 N 2 u 53.289 E) is a natural and deep (92 m) freshwater lake that is unusual because the water column has been stratified for a very long period without mixing (Michard et al., 1994). The sediment is covered by a 30 m-thick, cold (5 uC), anoxic water column that is in a steady state (Assayag et al., 2008). A molecular ecological study (Lehours et al., 2007) revealed that the methanogenic community in this water column consisted mainly of acetoclastic members of the genus Methanosaeta and species belonging to the order Methanomicrobiales (putative hydrogenotrophs). During a study on methanogens in the sediment of Lake Pavin, several hydrogenotrophic strains were isolated from enrichments containing unidentified species of the genera Methanosarcina, Methanosphaerula and Methanobacterium. We describe here the taxonomic investigation of one of these hydrogenotrophic methanogens, which was designated strain 17A1 T .In December 2007, samples of sediment from Lake Pavin were collected with a gravity-corer sampler and transported to the research labora...
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