Phylogenetic affinity of a wide, vacuolate, nitrate-accumulatlng Beggiatoasp. from Monterey Canyon, California, with Thioploca spp.

Environmentally dominant members of the genus Beggiatoa and Thioploca spp. are united by unique morphological and physiological adaptations (S. C. McH atton, J. P. Barry, Ii. W, Jannasch, and D. C. Nelson, Appl, Environ. Micro biol. 62:954-958, 1996), These adaptations include the presence of very wid e filaments (width, 12 to 160 mu m), the presence of a central vacuole comp rising roughly 80% of the cellular biovolume, and the capacity to internall y concentrate nitrate at levels ranging from 150 to 500 mM. Until recently, the genera Beggiatoa and Thioploca were recognized and differentiated on t he basis of morphology alone; they were distinguished by the fact that nume rous Thioploca filaments are contained within a common polysaccharide sheat h, while Beggiatoa filaments occur singly. Vacuolate Beggiatoa or Thioploca spp, can dominate a variety of marine sediments, seeps, and vents, and it has been proposed (H. Fossing, V, A. Gallardo, B. B. Jorgensen, M, Huttel, L, P, Nielsen, H, Schulz, D. E, Canfield, S, Forster, R. N, Glud, J. K. Gun dersen, J, Kuver, N, B. Ramsing, A. Teske, B, Thamdrup, and O, Ulloa, Natur e [London] 374:713-715, 1995) that members of the genus Thioploca are respo nsible for a significant portion of total marine denitrification. In order to investigate the phylogeny of an environmentally dominant Beggiatoa sp,, we analyzed complete 16S rRNA gene sequence data obtained from a natural po pulation found in Monterey Canyon cold seeps, Restriction fragment length p olymorphism analysis of a clone library revealed a dominant clone, which ga ve rise to a putative Monterey Beggiatoa 16S rRNA sequence. Fluorescent in situ hybridization with a sequence-specific probe confirmed that this seque nce originated from wide Beggiatoa filaments (width, 65 to 85 mu m). A phyl ogenetic tree based on evolutionary distances indicated that the Monterey B eggiatoa sp. falls in the gamma subdivision of the class Proteobacteria and is most closely related to the genus Thioploca. This vacuolate Beggiatoa-T hioploca cluster and a more distantly related freshwater Beggiatoa species cluster form a distinct phylogenetic group.