Mercury methylation has been extensively reported in the literature among “Firmicutes” and “Proteobacteria.” Nevertheless, results are hardly comparable because of differences in initial inorganic mercury concentrations used. The use of stable isotopic tracers now permits to study mercury transformations at concentrations close to environmental levels. Here, several strains, including strict fermentative and sulphate-reducing bacteria, were tested for their mercury methylation capacities and the results were compared with data available to date. Under such conditions, mercury methylation only occurs among the delta-Proteobacteria. The absence of relation between taxonomic/phylogenetic affiliation and mercury methylation capacities was pointed out and discussed for environmental studies.

A novel, mesophilic, strictly anaerobic, sulfate-reducing and propionate-oxidizing bacterium, strain Prop6T, was enriched and isolated from a municipal anaerobic sewage sludge digester. Cells were Gram-negative, catalase-positive, oval rods, motile by means of amphitrichous flagella, non-spore-forming and contained menaquinone MK-5(H2) as the major respiratory quinone. The genomic DNA G+C content was 51.7 mol%. The optimal NaCl concentration, temperature and pH were 2-5 g l-1, 35°C and 7.6, respectively. Strain Prop6T could only oxidize propionate, lactate and pyruvate (weakly) with sulfate, sulfite or thiosulfate, mainly to acetate. Strain Prop6T fermented pyruvate and lactate to acetate and propionate. The predominant cellular fatty acids were C14:0, C16:0, C16:17, C16:15, C17:1and C18:17. Phylogenetic analysis based on 16S rRNA gene sequences revealed that the isolated strain was a member of the genus Desulfobulbus with Desulfobulbus elongatus DSM 2908T, Desulfobulbus propionicus DSM 2032T and Desulfobulbus rhabdoformis DSM 8777T as closest relatives among validly described species. On the basis of genotypic, phenotypic and chemotaxonomic characteristics, it is proposed that the isolate represents a novel species, Desulfobulbus oligotrophicus sp. nov. The type strain is Prop6T (=DSM 103420T = JCM 31535T).