is a large, morphologically conspicuous, sediment-dwelling bacterium. Nothing is known concerning its phylogeny and it has eluded all attempts at laboratory cultivation. The limited physiological description of has been based on morphological features of the bacterium such as the presence of intracellular sulphur inclusions. cells were purified from a wetland region close to Rydal Water (Cumbria, UK). Scanning and transmission electron microscopy revealed that a number of morphologically distinct cell-types, based on cell surface features and the size and abundance of calcite and sulphur inclusions within the cells, were present in a single sample of purified cells. PCR was used to amplify almost full-length 16S rRNA gene sequences from DNA extracted from cells directly purified from sediments. The PCR products were cloned and partial sequences (approx. 400 bp) were determined for seven of the clones. Three different sequence clusters were recovered from the clone libraries. A near full-length (1489 bp) 16S rRNA gene sequence was determined for a representative clone of the most dominant sequence-type (52 % of the sequences). Comparative sequence analysis showed to form a deep branching lineage within the γ-subdivision of the Proteobacteria. was related most closely to the assemblage that includes sulphur-oxidizing symbiotic bacteria, purple sulphur bacteria, and sulpur- and iron-oxidizing thiobacilli. Phylogenetic inferences made using distance, parsimony and maximum likelihood methods all placed with this group of bacteria. Bootstrap support for a relationship with any particular lineage within the assemblage was weak. The seven clone sequences recovered from the cells however formed a monophyletic group well supported by bootstrap analysis (85-100% support depending on the analysis done). It was concluded that was related to organisms of the assemblage but constituted a novel lineage within this group of bacteria. cells were confirmed as the source of the 16S rRNA sequence obtained, by the use of a fluorescently-labelled 165 rRNA-targeted oligonucleotide specific for the rRNA sequence.


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